Skardu Seismic Microzonation for Master Plan 2040 (Volume-1)

SKARDU MASTER PLAN 2040

MM PAKISTAN (Pvt) Ltd.

SEISMIC MICROZONATION STUDIES

Volume-1

AUGUST 2022

PREPARED BY

SYED KAZIM MEHDI

Consultant in

Geophysics, Geology, & Seismic Studies

Cell: 0300-5478842 & 0342-2940921

TABLE OF CONTENTS

Page No.

1.0 INTRODUCTION 1

1.1 Tasks to Address as per Technical Proposal 2

2.0 METHODOLOGY FOR SEISMIC HAZARD MICROZONATION 2

3.0 REGIONAL TECTONIC FRAMEWORK 3

3.1 Seismotectonic of Skardu Region 6

4.0 SEISMOTECTONIC AND GEOLOGIC SETTING OF NORTHERN PAKISTAN 7

4.1 General 7

4.2 Eurasian Plate 10

4.3 Kohistan Island Arc 10

4.4 Indian Plate 12

5.0 MAJOR SEISMOTECTONIC FEATURES AROUND SKARDU VALLEY 13

5.1 Main Karakoram Thrust 14

5.2 Main Mantle Thrust 14

5.3 Main Boundary Thrust 15

5.4 Karakoram Fault 15

6.0 GEOLOGY OF SKARDU VALLEY 16

7.0 SEISMOTECTONIC OF SKARDU VALLEY 18

8.0 NEOTECTONIC STUDIES AROUND SKARDU VALLEY 19

9.0 BUILDING CODE OF PAKISTAN 20

9.1 Soil Profile Type 22

9.2 Site Foundation Condition 22

10.0 HISTORICAL SEISMICITY AND EARTHQUAKE CATALOGUE 23

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11.0 INSTRUMENTAL EARTHQUAKE CATALOGUES 27

11.1 NESPAK Earthquake Catalogue 28

11.2 Waseem et al. (2018) Earthquake Catalogue 28

11.3 Pakistan WAPDA Micro Seismic Monitoring System Earthquake 28

Catalogue

11.4 Focal Depth 29

12.0 CATALOGUE COMPILATION 29

12.1 Catalogue Compilation and Magnitude Conversion 30

12.2 Declustering of Earthquake Catalogue 31

12.3 Analysis of Earthquake Record 31

13.0 SEISMOTECTONIC ANALYSIS 34

13.1 Identification and Description of Seismic Sources 34

13.2 Recurrence Relationship and Seismicity Models 39

14.0 SEISMIC HAZARD ANALYSIS 34

14.1 Deterministic Procedure 34

14.2 Probabilistic Procedure 37

14.2.1 PSHA Methodology 37

14.2.2 Source Modeling – Area Sources 38

14.2.3 Earthquake Recurrence Model 39

14.2.4 Maximum Magnitude 40

14.2.5 Attenuation Relationship 40

15.0 PRINCIPLES OF SEISMIC MICROZONATION 41

15.1 Framework for Seismic Microzonation 42

15.2 Seismic Micro Zonation for Skardu City 2040 42

15.3 Migration of Population in Skardu City 2040 42

15.4 Seismic Microzonation of Skardu City 2040 44

15.5 Skardu Khas Seismic Microzone 44

15.6 Hussainabad Seismic Microzone 45

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15.7 Shigri Kalan Seismic Microzone 45

15.8 Results of PSHA 45

16.0 SOIL LIQUEFACTION AND MITIGATION FOR SKARDU CITY 2040 63

16.1 Soil Liquefaction 63

16.2 Soil Liquification and Peak ground acceleration (PGA) 64

16.3 Liquefaction potential index (LPI) 64

16.4 Traditional Architecture of structural units in Skardu 64

16.5 An Example Traditional House in Skardu 65

16.6 Chief Materials 67

16.7 Recent Constructions in Skardu Valley 68

16.8 Structural integrity of existing structures 68

16.9 Conclusions and Recommendations 69

17.0 MICRO SEISMIC MONITORING SYSTEM (MSMS) 69

18.0 CONCLUSIONS AND RECOMMENDATIONS 71

REFERANCES 73

LIST OF FIGURES

Figure-1.

Figure-2.

Figure-3.

Figure-4.

Figure-5.

Figure-6

Topographic location of Gamba/Hotto, Skardu & Hussainabad

Seismicity distribution of Pakistan

Generalized tectonic map of northern Pakistan, showing subdivisions of the

Himalayan Mountains

Tectonic map of Northern Pakistan displaying major faults

Generalized Tectonic Map of Northern Pakistan

Seismotectonic fault System of Northern Pakistan including Skardu

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Figure-7

Figure-8

Figure-9.

Figure-10.

Figure-11.

Figure-12.

Figure-13.

Seismotectonic Model of Northern Pakistan

Geological map of Skardu District

Seismotectonic Map North of Skardu Valley

Peak Ground Acceleration (PGA) variation in Pakistan

Spatial Distribution of Seismicity in and around Skardu City

Seismicity 200 km radius around Skardu City with respect to Faults

Zones in the Skardu Seismic Region

Figure-14. Seismic Microzonation of Skardu City 2040

Figure-15.

Total Hazard Curve for Skardu Khas & Hotto/Gamba areas when Vs30 = 760 m/sec

Figure-16.

Figure-17.

Figure-18.

Figure-19.

Figure-20.

Uniform Hazard Spectra for Skardu Khas & Hotto/Gamba areas

when Vs30 = 760 m/sec







Figure-21.

Total Hazard Curve for Hussainabad & Shigri Kalan

(Airport & Surrounding areas) when Vs30 = 760 m/sec

Figure-22.

.

‘

Figure-23.

Uniform Hazard Spectra for Hussainabad & Shigri Kalan




Figure-24.

Uniform Hazard Spectra for Hussainabad & Shigri Kalan


Figure-25.



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Figure-26

Figure-27

Figure-28

Figure-29

Figure-30

Figure-31

Figure-32

Figure-33

Figure-34

Figure-35

Figure-36.

Figure-37.



Seismic Microzonation Map Return Period = 475 years, Vs30 = 200 m/sec.









Traditional Two-level Dwelling in Skardu

Plan of a traditional house in Skardu with a Central Courtyard

Figure-38.

Traditional Interior Ceiling of Baltistan

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EXECUTIVE SUMMERY

As a part of the Skardu City Master Plan 2040, this report deals with the seismic microzonation

and further development of building and land-use planning strategies. Restrictions to ensure

mainstreaming of seismic hazards in the spatial planning and management of the Skardu City. The

proposed area of interest starting from Hotto/Gamba to Hussainabad Skardu is approximately

135000 kanal. Due to the presence of three major mountain ranges, the geology of Skardu Valley

is much diverse consisting of Metamorphic and Igneous rocks. Mostly all soil types are found in

the Valley. Typically, the Valley has Mesozoic and Palaeozoic -Precambrian soil types.

According to Building Code of Pakistan (BCP) 2007 and 2021, the Skardu City is placed in

seismically active Zone 3, while in its close vicinity there is more active area of Seismic Zone 4.

Historical earthquake data catalogue for Pakistan and composite instrumental seismic data

catalogue of Skardu Region are developed for the studies. The area of 200 km. radial distance

from Skardu is named as Skardu Region in this report. Study of historical catalogue indicates that

Skardu Region has experienced earthquake Intensity up to XI, while seismic events up to Mw 7.6

have originated from the area.

To determine the Soil Profile Types in accordance with Table 4.1 of BCP, the area was classified

into various seismic units. The resultant Soil Profile Types are S B = 760 m/sec, S C = 450 m/sec

and S D = 200 m/sec. As per Deterministic Hazard Analysis the Karakoram strike-slip fault is present

about 35 km NE of the Skardu City. A large amount of seismic activity having Mw ≥ 4.0 in the City,

have been located from this fault. Maximum magnitude assigned to the fault is Mw 7.7, while the

computed PGA (H) = 0.2 g and PGA (V) = 0.13 g [PGA = Peak Ground Acceleration].

Based on the geological reports/maps, geophysical borehole logs data reports, satellite imaginary

and desktop study of research papers, the Skardu City 2040 is divided into three Seismic

Microzones. The seismic Microzones are named as Skardu Khas, Hussainabad and Shigri Kalan.

Most useful way of presenting the result is in terms of horizontal hazard curves and uniform hazard

spectra. Figures for different return periods, i.e., for 475, 975 and 2475 years, relating estimated

ground motion to annual exceedance probabilities which are the inverse of return periods in years.

The Probabilistic Seismic Hazard Analysis (PSHA) was carried out using single site EZ-FRISK

software developed by Fugro Engineering Consultants, USA. The resultant values are provided in

Table-4 of this report. The resultant values of g are in accordance to PSHA of BCP 2021.

Large number of construction activity is under progress in the Skardu City. It should strictly adhere

to existing and state of art Building Code of Pakistan 2021. Areas with Soil Profile Types S B and

S C are good for construction purposes. While Soil Profile Types areas of S D may be avoided in

constructions goings-on.

Constitution of teams for review and enforcement of the Building Codes for government, semi

government, corporation and private residential buildings completed or under construction in urban

areas should be done. Capacity building of stakeholders, service providers and incident

respondents with backup mechanism under Skardu Development Authority (SDA) is needed.

Firming up disaster mitigation management plans for critical departments and to construct the

multilayered incident response teams backed up by emergency response centres for each

department is required.

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There is lack of ground strategies like construction of earthquake resistant structures, flood

management strategies, landslide and avalanche mitigation measures increase the higher chances

of damage by future calamities. The SDA needs immediate establishment of fully equipped disaster

authorities with well-trained disaster rehabilitation force at city as well as district level. The

transportation of rescue teams as well as rescue to supply after the disaster takes lot of time and

energy due to rugged and treacherous terrains of the region is required.

More geological, geotechnical investigations should be carried out to document the active near

source seismogenic structure which would help in formulating safer design decisions. Although

natural calamities cannot be stopped but proper strategies and mitigation measures would

substantially decrease the level of hazard and damage.

For the seismic safety monitoring purposes, a Micro Seismic Monitoring System (MSMS) may be

installed in and around the Skardu City. Plan for MSMS is given as Appendix C.

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1.0 INTRODUCTION

As a part of the Master Planning for Skardu City 2040, this report deals with the micro-seismic

zonation and further development of building and land-use planning strategies and restrictions to

ensure mainstreaming of seismic hazards in the spatial planning and management of the city. The

proposed area of interest (Figure-1), starting from Hotto/Gamba to Hussainabad Skardu is

(approximately 135000 kanal), as the people are shifting to the area without any proper master

plan and haphazardly doing their business which will create trouble in future.

Skardu city is situated at an elevation of nearly 2,500 metres in the Skardu Valley, at the confluence

of the Indus and Shigar Rivers (Figure-1). The Indus River running through the region separates

the Karakoram from the Himalayas Glaciers from the Indus and Shigar valleys broadened the

Skardu valley between 3.2 million years ago up to the Holocene approximately 11,700 years ago.

The city of Skardu is located along the bank of Indus River, surrounded by mountains with no

greenery and sand dunes. Near the city, the river is wide and still. The beauty of the valley is

enhanced by its fresh spring water, delicious fruits, the blue water of the Indus River, historical

sites, lakes, and pleasant weather, which attract tourists from around the world. The city of Skardu

is the main urban centre and headquarters of the Baltistan Division, a strategic northern region of

Pakistan bordering China, Afghanistan, and India.

Figure-1. Topographic location of Gamba/Hotto, Skardu & Hussainabad

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1.1 Tasks to Address as per Technical Proposal

• Seismic Hazard Risk Evaluation of Study Area

• To provide resilience improvement subject plan of the Skardu City 2040.

• To develop strategies related to the development of the building and land use planning in

the context of seismic zoning.

• To prepare strategies regarding restrictions to ensure mainstreaming of seismic hazard in

the spatial planning and management of the Skardu City 2040.

• Proposal for the installation of micro seismic monitoring system.

2.0 METHODOLOGY FOR SEISMIC HAZARD MICROZONATION

Seismic hazard micro-zonation of the study area Skardu City 2040 is required to define zones with

different level of seismic hazard due to earthquakes keeping in view topographical, geological and

local site characteristics and incorporating the results in future spatial planning and management

of the Skardu City 2040.

The main points of the methodology to be adopted for the seismic hazard micro-zonation of the

Skardu City 2040 is briefly described below:

1. Collection and review of available data about regional geology and tectonic setting of the

region covering about 200 km radius of the city center. Preparation of tectonic map, using

available information, to show the major tectonic features of the region.

2. Preparation of a historical and instrumental seismicity Catalogue of the region incorporating

data from different local and international sources. From this set of data, a comprehensive

earthquake Catalogue for 200 km radius of the Skardu City center will be developed.

3. Generate a seismotectonic map by combining the above two maps for the definition of all

the potential earthquake sources effecting the study area.

4. Review seismic parameters for existing Projects in and around Skardu City.

5. Based on the Building Code of Pakistan (2021) recommendations, Seismic Hazard

Evaluation using “Probabilistic Hazard Evaluation procedure (PSHA)” established on area

and/or fault sources, determination of recurrence relationship and activity rate of each

source model and determination of PGA at specific points with 10% probability of

exceedance in 50 years (i.e., a return period of 475 years). Using licensed state-of-the-art

Windows based “EZFRISK” seismic hazard evaluation software developed by Fugro

Consultants USA. Based on the results of PSHA, PGA with 10% probability of exceedance

in 50 years will be prepared for the rock level condition.

6. Seismic hazard evaluation using deterministic hazard evaluation procedures (DSHA) based

on most critical tectonic sources around the study area to establish deterministic scenario.

Based on the results of the DSHA analysis, seismic design parameters associated with

deterministic scenario will be suggested for the soft-rock, hard soil and soft soil level

conditions.

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7. Site characterization study using:

• Available surface geologic maps of the study area.

• Available geotechnical borehole data (SPT, CPT) of the study area (to be collected

with the help of Client).

• Estimation of depth to Engineering bedrock using available borehole data.

• Based on the above data, the Skardu City 2040 will be divided into micro zones

with different site characterization (soft rock, hard soil, soft soil).

8. Estimation of seismic design parameters for areas having different Local site

characteristics.

9. Estimation of liquefaction potential of soils and estimation of liquefaction hazard.

10. Estimation of landslide/slope failure hazard associated with seismic activity.

11. Integration of seismic associated hazard on GIS platform to develop seismic micro-zonation

map of the study area based on:

• Geomorphologic/geotechnical attributes

• Seismological attributes

12. Integration of the seismic hazard micro-zonation map and comments of all findings in the

concept master plan.

13. To prepare strategies related to the development of buildings and land use planning in the

context of seismic zoning.

14. Integration of the seismic hazard micro-zonation map and all findings in the final master

plan.

15. Planning for the installation of Micro Seismic Monitoring System (MSMS) in and around the

Skardu City.

3.0 REGIONAL TECTONIC FRAMEWORK

Pakistan is located along one of the most seismically active regions of the world due to its peculiar

regional tectonic environment (Figure-2). Northern Pakistan region is particularly active that has

experienced devastating earthquakes in the past causing damage to the physical infrastructures

and loss of human lives. The most recent strong earthquake that has occurred in the region is the

Mw 7.6 Kashmir-Hazara October 08, 2005 earthquake, resulting in 72,763 fatalities and 6,8697

injuries alone in Pakistan (Rossetto and Peiris 2009). Similar earthquakes are likely to occur in the

future, and it is thus important to evaluate the seismic performance of structures based on reliable

ground motion scenarios. Consideration of these facts, the past seismicity history and the presence

of active tectonic features warrant careful assessment of ground motions and calculation of

structures in a quantifiable way for the region. In North Pakistan, the western Karakoram region

has a remarkable history of seismicity, extreme climate, and a diverse topographical setting, such

as valleys, ridges, and slopes. The Northern parts of Pakistan are near to the collisional boundaries

of Eurasian and Indian plates margins and therefore seismically very active (Figures 3 through 6).

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Figure-2. Seismicity distribution of Pakistan (Dr. Sarfraz PhD Thesis 2016)

Northern and western sections of Pakistan are more sensitive to earthquake activity than the other

segments because they are surrounded by the micro plates of Afghanistan, Iran and India. Main

Central Thrust (MCT), Main Karakoram Thrust (MKT), Main Boundary Thrust (MBT) and Main

Mantle Thrust (MMT) are the major faults located in Northern Pakistan. The area also includes two

Syntaxial Bends, known as Nanga Parbat Haramosh Massif (NPHM) and Kashmir Hazara, where

the rocks strata are folded around this syntax and are subject to a 90 0 “rotation” from one side to

the other side (Figures 3 through 6). Seismic data indicates that movements along these faults and

Syntaxial Bends are the major sources of significant and destructive earthquakes.

Based on current knowledge of the region, the Skardu Valley is located in the collision zone of the

Indian and Eurasian plates; with “High Seismic Risk”, corresponding to Seismic Design Category

(SDC) “D” of the International Building Code 2006 and Seismic Zone III of Uniform Building Code

(UBC). Also Building Code of Pakistan (BCP) 2007 and 2021 has placed it in Zone 3 of significant

seismic danger.

Therefore, the Skardu City is facing a severe earthquake hazard potential. Moreover, within the

scenario of the October 08, 2005 Kashmir-Hazara earthquake of Pakistan it becomes important to

be very cautious regarding the seismic hazard assessment and seismic micro zonation for such

an important location while working on its Master Plan 2040.

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Figure-3 Generalized tectonic map of northern Pakistan, showing subdivisions of the

Himalayan Mountains (modified after Gansser 1981; Kazmi and Rana 1982).

5

Figure-4. Tectonic map of Northern Pakistan displaying major faults. [EERI Special

Report 2006], (EERI = Earthquake Engineering Research Institute, USA).

3.1 Seismotectonic of Skardu Region

The Skardu Region (area of 200 km radial distance around Skardu City), of the western Karakoram

has been known for instabilities due to moderate to major earthquakes. Seismically active fault

lines (Figures 3 through 6) are located in this region that could make the population vulnerable to

this disaster. The root cause of most seismic events can be associated to tectonic processes in

the upper portions of the earth crust. The earth crust is divided into several plates. Build-up of

strain/strain within these plates or margins is due to the deformations taking place as results of

movements along or relative to the interfaces or margins of the plates.

In the Skardu Region of Gilgit-Baltistan (Western Himalayas), the seismic activity is associated

with the micro earthquakes and macro earthquakes of Mw ≥ 4.0, and largely coincides with the

surface trace of the Himalayan Main Central Thrust (MCT) rather than with the Himalayan Main

Boundary Thrust (MBT) which represents the structural boundary.

Along the Skardu Valley a large amount of seismicity is also contributed from the Karakoram strikeslip

fault present about 55 km NE of the city. Many macro events having Mw ≥ 4.0, have been

located from this fault. Direction of the horizontal compression in the region has been inferred from

the focal mechanism solutions. The direction of crustal stress in the Skardu Valley is NE-SW,

perpendicular to the line of plate collision and the MBT. In the Hindu Kush region, the earthquake

mechanism is generally thrust faulting occasionally normal faulting whereas in the Kashmir, the

earthquakes mainly show thrust faulting mechanism with a clear NE-SW compression.

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4.0 SEISMOTECTONIC AND GEOLOGIC SETTING OF NORTHERN PAKISTAN

4.1 General

The geodynamics and geology of the Karakorum-Himalayan region in Northern Pakistan are

characterized by the interactions of three principal tectonic units (Figure-5):

• The Asian Mass (Eurasian Plate);

• The Kohistan Island Arc; and

• The Indian Mass (Indian Plate).

These units have distinctly different lithology and structural settings and are separated by two major

branches of the Indus suture (Tahirkheli, et al., 1979; Treloar, et al., 1990; Khan, et al., 1997). Both

sutures are marked by the occurrence of a mélange including ultramafic rocks, the southern one

also having a wedge of garnet granulite’s considered to have recrystallized at a depth of more than

40 km.

The geodynamic framework of Northern Pakistan is characterized by the collision and coalescence

of Eurasian and Indian Continental Plates, which were once separated by the oceanic domains,

and creation of the Kohistan Island arc in the late Cretaceous. The collisional process started in

the late Eocene to early Oligocene with the formation of the Himalayan Ranges and this process

still continues. Relative to the Eurasia, the Indian plate is still moving northwards at a rate of around

4 cm/year. The subduction of the Indian plate beneath the Eurasian plate has resulted in folding

and thrusting of the upper crustal layers near the collisional boundary. The thrusting has been

depicted from north to south in the shape of MKT (Main Karakoram Thrust), MMT (Main Mantle

Thrust), MBT (Main Boundary Thrust) and SRT (Salt Range Thrust) the locations of which are

shown in Figures 3 through 6.

Structural geometry shows that the duplex stacks in nappe structures became younger away from

the suture zone in the opposite direction that the footwall plate is moving. Thus, for the Northern

Pakistan region, the older thrusts are near the Main Mantle Thrust (MMT) or suture zone and the

youngest thrusts are farther south along the Salt Range Thrust, well within the India plate (Figures

3 through 6). The Indian subcontinent has been colliding with the Eurasian subcontinent over the

last 30 to 40 million years (Aitchison et al., 2007). During this period, the continental lithosphere

longer than 2,000 km has been shortened into the massive mountain ranges and elevated plateaus

of central Asia.

The Himalaya-Karakoram-Tibet orogen System is one of the most fascinating tectonic zones on

the Earth as a natural laboratory and cradles for various geodynamic concepts like continentalisland

arc collision, extrusion tectonics, channel flows in the convergent orogenic belts, inverted

metamorphism, monsoon-control erosion affecting the tectonics, present day crustal deformation

and seismicity. The high mountainous belt is the result of collision between the Indian-Eurasian

plates during the late Cretaceous and Cenozoic.

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Figure-5. Generalized Tectonic Map of Northern Pakistan; by GSP.

The most complete and accessible cross-section through various tectonic units of the Himalayan,

Trans-Himalayan and Karakoram mountains is in the Northern Pakistan where one can visualize

the late Mesozoic subduction of the Neo-Tethyan oceanic lithosphere along the Northern Suture

Zone (NSZ) and the Indus Tsangpo Suture Zone (ITSZ), which is followed by intense crustal

shortening in the Himalaya during the Cenozoic continent-continent collision (Honegger et al.,

1982).

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Figure-6 Seismotectonic fault System of Northern Pakistan including Skardu

From Searle et al. (1999).

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The geology of Northern Pakistan provides insight into the evolution of the south Asian margin

since the Paleozoic and is dominated structurally by the Northern Suture Zone (NSZ), an 800 km

long dextral strike-slip fault that bounds western Tibet, extending from the Pamirs, in Northern

Pakistan to Gar Basin in southwest Tibet. Though numerous details are now available of the

southern segment of the Himalaya (Gansser, 1964), but still there is a lack of literature about this

important segment, where the linkage between the Himalayan and Karakoram mountains can be

visualized through the contacts of the Indian and Asian plates particularly north of Skardu Valley.

The geology of Northern Pakistan is a superb example of continental collision tectonics. In this

area, the three of the world’s greatest mountain ranges converge, the Himalayas, the Karakoram,

and the Hindukush. The mountain building process that formed these ranges commenced in

Cretaceous time when Indian plate started moving and was carried northward (Scotese et al.,

1988). During that time (i.e., Early Cretaceous) Karakoram terrane sutured with eastern Hindukush

along the Tirich Mir fault (Zanchi et el., 2000; Hildebrand et al, 2001).

Soon after, the intra-oceanic Kohistan arc formed over a subduction zone that dipped beneath the

arc, either to the south or to the north (Khan et al. 1997). It is widely accepted that the northward

movement of India was concurrent with the accretion to Asia of an intra-oceanic arc system, the

Kohistan arc that collided with Asia along the Shyok Suture or MKT. The southern margin of Asia,

including the Kohistan arc, then became an Andean type convergent margin, until India collided

with Asia. Thrusting of the Kohistan terrane southward over the northern Indian plate margin along

the Main Mantle Thrust (MMT) probably took place in Late Cretaceous or Paleocene time and was

completed by 55Ma, forming the Indus Suture Zone (Searle et al., 1999).

A detailed description of the salient features of the Eurasian plate, Kohistan Island and the Indian

plate are given below:

4.2 The Eurasian Plate

The physiographic divisions of the southern part of the Eurasian Plate in Northern Pakistan include

the Northern Karakorum Tethyan Zone, the Karakorum Batholith, and Volcanic and Metasediments

south of the Karakorum Batholith. The Eurasian Plate is bordered to the south by the Northern

Suture, which consists of an almost chaotic arrangement of large lenses, each several kilometres

long and several tens of meters wide, of highly varied sedimentary, metamorphic, and igneous

rocks in a matrix of chloritoid slates. The whole assemblage has the appearance of a major

mélange with no simple repetitions, as expected in an imbricate zone. This tectonic zone is

considered to mark the suture between the Kohistan sequence to the south and the Eurasian Plate

to the north. There is no evidence of blue-schists, of obducted high pressure granulite’s or of an

ophiolite, but instead large tectonic lenses of a mélange.

4.3 Kohistan Island Arc

The Kohistan Island Arc was formed in the mid-Cretaceous and sutured to Asia around 100 to 85

million years ago. India later collided with the arc after continued subduction beneath the arc

complex, now accreted to the active continental margin. Nearly 50 to 55 million years ago, the two

continental plates collided at this junction.

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Tremendous amount of pressure created caused the Earth’s crust to buckle, producing large

horizontal and vertical displacement and also producing the mountains of the Karakoram.

The principal rock units of the Kohistan Island Arc include, from south to north include:

• Jijal Complex: granulite, mafics, and ultramafics

• Kamila Amphibolite Complex: mostly norites

• Chilas Complex: mafic and ultra-mafic layered complex of gabbros, norites, and dunnite

intersected by dikes and seams of anorthosite and chromite

• Kohistan Batholith: various calc-alkaline intrusive

• Kohistan Arc Sequence: various meta-sedimentary units and volcanic units typical of an

island arc and fore-arc setting.

Kohistan is an intra-oceanic island arc bounded by the Main Mantle Thrust (MMT) to the south and

the Main Karakoram Thrust (MKT) to the north. This E-W oriented arc is wedged between the

northern promontory of the Indian crustal plate and the Karakoram block. Gravity data modelling

indicates that the MMT and MKT dip northward at 35˚ to 50˚ and that the Kohistan arc terrain is 8

to 10 km thick (Malinconico, 1989).

Seismological data suggests that the arc is underlain by the Indian crustal plate (Seeber and

Armbruster, 1979, Fineti et al., 1979). The northern and western part of the arc, along MKT, is

covered by a sequence of Late Cretaceous to Paleocene volcanic and sedimentary rocks. The

central part of the arc terrain is mainly composed of Kohistan Batholith which comprises an early

(110-85 Ma) suite of gabbro and diorite, followed by more extensive intrusions of gabbro, diorite

and granodiorite (85-40 Ma) which are intruded by much younger dykes and sills of leucogranite

(30-26 Ma).

The southern part of Kohistan is comprised of a thick sequence of mafic and ultramafic rocks.

These rocks may be divided into three tectono-metamorphic complexes separated by major thrust

zones. The Chilas Complex forms the northern and upper unit. It comprises layered norites and

gabbros metamorphosed to granulite facies. It is characterized by a series of south-verging folds.

It has been thrusted southwards over the Kamila Amphibolites Complex. The latter consists of

amphibolites, meta-gabbro and orthogneisses. This sequence comprises a highly tectonised shear

zone. Southward, it is thrusted over the Jijal Complex which forms a tectonic wedge between the

Kamila Shear zone and the MMT.

The Jijal Complex is largely comprised of garnet-pyroxene-granulites and ultramafic rock

(Tahirkheli and Jan, 1979; Coward et al., 1986; Khan et al., 1993; Treloar et al., 1990; Miller et al.,

1991).

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4.4 Indian Plate

The bedrock suites south of the Kohistan Island Arc and the southern suture zone include those

forming the pre-collisional stratigraphy of the Indian Plate plus the syn- and post-tectonic material

eroded from the mountain ranges of the Himalayas, Karakorum, Hindukush, and Pamirs.

The principal geologic units are:

➢ Salt Range. The Salt Range defines the Frontal Thrust of the Himalayas, a thin-skinned

structure riding on an evaporite decollement. The topographic relief of the Salt Range is

produced by blind thrusts and ramp anticlines.

➢ Molasse. Molasse sequences of detrital sediments form the Margalla Hills and the

Punjab Plains. All tectonism is thin skinned with numerous southward-propagating

thrusts that have produced numerous imbricate zones. The sedimentary sequences

making up the Murree Series found in the project area belong to this grouping.

➢ Hazara Sediments. The Hazara metasedimentary belt is largely composed of

Precambrian to Early Mesozoic sediments. The Precambrian sequence is composed of

quartz schist, graphitic schist, marble and gneiss overlain by thick sequence of slate,

phyllite and greywacke sandstone. The Precambrian sequence is unconformably

overlain by quartzite and argillite.

➢ Mansehra Batholith. Imbricated slices of this granitic batholith, intruded into the

metamorphic cover, are exposed in the Hazara Syntaxis. It is Cambrian in age and

obviously pre-exists collision.

➢ Metamorphic Cover. This consists of late Precambrian-early Cambrian metasediments,

which have undergone a Palaeozoic low-grade metamorphism and are overlain by precollisional

Mesozoic sediments. These were further metamorphosed and thrusted in the

foothill of the MMT synchronous with full collision.

➢ Nanga Parbat Group. Rocks of the Nanga Parbat Group represent units belonging to the

cratonic Gondwana basement, exposed in the Nanga Parbat-Haramosh Massif syntaxis.

The Proterozoic gneisses of the Indian Plate have their northernmost exposure in the

Nanga Parbat Syntaxis and represent the lowest structural levels of the Indian Plate

observed. They have been mapped and subdivided into three litho-stratigraphical groups

(Madin et al., 1989).

12

Figure-7. Seismotectonic Model of Northern Pakistan (Tectonic Aneurysm Model by

National Science Foundation USA,

www.ees.lehigh.edu/groups/corners/index.shtml)

5.0 MAJOR SEISMOTECTONIC FEATURES AROUND SKARDU VALLEY

It has been established that the major faults of Pakistan appear to be seismically quiet except at

the times of large earthquakes (e.g., Nakata et al., 1991). It seems that this silence (or seismic

gap) is at least true for the Himalayas. It represents a problem while conducting seismic hazard

evaluation as we can find a seismic gap in an area and it may be found inactive for larger time

periods than the monitoring record. Also, while a thrust regime clearly dominates in several places

of the study area, it is often difficult or impossible to associate specific seismic activity with specific

fault traces, and this leads to the conclusion that many faults may be blind.

The boundaries of major Lithological units within the Kohistan Island Arc (KIA) area are known to

be faulted based on geological mapping. The average rupture length of potential earthquake faults

in the Kohistan province is considered to be in the range of 100 km, based on examination of map

trace lengths and field observations of features during geo-tectonic investigations. The Kohistan

Oceanic Arc is bounded in the north by the Main Karakoram Thrust (MKT) and in the south by the

Main Mantle Thrust (MMT) (Figure-6). Along the MKT the region is sutured to the Asiatic

mass/Asian Plate, including the Eurasian Continent and Karakoram micro-continental blocks. The

territory of Kohistan covers about 36000 km 2 .

Major faults in and around Skardu Valley are described on the next pages:

13

5.1 Main Karakoram Thrust (MKT)

The Main Karakoram Thrust (MKT) represents the collision zone of the southern margin of the

Eurasian plate in Asia and extends into the Baltistan area through Hashupa and Machie in the

Shigar and Shyok valleys, respectively. This fault forms the northern boundary of the Kohistan

Island Arc and runs eastward to join Indus suture zone in upper Himalayas and terminates at its

junction with the Karakoram fault. In the Chitral and Gilgit area, the rocks of the Karakoram

Batholith are thrusted over the rocks of Kohistan Batholith along the MKT (Figure-7).

MKT is a high angle, seismically active thrust with a large number of earthquakes of low to medium

Intensity. It is considered that rupture along the MKT during an earthquake could take place over

a large area and could involve a relatively long portion of the fault system. This equates to known

ruptures on smaller boundary structures elsewhere in the Himalaya and the fact that the fault zone

is comparatively straight over significant distances (>100 km).

The MKT is the collision zone of the southern margin of the Eurasian plate and extends into

Baltistan through the Hashupa, Shigar, and Shyok valleys, respectively. MKT is a seismically active

thrust fault that has a high angle along which many earthquakes occurred. It represents the collision

zone of the southern margin of the Eurasian plate in Asia and extends into the Baltistan area

through Hashupa and Machie in the Shigar and Shyok valleys, respectively. This fault forms the

northern boundary of the Kohistan Island Arc and runs eastward to join Indus suture zone in upper

Himalayas and terminates at its junction with the Karakoram fault (Figure-7). In the Chitral and

Gilgit area, the rocks of the Karakoram Batholith are thrusted over the rocks of Kohistan Batholith

along the MKT.

5.2 Main Mantle Thrust (MMT)

The MMT is the suture zone between the Indian plate and the Kohistan Island Arc. This zone is

marked by the presence of upper mantle and lower crustal rocks on the Kohistan side and exhumed

deep lower crustal rocks such as blue schist and eclogite on the Indian side. The MMT extends

from Nawagai (Mohmand Agency) in the west to the north of Narran (Kaghan Valley) in the east,

where it takes a north-eastward bend towards the east of Bunji and gets truncated by Raikot Fault.

The MMT was originally defined as the tectonic boundary between the metamorphic shield and

platform rocks of the Indian plate hinterland and dominantly mafic and ultramafic rocks of the

Kohistan-Ladakh arc complex in Pakistan (Tahirkheli et al., 1979).

DiPietro et al. (2000, 2008) suggest that the MMT contact can be defined as a series of faults of

different age and tectonic history that collectively define the northern margin of the Indian plate in

Pakistan. On this basis, the faults that define the MMT vary in age from Quaternary to possibly as

old as Late Cretaceous. Discontinuous lenses of ophiolite mélange, which overlie the MMT contact

and intervene between the Indian plate and the Kohistan Island Arc, are considered to be part of

an MMT zone that is equivalent with the Indus suture zone. Auxiliary structures associated with the

MMT include imbricate thrusts and shears parallel to it, including the Kamila Shear Zone. Towards

the east, the major north-south striking Raikot Fault zone, which together with its associated

structures, exhibits remarkable neo tectonic features with recently located earthquakes between

November 2002 and January 2003 (DiPietro et al., 2000). Ruptures on the MMT are thought to be

limited to comparatively short segments of the system of faults, shears, and sutures. This

assumption is supported by the mapping of the fault trace, which is remarkably sinuous.

14

Majority of recorded earthquakes in this region occur deeper than 20 km and potentially reflect

movement on the fault plane of the MMT. In area east of Kharg along Indus Kohistan, where large

ophiolite slices are absent, the MMT is represented by the Kohistan-Raikot Fault system and by

faults and mylonite zones that define the northern and eastern flanks of the Nanga Parbat-

Haramosh massif (Figures 6 & 7). West of Kharg, the MMT is represented by the Shergarh fault at

Kharg, the Kohistan fault in the Indus syntaxis, the Kishora fault in Swat, and the Kohistan fault

near Chakdara. Further west, the MMT comprises the Nawagai fault along the west side of the

Malakand slice, imbricate faults along the northern margin of the Dargai melange, the Dargai fault

at Qila and Nawe Kili, and the Nawagai faults to the Afghan border (Figure 7). West of Kharg, the

MMT would be bounded on the north side by the Kohistan fault and on the south side by the

Shergarh-Kishora-Dargai-Nawagai fault system. All of these features are described in more detail

in DiPietro et al. (2000).

5.3 Main Boundary Thrust (MBT)

The most significant and active tectonic feature of regional extent is the Main Boundary Thrust

(MBT). It is the main frontal thrust of Himalayan Range, which runs along the Himalayan arc for

almost 2500 km from the Assam in the east to Kashmir and Parachinar in the west. MBT along

with other associated thrusts forms a sharp conspicuous Hazara-Kashmir Syntaxis. This syntaxial

bend is the most dominant tectonic feature of the area as all local major fault systems and geologic

structures follow its trend. On the west side of syntaxial knot, the MBT initially follows a rather

southwest trend and then extend westward reaching Parachinar.

Near its surface trace, the MBT dips northward at a steep angle, which becomes sub-horizontal

with depth. Islamabad-Rawalpindi area is located at a close distance south of the western limb of

the MBT.

A number of large to major earthquakes have occurred along Himalayan Arc east of the Hazara-

Kashmir syntaxis during the last two centuries, which places it amongst the most active regions of

the world. A lot of seismicity recorded during the last century is associated with surface and

subsurface extensions of MBT and other associated thrusts. Based on this data, Seeber et al.

(1981) have shown that great earthquakes occurring along Himalayan Arc are probably related to

slips taking place along this quasi-horizontal surface (detachment).

5.4 Karakoram Fault

The Karakoram fault is an oblique-slip fault system in the Himalayan region across India and Asia.

The slip along the fault accommodates radial expansion of the Himalayan arc, northward

indentation of the Pamir Mountains, and eastward lateral extrusion of the Tibetan plateau.

Current plate motions suggest that the convergence between the Indian Plate and the Eurasian

Plate is around 44±5 mm per year in the western Himalaya-Pamir region and approximately

50±2 mm per year in the eastern Himalayan region. The creation of the Karakoram fault started

with the closing of the ancient Tethys Ocean seaway which once separated the two modern

continents of Asia and India. The Karakoram fault itself does not trace a plate boundary, except for

where it possibly ends in the Indus-Yarlung Suture Zone. The original thrusting occurred by linking

existing thrust faults in what is now the Pamir Mountains starting between 17 and 20 million years

ago.

15

The Karakoram fault was a right lateral slip fault starting approximately 20 million years ago.

Approximately 14 million years ago the fault changed to a predominately normal fault. This

conclusion is based on argon dating. Around 10-11 million years ago the Karakoram fault had

become trans-tensional and extended southwest into Tibet. The southwest extension is marked by

the Karakoram fault crossing the active South Kailas Thrust in the vicinity of present-day Mount

Kailas. It is suggested that a late Cretaceous-Eocene granite batholith had been offset 1000 km

dextrally along the Karakoram fault based on mapping in the central Karakoram, in nearby Ladakh-

Zanskar, and in south Tibet. Some researchers suggest that this might be incorrect due to

associating granite that was never part of the same batholith. Others researchers work have shown

600 km of right lateral slip since 23 million years ago, and possibly starting 34 million years ago,

based on U-Pb dating. Slip in this model has been transferred into the Indus-Yalu suture zone, as

well as large scale boudinage. Research in the early 1990s suggested that this slip was transferred

into the South Tibetan Detachment. Another suggestion is that the Karakoram fault is offset at least

500 km as measured by the offset of late Palaeozoic granites in the Kunlun batholith. Most

researchers tend to agree with the lower slip estimates. Currently some researchers believe that

the Karakoram fault merges and terminates into the Indus-Yalu suture zone at Mount Kailas

6.0 GEOLOGY OF SKARDU VALLEY

Due to the presence of three major mountain ranges, the geology of Skardu Valley is much diverse

consisting of Metamorphic and Igneous rocks. Most of the soil types are found in the Valley. Mostly

the Valley has Mesozoic and Palaeozoic -Precambrian soil. Skardu Valley is located along the

Kohistan-Ladakh terrane, formed as a magmatic arch over a Tethyan subduction zone that was

later accreted onto the Eurasian Plate (Figure-8). The stone in the Skardu region is Katzara schist,

with a radiometric age of 37 to 105 million years. Numerous complex granitic pegmatites and a few

alpine-cleft metamorphic deposits are found in the Skardu Valley and its tributaries. Adjacent to it,

Shigar Valley contains the Main Karakorum Thrust separating the metasediments (chlorite to

amphibolite grade) on the Asian plate from the southern volcanoclastic rocks of the Kohistan-

Ladakh Island arc. Active erosion in the nearby Karakoram Mountains has resulted in enormous

deposits of sediment throughout the Skardu Valley.

A variety of gemstones is being mined in the Skardu Valley. These include beryl, tourmaline,

garnet, apatite, topaz, fluorite, zoisite, and axinite, mostly occurring in complex or zoned pegmatites

and metamorphic rocks. The valley remains a hub for global mountaineers from April to October

during summer, while winter remains snowy and freezing in the area. The majestic valley also has

impressive glaciers in Baltoro, Gyari and Gyong in the Siachen region. The city of Skardu is located

along the bank of Indus River, surrounded by mountains with no greenery and sand dunes. Near

the city, the river is wide and still. The valley is often snowbound in winter. Roads in and out of

Skardu can be blocked for several days and air travel is the only feasible alternative to reach other

parts of the country.

The igneous rocks of this complex display several phases of tectonic deformation during

which a penetrative tectonic fabric was generated. During this ttectonic genesis the basic

rocks were deformed into a series of recumbent south-verging isoclinal anticlines

separated by tight narrow synclines. The sub-horizontal fold axis and the northerly dipping

regional tectonic layering mostly trend roughly east west.

16

Figure 8: Geological map of Skardu District. (With thanks from Department of

Geosciences, Baltistan University, Hussainabad, Skardu)

The exposed rocks range in age from pre-Cambrian to recent and are composed of igneous and

metamorphic rocks of various types. The general structural trend in the Karakoram ranges along

the Skardu Valley in the NW-SE direction (Figure-8). The topography of the area is demarcated by

steep-walled glacially scoured valleys dominated by arete and horn geometries. Tectonically the

area is in the active collision zone of the Indian and Eurasian plates (Figure-6). The crustal

shortening, active faulting, and subduction are continuing with convergence and uplift rates of ~4–

5 cm/year and ~7mm/year, respectively.

The geological setting of Kohistan Island Arc was depicted by many researchers in the form of

regional geologic maps, like those developed by Bard et al. (1983) and by Petterson (2018). Based

on these maps, Burg (2006) developed a representative subsurface cross-section, which is shown

in Figures 7 & 8. These maps show that the subduction tectonics of the Kohistan Island Arc are

similar to that present in the Himalayas, where the north-dipping Main Boundary Thrust (MBT) and

Main Central Thrust (MCT) become sub-horizontal at depth. The MMT is a regional feature similar

in nature to that of the MBT below the Himalaya. In the Himalayas, large earthquakes along the

MBT and the MCT have been mainly associated with slip along ramps in the decollement dipping

at shallower angle towards north. However, the MMT is of small regional extent compared to the

MBT, and the Kohistan Island Arc is not as active as the Himalayas.

17

7.0 SEISMOTECTONIC OF SKARDU VALLEY

Skardu Valley is situated on the Kohistan Island Arc (~100 Ma), which is sandwiched between the

northern Asian/Eurasian continental plate and the southern Indian plate and bounded by the MKT

in the north and west and by the MMT to the south and east (Figure-9).

Due to the limitations in the available earthquake data of the region, it is difficult to correlate specific

earthquakes with specific faults, and it is possible that some faults may experience blind ruptures.

Some major faults in Pakistan, particularly those near the Himalayas, may be seismically quiet

except at the times of large earthquakes (e.g., Nakata et al., 1991). However, the Skardu Valley is

not located near the Himalayas and given the earthquake catalogue in the region around the Valley,

the occurrence rate of seismicity is not considered to be episodic and is assumed to be stationary.

Figure-9 Seismotectonic Map North of Skardu Valley (after Hanson 1986 & 1989)

18

The Skardu Valley is sited very close to the boundary between Kohistan Island and the Eurasian

plate marked by MKT or Shyok suture zone. The most critical tectonic feature is the MKT. Other

important tectonic features of regional scale extent are the Karakoram fault (KF) in the northeast

and Ladakh side of Main Mantle Thrust (MMT) in the south. This area hosts some of the highest

peaks and are amongst the most complex and difficult terrains of the world, demonstrating a great

variety of rock types and structures (Figure-9). The seismic map of Pakistan indicates that Skardu

Valley and surrounding areas lies in a very active seismic zone. The seismic factor in this

neighbourhood according to Building Code of Pakistan, Seismic Provision-2007, has been

evaluated as Zone-III (Figure-10) of noticeable seismic danger with acceleration values of 0.24 to

0.32 g. Along to the immediate north and north-west lies the Zone-IV of significant seismic danger

with acceleration values of ≥ 0.32 g.

Some major seismic events that were widely felt and caused some destructions in the Skardu

region, during the recent past are the 1974 Pattan earthquake of mb 6.0, two Bunji earthquakes of

mb 5.3 and 6.0 that occurred in 2002, two Batagram earthquakes of mb 5.3 and 5.5 that took place

in 2004 and the October 8, 2005 Kashmir Hazara earthquake with Mw 7.6. The most recent are

the Gilgit earthquakes of December 30, 2019 with mb 5.1 and Skardu earthquake of March 2022.

It is believed that in August 1871 a shallow focused earthquake Mw = 6.3 with epicentre in Gilgit

city was felt widely (Jacob 1979). It’s computed Intensity at Gilgit city was VIII, while at Skardu

Valleys was VII on Modified Mercalli Scale (MMS). However, later on till date, no earthquake with

Mw ≥ 6.0 has been located from the Skardu Valley.

8.0 NEOTECTONIC STUDIES AROUND SKARDU VALLEY

The neotectonics study of a region provide useful evidence of paleo-seismic activity of that region.

A number of studies were done by various researchers on the deformation of Late Quaternary

sediment in upper Himalayas. Binita Phartiyal and Anupam Sharma (2009) while studying the softsediment

deformation structures in the Late Quaternary sediments of Ladakh found evidence for

multiple phases of seismic tremors in the northwestern Himalayan region.

A number of neotectonics studies have been done by various researchers on the deformation of

Late Quaternary sediment in upper Himalayas. Binita Phartiyal and Anupam Sharma (2009) while

studying the soft-sediment deformation structures in the Late Quaternary sediments of Ladakh

found evidence for multiple phases of seismic tremors in the northwestern Himalayan region. The

exposed Quaternary sections of Ladakh show evidence of seismicity during the late Quaternary.

Multiple levels of soft-sediment deformation structures (seismites) are recorded from the

Quaternary sediments of the Spituk-Leh, along Indus Suture Zone (ISZ) and the Khalsar palaeolakes,

along Shyok Suture Zone (SSZ) and Karakoram Fault (KF).

The studied sections by various Geologists and Seismologists are evidences of two major

tectonically formed paleo-lakes. The release of stress along the ISZ, SSZ and KF, may have been

responsible for inducing seismicity in the area during the late Quaternary times which may have

caused liquefaction as a direct consequence of permanent deformation of ground surface due to

earthquakes of large magnitudes (Intensity > V). The evidence of neo tectonic features such as,

tilting of the recent deposits along the river terraces, folding and faulting within the recent deposits,

coarsening upwards of the deposited materials, active landslides along the MKT, crushed and

brecciated zone and structural features were found. These evidences confirm the activeness of the

Main Karakoram Thrust (MKT).

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9.0 BUILDING CODE OF PAKISTAN

A building code is a set of rules that specify the standards for constructed objects such as buildings

and non-building structures. Buildings must conform to the code to obtain planning and

construction permission from concerned authorities. The main purpose of building codes is to

protect public health, safety and general welfare as they relate to the construction and occupancy

of main buildings and structures.

Seismic building codes result in earthquake-resistant buildings, but not earthquake-proof buildings.

Seismic codes are intended to protect people inside buildings by preventing collapse and allowing

for safe evacuation. Structures built according to the building code should resist minor earthquakes

undamaged, resist moderate earthquakes without significant structural damage, and resist severe

earthquakes without collapse.

Pakistan lies on a seismic junction of three major tectonic plates of the world including Eurasian,

Indian and Arabian. The devastating Kashmir-Hazara Earthquake of October, 2005 resulted in

87,000 casualties, 780,000 buildings were destroyed and 2.5 million people became homeless with

economic loss around US $ 2.3 billion. During that depth of difficulty and National disaster,

PEC took one step further and constituted a high profile Task Force comprising of eminent experts

from academia, industry, Ministry of Housing & Works, Ministry of Science and Technology,

National Engineering Services Pakistan (NESPAK) along with allied stakeholders, partier

organizations and the never ending assistance provided by the

International Code Council (ICC) and the American Concrete Institute (ACI) for development of

first-ever Building Code of Pakistan – Seismic Provisions in 2007.

Over a period of decade, the frequency of earthquakes has been gradually increasing in Pakistan

as per real-time recorded seismic data by Pakistan WAPDA and Pakistan Metrological Department

In fact, there were five major earthquakes ranging from 6.4 Mw to 7.5 Mw hit almost all metropolis

cities in Pakistan. Hence, the mandatory revision of the Code was very much needed in view of

seismic-resilient design with durable structural strength, modern typologies of building construction

and safe practices. For revision of the Code, PEC joined hands with the World Bank, NED-UET,

NDRMF, ICC and also constituted a high-profile Task Force on updating of Building Code of

Pakistan (2021). Figure-10 presents a PGA map of Pakistan from the BCP-2021.

Building Codes of Pakistan Seismic Provisions (BCPSP, 2007 & 2021), specified that a precise

seismic factor (to design a building) should possibly be higher than that concentrating in the seismic

zoning map of Pakistan. They stated design for PGA of 0.125g (acceleration due to gravity)

horizontal component acceleration value for average structures, and for 0.2g without collapse for

significant structures. For profound structures, micro level site-specific designs are vital that take

into account the strength of the underlying soil and bedrock and the region from credible

earthquake sources.

Construction of all building types is required to be followed according to rules and guidelines

mentioned in the Building Code of Pakistan (BCP) seismic provision 2007 and updated November

2021. Site specific studies are required for special types of all the large civil structures like all parts

of Hydro Power Projects (HPP), all parts of small and large storage dams, bridges and high-rise

buildings.

20

Figure-10 Peak Ground Acceleration (PGA) variation in Pakistan (BCP 2021)

21

9.1 Soil Profile Types

Each Structural site shall be assigned a soil profile type based on properly substantiated soil

engineering characteristics using the site categorization procedure. Building Code of Pakistan

seismic provision 2007 has defined the soil profile types in following BCP Table-2.

9.2 Site Foundation Condition

For the seismic hazard assessment, the site foundation condition is best defined by Vs30 which is

defined as the average shear-wave velocity (Vs) of subsurface material for the upper 30-m depth.

This is calculated from a Vs measurement presented in various formats. Because of the

gravitational influence, the property of ground materials is usually presented by, a "layered-earth

model" in which the earth's properties change only vertically and are represented by a collection

of distinctive layers. Each layer is then considered a homogeneous material with the same seismic

properties in S and P waves’ velocity (Vs and Vp) and density. In addition, because of the more

rapid property change at shallower depths, the thickness of each layer in a layered-earth-model

tends to be smaller at the top and increases with depth.

Calculation of the average Vs for a certain depth range (for example, top 30 m) can be

accomplished in two different ways: (1) based on relative thickness-contribution of each layer

(Method 1), and (2) based on the definition of velocity ─ total thickness (∑di) divided by total travel

time (∑ti) that is calculated by summation of thickness (di) divided by velocity (Vsi) of each layer

(Method 2).

The calculation of Vs30 in (m/sec) using the Method 2 is done by the formula:

Vs30 = ∑di / ∑ti = 30 / ∑(di/Vsi) (meters/sec) (1)

22

10.0 HISTORICAL SEISMICITY AND EARTHQUAKE CATALOGUE

Until the advent of instrumental records from seismological observatories at the beginning of the

20th century, intensity data collected from historical records were the only source of earthquake

information. The importance of intensity data is that it establishes some understanding of the level

of damage that can be expected to occur in a given region.

Pre-instrumental seismicity records include general accounts of injuries, loss to life, and damage

to property and infrastructure. Historical pre-instrument earthquake information has been

researched for other projects in Pakistan (such as for Tarbela Dam Project, Dasu Hydroelectric

Project and Diamer-Basha Dam Project) and several important historical earthquake catalogues

have been identified (Oldham, 1883; Heuckroth and Karim, 1970; Ambraseys et al., 1975;

Quittmeyer and Jacob, 1979).

Historical earthquake data helps to identify the seismicity patterns of an area and, in regions where

numerous earthquakes have occurred, can provide a basis for calculating the estimated probability

of future earthquake motion at the site considered. This is based on the assumption that events

similar to those which have occurred in the past could reoccur at or near the same location. The

lack of historical earthquakes, however, does not necessarily imply that the area considered is

aseismic.

It can be readily recognized that Northern Pakistan has been a region characterized by consistent

occurrence of historically documented earthquakes, although it is also evident that the records are

clearly incomplete. Since the 1700’s, the historical earthquake data for the northern areas of

Pakistan are few and mainly concentrated on the centres of colonial administration. Important

earthquakes for which damage data are available include:

➢

Aristobulus of Cassandreia described that the first known historical account of

seismicity of northern part of Pakistan in the fourth century B.C. He

accompanied Alexander on his expedition to India, who pointed out that the

country above the river Jhelum was subjected to earthquakes, which caused

the ground to open up so much that even the riverbeds were changed

(Ambraseys et al., 1975).

‣ An important historical earthquake occurring in northern Pakistan was the

destructive earthquake of 25 A.D., which ruined the city of Taxila, to which the

intensity of IX-X has been assigned (Ambraseys et al., 1975). The effect of this

earthquake can still be seen among the excavated remains of Jandail, Sirkap

and Dharmaraja near Taxila. The building methods after this earthquake

changed, including reduction in the height of buildings, improvements in

masonry bracing density, and making the foundations more secure, which can

still be observed in current structures in the area.

‣ On March 25, 1869, a large earthquake occurred in the Hindukush region and

was strongly felt at Kohat, Peshawar, Lahore, Gilgit-Baltistan and at Khodjend and

Tashkent with shaking that lasted 20 seconds.

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‣ On May 22, 1871, a damaging earthquake was noted at Gilgit and Skardu, and

followed with many aftershocks. This earthquake was strong enough to be felt as

far away as Meerut and Agra in India.

‣ On January 20, 1902, a large earthquake caused damage in the Chitral area and

was felt widely in the Punjab and up to Shimla.

‣ On July 8, 1909, an earthquake caused destruction in the region of Mankial and

Kalam in the Swat valley where Lady Minot’s Hospital was damaged and many

houses collapsed, killing 10 people and cattle. The damage area extended to Dir,

Karori, and Alipura, and the earthquake was felt in Gilgit, Skardu, Besham, and to

the north up to Tashkent.

‣ The epicentral intensity of the historical earthquakes that occurred after the 1700’s

is estimated to be not greater than VIII on the Modified Mercalli (MM) intensity scale.

10.1 Significant Earthquakes

Some important earthquakes that were felt in and around the Skardu City during last 50 years are

described below:

Harman Valley Earthquake of September 3, 1972

An earthquake of magnitude Mw 6.3 at a depth of 45.2 km struck the Harman Valley of Gilgit

Baltistan district on September 3, 1972. The epicentre of that earthquake was located in Kohistan

Island Arc at Latitude 35.94 degrees North and Longitude 73.33 degrees East, approximately 55

km northwest of Basha. In the epicentral area, the Intensity was felt as VII, while around Diamer

Basha and Skardu area, the Intensity was observed as IV. It has not been associated with any

tectonic structure within the area. The Fault Plane Solution (FPS) for the earthquake indicates a

thrust fault mechanism (NEAC, 2004).

Pattan Earthquake of December 28, 1974

The destructive Pattan earthquake of December 28, 1974 occurred in the isolated Pattan, Hazara,

and Swat districts of Northern Pakistan. The epicentre was located at 35.0 degrees North and 72.8

degrees East. The magnitude Mw 6.2 earthquake had a shallow focal depth and was followed by

numerous aftershocks. An official estimate of the number killed was 5,300 with approximately

17,000 injured. Most of the destruction was centered on the village of Pattan, located about 160

km north of the capital city of Islamabad. The Pattan village was almost completely destroyed. The

epicentral region is characterized by steep-walled narrow canyons and valleys. Most of the

population was concentrated along the rivers. Much of the destruction was caused by the

numerous landslides and rockfalls which came tumbling down from high above. The main road

leading into the area was blocked for about 40 km by landslides and rockfalls, hampering relief

efforts. The government flew in emergency supplies by helicopter until the roads were reopened

on January 13, 1975. The Fault Plane Solutions (FPS) for this earthquake indicates a thrust fault

mechanism (NEAC, 2004).

24

Although the 1974 epicentre is located in the vicinity of the Duber Kale fault, the strike of its fault

plane and the sense of motion are incompatible with that of the Duber Kale fault. The direction of

slip agrees with the overall deformation in the area. This is a thrust event with the greatest principal

stress striking NNE and the fault plane striking WNW. Maximum Intensity in the epicentral area

was VIII (Ambraseys et al., 1975).

Darel Valley Earthquake of September 12, 1981

An earthquake with shallow depth (29.7 km) and magnitude Mw 6.1 struck the Darel valley of

Kohistan district on September 12, 1981. The epicentre of that earthquake was located at Latitude

35.22 degrees North and Longitude 73.48 degrees East, approximately 75 km northwest of Skardu.

In the epicentral area, the Intensity was felt as VI, while around Skardu area, the Intensity was

observed as IV. It has not been associated with any known tectonic structure within the area.

Astore Valley Earthquakes of November 2002

On November 1, 2002, a moderate earthquake of magnitude Mw 5.3 (at a depth of 33 km) occurred

in the Gilgit Baltistan areas of Pakistan. The epicentre was located about 55 km south of Skardu

city. The earthquake occurred at 33.56 degrees North and 74.64 degrees East, in the Astore Valley,

where the Intensity was observed as VI. Around Skardu city area, the observed Intensity was IV–

V (Iqbal, 2011).

On November 20, 2002, the region was rocked again by another large shallow earthquake with a

magnitude Mw 6.3 at a depth of 15 km. The epicentre was located at 35.52 degrees North and

74.66 degrees East, in the north-western part of Astore Valley. In the epicentral region, the Intensity

of earthquake was observed as VIII. The main shock was followed by a large number of

aftershocks, few of which were large enough to increase the damage caused by the main shock.

The occurrence of aftershocks lasted 40 days. The epicentre of the main shock was located at

about 75 km south of Skardu city, where Intensity was observed as VII. Around the Skardu City

the observed Intensity was estimated as V.

The FPS of both earthquakes indicate a dip of 480 with normal faulting (Iqbal, 2011). The

epicentres of these earthquakes lie near the interface of Kohistan Magmatic Arc and the Nanga

Parbat Haramosh Massif (NPHM), along the MMT-Raikot fault, near the town of Bunji. The area

also represents the northern boundary of the under thrusting Indian plate. This feature raises the

significance of the tectonic studies and study of active seismicity of the Skardu Valley. The entire

area is faulted and fractured and seismically active with history of quite a few moderate-damaging

earthquakes (Kazmi and Jan, 1997).

Bunji Earthquake of November 3, 2002

On November 3, 2002, a moderate earthquake with a magnitude of Mw 5.3 and shallow depth of

17 km occurred in the Gilgit Baltistan Areas of Pakistan. The epicentre was located in the Bunji

area, about 75 km southeast of the Skardu city. In and around the epicentre area, the Intensity of

this earthquake was observed as VI. The epicentre of the earthquake was located at 35.10 degrees

North and 74.70 degrees East with as many as five aftershocks with magnitudes Mw ranging from

3.7 to 4.2. In the Skardu Valley the observed Intensity was IV.

25

Kashmir Hazara Earthquake of October 8, 2005

A powerful earthquake with a magnitude of Mw 7.6 struck the northern part of Pakistan on October

8, 2005 and caused widespread damage in Azad Kashmir and adjoining areas of Hazara area of

Pakistan. The epicentre of this large earthquake was located about 10 km northeast of

Muzaffarabad. This earthquake was felt for several minutes in Pakistan, Northern India, and

Afghanistan. It was also felt in Skardu City with Intensity of VII.

The heaviest damage was recorded in the towns of Balakot, Batal, and Batagram in Hazara and

Muzaffarabad, Bagh, and Rawalakot in Azad Kashmir where the entire population was severely

affected. Building collapse was also reported in Abbottabad and Islamabad. Serious cracks were

observed in many high-rise buildings in Islamabad. The death toll due to this earthquake exceeded

80,000 people and millions were rendered homeless due to collapse of houses. The earthquake

was followed by a series of more than thousand aftershocks, hundreds of them exceeding

magnitude 4.0. The fault plane solution for the main shock given by Harvard Moment Tensor

Solution shows a predominant thrust motion and its strike is compatible with the previously known

strike of the Himalayan Frontal Thrust (HFT).

Gilgit Earthquakes of December 30, 2019

On December 30, 2019, an earthquake with magnitude Mw 5.4 was felt in the area of Gilgit

Baltistan. The epicentre of this earthquake was located at Latitude 35.59 degrees North and

Longitude 74.62 degrees East, approximately 23 km SSE of Gilgit. The city of Gilgit was shaken

with an Intensity of VI due to this earthquake. The Intensity of earthquake in the Skardu Valley area

was observed as V. Within two days, the earthquake was followed by a number of aftershocks

ranging between Mw 3.8 and 5.1.

Shounter Valley Earthquake of January 12, 2020

On January 12, 2020, a shallow-focused (10 km) earthquake of magnitude 5.1 was felt in most

parts of Gilgit Baltistan areas. The epicentre of this earthquake was located at Latitude 35.01

degrees North and Longitude 74.47 degrees East, about 52 km SE of Chilas, along the Shounter

valley. Its epicentral Intensity was VI, while in the area of Skardu City its observed Intensity was

IV, (emsc-csem.org of European-Mediterranean Seismological Centre).

Astore Valley Earthquakes of December 2021

On December 27, 2021, a moderate earthquake of magnitude Mw 5.3 (45 km) occurred in the

Astore valley of Gilgit Baltistan. Its epicentre was located about 65 km southeast of Skardu. The

earthquake occurred at Latitude 35.55 0 N and Longitude 74.83 0 E. Along the epicentral area the

Intensity was observed as VI, while in/around Skardu City it was V.

Skardu Earthquake of March 2022

A moderate earthquake with magnitude Mw = 5.3 was felt by many people in and around Skardu

City with Intensity V on March 16, 2022. Its epicenter 35.72 N and 75.22 E was located, 59 Km

NW of Skardu at a depth of 10 km. In the Skardu Valley its observed Intensity was VI.

26

11.0 INSTRUMENTAL EARTHQUAKE CATALOGUES

The instrumental recording of earthquakes in the region started in 1904, but very few seismic

stations were established in the South Asian region until the 1960’s. During the 1960’s, the

installation of high-quality seismographs under the World-Wide Standard Seismograph Network

(WWSSN) established by the U.S. Coast and Geodetic Survey in 1960 greatly improved the quality

of earthquake recording in this region and has resulted in a better understanding of the seismicity

of Pakistan. However, based on the limited number of local seismographs, the complete

understanding of the seismicity including the smaller magnitude events is still limited and lacking

for the region. In Pakistan and most other parts of the world, the instrumental seismic record is too

short and incomplete to develop a sample that is truly representative of the spatial and temporal

distribution of earthquakes over a large period. Nevertheless, available information has been

gathered for the period covering the last century and was used to perform an assessment of the

seismic hazard for the Skardu City area.

Earthquake catalogues have been one of the vital products of seismology. Homogeneous and

complete earthquake catalogues are compiled for different purposes and specific to certain areas

of seismology such as seismic risk, earthquake physics, and hazard analysis (Kagan, 2003;

Woessner and Weimer, 2005). Catalogue accuracy is one of the most important considerations

while quantifying any earthquake catalogue because of its influence on the obtained results

(Kagan, 2003). Obtaining accurate source parameters is a task which is simple in theory but a

really challenging one in practice given the limited density and quality of the seismographic

instruments in a region such as Pakistan.

Earthquake catalogues and reports, as well as online databases are the standard sources used to

collect the necessary information for compiling an earthquake catalogue. Some catalogues offer

high quality hypocentres, while others enclose lower quality hypocentres through carefully

researched damage reports and other information (Allen et al., 2009). Improvements in seismic

observation and catalogue reporting can be done by examining the catalogue properties (Kagan,

2003). Typically, a ranking preference is developed for multiple contributing catalogues in a region

in which a given earthquake is reported from multiple catalogue sources.

For the present study, a composite list of seismic events that occurred in the Project region and

recorded by seismographs has been prepared. It is based upon earthquake catalogues of northern

Pakistan prepared by NESPAK (2020), Waseem et. al. (2018), Pakistan Metrology Department

and the WAPDA Micro Seismic Monitoring System (MSMS) installed around large dams in

Northern Pakistan.

11.1 NESPAK Earthquake Catalogue

After the devastating Kashmir Hazara Earthquake of October 8, 2005, the Building Code of

Pakistan (Seismic Provisions, 2007) were prepared by seismic and geotechnical experts of

NESPAK for the Ministry of Housing & Works, Pakistan. For the purpose of preparing a PSHA

based seismic zoning map of Pakistan, a composite earthquake catalogue of instrumentally

recorded earthquakes for Pakistan was compiled by NESPAK. This catalogue is being updated

every year for use in the seismic hazard evaluation of the projects. For present study, the catalogue

was updated through December 2021.

27

For this catalogue, the instrumental earthquake data was collected from two sources. The first one

is based upon earthquakes recorded by regional seismic networks and the other is compiled from

a local network data catalogue. The regional data was compiled from earthquake listings of

International Seismological Centre (ISC) England, National Earthquake Information Centre (NEIC)

of US Geological Survey, Pakistan Meteorological Department (PMD), and earthquake listing

compiled by Quittmeyer and Jacob (1979). As the ISC listing is based on a regular re-evaluation

of the epicentral data, this listing was given preference over the others.

11.2 Waseem et al. (2018) Earthquake Catalogue

During 2018, a comprehensive earthquake catalogue including both historical and instrumental

events was published by Waseem et al. (2018) of the National Centre for Excellence in Geology,

Peshawar, Pakistan, for their various research projects. The earthquake catalogue compiled in this

study for the region (quadrangle bounded by the geographical limits 40–83° N and 20–40° E)

includes 36,563 earthquake events, which are reported as 4.0– 8.3 moment magnitude (Mw) and

span from 25 AD to 2016.

The catalogue includes earthquakes from Pakistan and neighbouring countries. For this catalogue,

earthquakes reported by local and international agencies as well as individual catalogues are

included. The events from this catalogue were included in the seismicity catalogue only for events

which were not reported in the NESPAK or local MSMS catalogues.

11.3 Pakistan WAPDA Micro Seismic Monitoring System Earthquake Catalogue

For the seismic safety monitoring of Tarbela Dam Project, Pakistan WAPDA installed a thirteen

station Micro Seismic Monitoring System (MSMS) in and around Tarbela Dam Project. After going

through various upgradations, it is operating till date. During the year 2010 two of its local stations

were relocated in Skardu and Chitral, to monitor the seismicity of the Satpara and Golan Gol Hydro

Power Projects.

A MSMS comprising of ten seismic stations was installed in and around the proposed Diamer

Basha Dam Project Pakistan WAPDA during August 2007. Additionally, a three station MSMS was

commissioned around the proposed Bunji Hydro Power Project during 2010. Both MSMS remained

in operation until December 2016. The microseismic events recorded by theese networks are

available from September 2007 through December 2016, although not on a continuous basis. All

these thirteen MSMS stations seismic data is available for seismicity SW off Skardu City.

The earthquake data recorded by WAPDA local networks was collected and included in the

regional catalogue and given preference due to high accuracy of local networks. The duplicate

events were removed from the seismicity catalogue based on this ranking preference for the

contributing sources.

28

11.4 Focal Depth

In addition to the spatial distribution of earthquake epicentres, the distribution as a function of depth

of the observed earthquakes can be a critical parameterization in a seismic hazard analysis. The

distribution of earthquakes as a function of depth can help to define the depth of the seismogenic

zone in the Earth’s crust. This depth range of the seismogenic zone is observed to vary around the

world and is considered to be related to the larger regional tectonics. Often, local seismograph

networks make it possible to resolve hypocentral depths to within 1 to 2 km in depth. However, this

is true for areas in which the local seismic networks are well distributed as is the case in California

and Japan. For Pakistan, the same high-density distribution of seismic instrumentation is not

available; hence, the hypocentral depth uncertainty associated with the observed seismicity is

expected to be significantly larger.

For event locations and hypocentral depths reported from global teleseismic instrumentation

catalogues, (e.g., the ISC and the USGS) many hypocentral depths are assigned the default value

of 33 km. This assignment is based on the limitation of depth estimations from seismic stations

which are not closely distributed. Specifically, at teleseismic distances, the depth can only be

estimated accurately if the near-source surface reflection, the depth phase pP, can be identified.

Without special analysis, pP can only be identified as separate from the direct phase P, once its

delay corresponds to that of about 50 km depth. Hence, the teleseismic uncertainty in the depth

estimate of ‘shallow’ earthquakes is roughly ±25 km. Therefore, the depths given in the ISC and

USGS catalogues may not provide the robust estimate of the depth of future seismic energy

release.

12.0 CATALOGUE COMPILATION

A reliable and complete catalogue should include all earthquakes that have occurred in the region

reported by reliable sources and the earthquake magnitudes should be converted into a single and

consistent magnitude scale. For the Seismic Hazard Analysis and Micro Zonation of Skardu City

2040, a composite earthquake catalogue was developed. The composite earthquake catalogue

was organized by combining all the earthquake catalogues described above.

The NESPAK catalogue was taken as main source of earthquake data because it contained

seismic events reported by various international and national agencies. Those seismic events,

which were missing from the NESPAK catalogue but contained in the Waseem et. al. (2018)

catalogue, were added in the composite catalogue. Seismic events with magnitudes ≥ 3.0 from the

WAPDA MSMS catalogue, not already included in the other two catalogues, were added in the

composite catalogue.

In the process of compiling the composite catalogue, care was taken to avoid including duplicate

seismic events. This composite catalogue includes all earthquake data from all available modern

instrumental catalogues (International & National) covering the period from January 1900 through

May 2022 (Appendix-B).

29

12.1 Catalogue Compilation and Magnitude Conversion

The seismic events with magnitude 3 and greater occurring within about 200 km radius of the

Skardu City, from January 1900 though May 2022, have been included in the composite catalogue

prepared for the present studies. The contributing reporting agencies have given a variety of

magnitudes: body-wave magnitude (mb), surface-wave magnitude (MS), Richter/local magnitude

(ML) or duration-magnitude (MD), etc.

In the case of a single event reported in different magnitude scales, the Mw scale is considered

superior to all other scales. Otherwise, mb and MS scales are preferred over ML scale. Since

attenuation relationships are based on magnitude of a given type, a single and consistent

magnitude type must be used.

As the majority of recent attenuation equations used in seismic hazard analysis are based on

moment magnitude (Mw), all the other magnitude types were therefore converted to moment

magnitude (Mw) using the following equations:

Conversion from MS and mb to Mw was achieved through following four equations suggested by

Scordilis (2006):

Mw = 0.67 MS + 2.07 for MS < 6.1 (2)

Mw = 0.99 MS + 0.08 for MS >6.1 (3)

Mw = 0.85 mb + 1.03 for mb < 6 (4)

Mw = 1.69 mb – 4.01 for mb > 6 (5)

For ML up to 5.7, the value of ML was taken equal to Mw as supported by operators of local

networks in Pakistan (Personal communication). It should be noted that the conversion of

earthquakes from ML to Mw is more uncertain for magnitudes less than 4 and that the recurrence

parameters developed for this study considered only events with magnitude greater than 4.

Conversion of ML to Mw beyond magnitude 5.7 was done by using the following five equations

suggested by Ambraseys and Bommer (1990) and Ambraseys and Bilham (2003):

0.82 (ML) – 0.58 (MS) = 1.20 (6)

MS = (0.82 (ML) – 1.20) / (0.58) (7)

Log Mo = 19.09 + MS for MS ≤ 6.2 (8)

Log Mo = 15.94 + 1.5 MS for MS > 6.2 (9)

Mw = (2/3) Log (Mo) – 10.7 (10)

30

where mb is the body–wave magnitude, MS is the surface-wave magnitude, ML is the local

magnitude, Mw is the moment magnitude, and Mo is the seismic moment. Using a two-step

process to estimate Mw for events with ML greater than 5.7 is not ideal, as it increases the

uncertainty generated in the catalogue due to magnitude conversion, but it is necessary due to the

limitations of available earthquake data in Pakistan.

12.2 Declustering of Earthquake Catalogue

In this seismic hazard analysis, it was assumed that earthquakes occur independently of each

other. Foreshocks and aftershocks are both temporally and spatially dependent on a mainshock.

Therefore, the earthquake catalogue was declustered (i.e., dependent events were identified and

removed), resulting in a catalogue composed of independent events. The composite catalogue for

this Project was declustered using the Reasenberg (1985) algorithm to remove dependent events

(aftershocks and foreshocks). The Reasenberg algorithm identifies events that occur within time

and spatial windows, termed clusters, with the largest event in the cluster being named the main

shock and the smaller earthquakes labelled as foreshocks or aftershocks. These clusters are then

replaced with the main shock earthquake. The resulting catalogue is assumed to be events

independent in space and time. This algorithm works well with catalogues that have a large number

of smaller earthquakes (such as the catalogue compiled for this study), typically removing more of

these events than other methods. A plot of the instrumentally recorded earthquakes in the project

region is provided in Figure 8. Note the figure shows the declustered seismicity catalogue.

12.3 Analysis of Earthquake Record

The spatial distribution of seismic events recorded in and around the Skardu City and given in

Appendix-B is plotted on Figure-11.

The distribution of observed seismicity on the seismicity map clearly shows that the Skardu Valley

is located in a region of high seismicity. The concentration of earthquakes southwest of the project

area is related to seismically active Himalayan frontal zone along which Kangra earthquake of 1905

and Kashmir-Hazara earthquake of 2005 occurred.

The concentration of seismicity in the west of the Skardu Valley is from highly active zone of Raikot-

Sassi fault zone of the Nanga Parbat-Haramosh Syntaxis (NPHS). The small concentration of

recent earthquakes (November 2002 to March 2003) is located in the Raikot area on the western

flank of the Nanga Parbat-Haramosh structure and possibly extending within the massif. The main

shocks include:

• 1st November, 2002; magnitude (Mw) 5.5; and

• 20 th November, 2002; magnitude (Mw) 5.9.

About 89 aftershocks were also recorded in this period the alignment of which shows predominantly

north-south trend. Along MKT, a number of small to medium earthquakes are located towards the

northwest of the Skardu Valley showing recent activity of MKT. The earthquake activity on

northeast of the study area is mainly related to Karakoram fault.

31

The observed seismicity (Figure-11) shows that the Skardu Region is seismically active due to

tectonic processes associated with the interaction of the Eurasian and Indian tectonic plates.

Figure-11. Spatial Distribution of Seismicity in and around Skardu City

The seismic events in the Skardu Region mostly shows E-W trending folds and faults. The

deformation within this zone is primarily the result of thrusting and of deep crustal decollement

processes associated within the collision of the plates. The map indicates that most of the seismic

activity is aligned along known faults that are controlling the seismotectonic of Skardu Region.

However, in the seismic activity map, many of the located seismic events may not be associated

to the surface tectonic faults and may be attributed to features present at shallow depths. Within

some areas of the seismic activity map the observed seismicity is relatively low and do not consist

of higher magnitude events. This implies that the regional tectonic features in the Skardu Region

are seismically active at moderate to high level magnitudes, due to stresses developed as a result

of collision of the tectonic plates (Figures-11 & 12)

This map (Figures-11 & 12) displays the presence of seismic activity in east, north and south of

the Skardu Valley which could be associated with faults present in this region. Along the North-

East are the seismic events caused by the Main Karakoram Thrust (MKT). The cluster of seismicity

South-West off Skardu Valley is related to earthquake activity along the Indus Kohistan Seismic

Zone. This cluster of seismic events also includes the aftershocks of mega Mw 7.6 Kashmir Hazara

earthquake of October 08, 2005. Along the South-East off Skardu Valley, the seismic activity is low

to moderate and related to the active faults present in the area (Figures11 & 12). It is therefore

assumed that the Skardu Seismic Region (radial distance of 200 km around Skardu) is seismically

active and generating earthquakes of Mw ≥ 4.0.

32

Figure-12. Seismicity 200 km radius around Skardu City with respect to Faults.

33

13.0 SEISMOTECTONIC ANALYSIS

From the available tectonic and seismic data and neo tectonic studies carried out in the Skardu

Valley area, a preliminary understanding about the seismotectonic set up can be developed. A

seismotectonic map of the Skardu Valley region (200 km radial distance off Skardu), with active

faults is presented in Figure-12.

13.1 Identification and Description of Seismic Sources

The available seismic and tectonic data provides several evidences of the seismic activity along

the major faults i.e., Main Mantle Thrust (MMT) and Kohistan Fault passing south of the site, the

locations of which are shown on Figure-8.

Based on this understanding of the seismotectonic setting and faults of the area, the major

seismogenic features which may significantly influence the seismic hazard for Skardu City are:

• Main Karakoram Thrust (MKT),

• Indus – Tsangpo Suture (ITS), and

• Karakoram Strike-Slip Fault (KF)

14.0 SEISMIC HAZARD ANALYSIS

Seismic Hazard Analysis involves the quantitative estimation of ground motion characteristics at a

particular site and conducted by probabilistic or/and deterministic methods. In recent years a good

deal of work has been carried out throughout the world to study the seismicity of various areas to

estimate the earthquake hazard potential for establishing design criteria for the construction of

high-rise structures and multi-story buildings etc.

For Seismic Hazard Analysis (SHA) of the Skardu City, the guidelines provided by the International

Commission on Large Dam (ICOLD) for selecting seismic parameters (Bulletin 148, 2016) and the

U.S. Army Corps of Engineers, ER 1110-2-1806 (2016), Earthquake Design and Evaluation for

Civil Works Projects has been followed. A brief description of the methodology of the approaches

to be used for the seismic hazard analysis in accordance with ICOLD guidelines is given below.

14.1 Deterministic Procedure

In the deterministic procedure, critical seismogenic sources (active or potentially active faults) that

represent a threat to the area of study are identified and a maximum magnitude is assigned to each

of these faults.

The capability of the faults is ascertained through observation of historical and instrumental seismic

data and geological criteria such as rupture length – magnitude relationship or fault movement –

magnitude relationship.

34

The maximum seismic design parameter is then obtained by considering the most severe

combination of maximum magnitude and minimum distance to the project site, independently of

the return period.

The main tectonic features around the Skardu City which could be controlling the maximum

earthquake hazard are as follows:

• Main Karakoram Thrust (MKT),

• Indus – Tsangpo Suture (ITS), and

• Karakoram Strike-Slip Fault (KF)

Empirical correlations have been developed between maximum potential of a fault and key fault

parameters like rupture length, fault area, fault displacement and slip rate. Out of these fault

parameters, only fault lengths are known with sufficient accuracy. For the faults around the site,

the half rupture length of the faults has been taken for determination of maximum potential

magnitude (in moment magnitude M W scale). Those segments were calculated using Wells &

Coppersmith (1994), Nowroozi (1987) and Slemmons et al. (1982) relationships between fault

rupture length and magnitude potential which are given in Table-1.

Table-1

Fault Maximum Magnitude Potential Selected

Tectonic Feature Rupture Nowroozi Wells & Slemmons (Mw)

Length (km) (1997) Coppersmith

(1994)

Et. al.

(1982)

Main Karakoram Thrust 400 7.8 7.8 7.9 7.8

Indus Tsangpo Suture

(ITS)

300 7.5 7.6 7.7 7.6

Karakoram Fault 400 7.7 7.6 7.7 7.7

Geological Maps, research papers on Skardu Geology and available bore hole logs of the Skardu

City were studied. It is concluded that the average shear wave velocity in top 30 meters of the site

profile (V s30) is assigned to be 200 m/sec, 450 m/sec and 760 m/sec along various locations of the

Skardu City.

The horizontal Peak Ground Acceleration (PGA) at the site caused by the earthquake of maximum

magnitude occurring at the closest distance to fault was then calculated by using the following five

latest attenuation relationships of PEERC developed for NGA West-2 Model (2014) Project, from

strong motion data of USA and worldwide. These relations were used so, because due to absence

of enough strong motion data for the south Asian region, no attenuation relation for this region is

available till date.

35

For all the seismic sources, thrust rupture mechanism and bedrock site conditions have been

assumed.

1. Abrahamson & Silva & Kamai

2. Boore & Stewart & Seyhan & Atkinson

3. Campbell & Bozorgnia

4. Chiou & Youngs

5. Idriss

The peak horizontal ground acceleration at the site caused by the earthquake of maximum

magnitude occurring at the closest distance to fault was then calculated by using the latest

attenuation relationships developed by various researchers from strong motion data from USA and

worldwide. These relations were used so, because due to absence of enough strong motion data

for the south Asian region, no attenuation relation for this region is available.

As shallow crustal earthquakes are more important for the assessment of seismic hazard to the

project, therefore equations applicable for shallow crustal earthquakes were employed. The

attenuation equations developed under the Next Generation Attenuation (NGA) Project of Pacific

Earthquake Engineering Research Center; University of California at Berkeley are used to obtain

the median values of peak horizontal ground acceleration (PGA) for all fault sources.

The NGA equations are developed using a large worldwide database of strong motion recording

of earthquakes of magnitude 4-8 and distances ranging from 0-200 km. The NGA equations are

preferable over the older equations for the evaluation of seismic hazard particularly in the near field

as these are based on a broad spectrum of data recorded in the near field. The same attenuation

equations were used for the PSHA.

For all the seismic sources, thrust rupture mechanism has been assumed. It is assumed that dam

foundation will mainly be placed on gravelly soil therefore the site condition was taken as gravelly

soil with average shear wave velocity for top 30 meters (V s30) of 500 m/sec. The depth to basement

rocks (Z 25) is taken as 5 km. The peak horizontal ground acceleration (PGA) values obtained at

dam site is given in Table-2.

The 50-percentile (median) values of the peak horizontal ground acceleration (PGA) obtained by

five NGA attenuation relationships are given in Table-2. As shallow crustal earthquakes are more

important for seismic hazard in the project region, the attenuation equations applicable for shallow

crustal earthquakes were employed.

When critical faults are ≤ 40 km away the vertical PGA (V) are taken 2/3 of horizontal, and while

for ≥ 40 km away the ½ of horizontal PGA (H) [US Federal Guidelines for Dam Safety].

36

Table-2

Tectonic Feature

Maximum

Magnitude

(Mw)

Closest

Distance

to Fault

(km)

PGA (H)

Median

5% Damping

g

PGA(V)

Median

5% Damping

g

Main Karakoram Thrust 7.8 90 0.18 0.090

Indus Tsangpo Suture (ITS) 7.6 65 0.14 0.07

Karakoram Fault 7.7 35 0.20 0.130

14.2 Probabilistic Procedure

14.2.1 PSHA Methodology

In probabilistic seismic hazard assessment (PSHA), the seismic activity of seismic source (line or

area) is specified by a recurrence relationship, defining the cumulative number of events per year

versus the magnitude. Distribution of earthquake is assumed to be uniform within the source zone

and independent of time.

The principle of the analysis, first developed by Cornell (1968) and later refined by various

researchers, is to evaluate at the site of interest the probability of exceedance of a ground motion

parameter (e.g., acceleration) due to the occurrence of a strong event around the site. This

approach combines the probability of exceedance of the earthquake size (recurrence

relationship), and probability on the distance from the epicenter to the site.

Each seismic source zone is split into elementary zones at a certain distance from the site.

Integration is carried out within each zone by summing the effects of the various elementary

source zones taking into account the attenuation effect with distance. Total hazard is finally

obtained by adding the influence of various sources. The results are expressed in terms of a

ground motion parameter associated with return period (return period is the inverse of the annual

frequency of exceedance of a given level of ground motion).

The seismic hazard model used in the present analysis was developed based on findings of the

seismotectonic synthesis. The seismic hazard model relies upon the concept of seismotectonic

zones and does not include linear or discrete fault sources. Each seismic source zone is defined

as a zone with homogenous seismic and tectonic features, inferred from geological, tectonic and

seismic data. These zones are first defined, and then a maximum earthquake and an earthquake

recurrence equation are elaborated for each of these seismic source zones.

37

The seismic parameters attached to the various seismic source zones are: a recurrence

relationship relating the number of events for a specific period of time to the magnitude; the

maximum earthquake giving an upper bound of potential magnitude in the zone; and an

attenuation relationship representing the decrease of acceleration with distance.

The probabilistic seismic hazard evaluation requires a detailed analysis of distribution of observed

seismic data to the seismic sources, determination of b-value and activity rate of each seismic

source and assigning maximum magnitude potential to each seismic source.

14.2.2 Source Modeling – Area Sources

For the definition of seismic sources, either line (i.e., fault) or area sources can be used for source

modeling. Because of uncertainty in the epicenter’s location, it is not possible to relate the

recorded earthquakes to the faults and to develop recurrence relationship for each fault and use

them as exponential model. The Skardu Region was therefore divided into three seismic area

source zones based on their homogeneous tectonic and seismic characteristics, keeping in view

the geology, tectonics, seismicity and fault plane solutions of each area source zone. These

seismic area source zones in the Northern part of Pakistan are shown in Figure-13.

Each of these area sources was assigned a maximum magnitude based on recorded seismicity

and potential of the faults within the zone and a minimum magnitude based on threshold

magnitude observed in the magnitude-frequency curve for the zone. As the shallow earthquakes

are of more concern to seismic hazard, the minimum depth of the earthquakes is taken as 5 km

and maximum depth as 70 km for all area sources. The source zone parameters used in

probabilistic hazard analysis are given in Table-3.

Table - 3 Seismic Area Source Zones Parameters for Probabilistic Analysis.

No. of Minimum Activity

Zone Seismic Source Earthquakes Magnitude Rate b Maximum

No Zone name above Min.

Magnitude

Mw /Year Value Magnitude

Mw

1 Karakoram 445 3.9 7.492 1.138 7.8

2 Kohistan 180 4.4 3.030 1.258 7.6

3 Eastern

Himalayas

331 4.3 5.572 0.991 8.0

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Figure-13. Zones in the Skardu Seismic Region

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14.2.3 Earthquake Recurrence Model

A general equation that describes earthquake recurrence may be expressed as follows:

N (m) = f (m, t) (11)

Where N (m) is the number of earthquakes with magnitude equal to or greater than m, and

it is time period.

The simplest form of equation (1) that has been used in most engineering applications is

the well-known Richter’s law which states that the cumulated number of earthquakes

occurred in a given period of time can be approximated by the relationship:

Log N(m) = a – b m (12)

Equation (2) assumes spatial and temporal independence of all earthquakes, i.e., it has the

properties of a Poisson model. Coefficients ‘a’ and ‘b’ can be derived from seismic data related to

the source of interest. Coefficient ‘a’ is related to the total number of events occurred in the source

zone and depends on its area, while coefficient ‘b’ represents the coefficient of proportionality

between log N (m) and the magnitude. The composite catalogue of earthquakes prepared for the

Skardu Region (area of 200 km radial distance from Skardu City), provided the necessary database

for the computation of b-value for each seismic area source zone.

The composite earthquake list contains limited number of earthquakes prior to 1961 and only few

of these earthquakes have been assigned magnitude values. Due to installation of WWSSN, the

earthquake recording in this region improved and a better and complete recording of earthquake

data are available after 1961. A basic assumption of seismic hazard methodology is that

earthquake sources are independent. Thus, catalogues that are used to estimate future seismic

activity must be free of dependent events such as foreshocks and aftershocks. To the extent

possible such events were also eliminated manually, however, there are insufficient data to apply

rigorous procedures such as that of Gardner and Knopoff (1974) to eliminate foreshocks and

aftershocks from the composite catalogue.

The completeness analysis of the overall data for the region showed that earthquake data up to

magnitude 4.0 is complete after 1961.The converted moment magnitude for the period between

January 1962 and May 2022 (59.4 years) was therefore used in the PSHA after excluding the

aftershocks. A separate list of earthquakes occurring in each area source zone was prepared

through GIS software and magnitude-frequency curves were prepared for each seismic area

source. The b-value for each seismic area source zone was calculated using linear regression

through least square method. The minimum magnitude for each area source zone was selected

from the magnitude-frequency curve based on completeness checks suggested by Woeffner and

Weimer (2005). The b–values, minimum magnitude and the activity rates for the six seismic area

source zones used in the probabilistic analysis are given in Table-3.

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14.2.4 Maximum Magnitude

To each seismic area source zone, a maximum magnitude potential was assigned based on the

maximum observed seismicity in the historical seismic record and enhancing by 0.5 magnitude the

maximum observed magnitude in the instrumental seismic record for that area seismic source zone

or determining the maximum magnitude of the longest active fault in the area using Well &

Coppersmith equation (1994). The maximum potential magnitude used for each seismic area

source zone is given in Table-3.

14.2.5 Attenuation Relationship

Attenuation equations have been developed between maximum potential of a fault and key fault

parameters like rupture length, fault area, fault displacement and slip rate. Out of these fault

parameters, only fault lengths are known with sufficient accuracy. Because of lack of sufficient

strong–motion data covering a larger range of magnitudes and distances, attenuation relationships

for the South Asian Region cannot be developed.

The present earthquake hazard study requires the availability of earthquake ground motion models

for peak ground acceleration and spectral acceleration, for the frequency range of engineering

interest. Available models include near field excitation as well as the attenuation with distance, and

the scaling with magnitude here is essentially developed for estimating the effects of an earthquake

which is not yet been observed in the region considered.

A number of attenuation equations have been developed from strong motion data collected in other

parts of the world. As shallow earthquakes are of more concern for hazard analysis of the Skardu

City Seismic Microzonation, attenuation equations developed for such conditions were considered

for use in the hazard analysis.

For probabilistic hazard analysis, following latest available NGA equations developed under Pacific

Earthquake Engineering Research (PEER) Centre, USA, were used as these equations are valid

for tectonically active regions of shallow crustal faulting worldwide and also recommended by

Bommer et al. (2016).

1. Abrahamson and Silva (2014),

2. Boore & Atkinson (2014)

3. Campbell & Bozorgnia (2014)

15.0 PRINCIPLES OF SEISMIC MICROZONATION

A natural hazard is defined as the probability of a potentially damaging phenomenon occurring

within a specified period of time and within a given area (Varnes, 1984). In this context, seismic

hazards represent the probable occurrence of earthquakes and seismically induced processes,

which include ground motions, liquefaction and land sliding. Geotechnical hazards are described

as the influence of natural hazards on engineering objects. Earthquake hazard maps may include

one or more of the aforementioned seismic hazards (Levson et al., 2003).

41

Seismic microzonation is the generic name for subdividing a region into individual areas having

different potentials hazardous earthquake effects, defining their detailed seismic behaviour for

engineering design and land-use planning. The practice of earthquake engineering comprises the

identification and mitigation of seismic hazards. Seismic Microzonation has typically been

recognized as the most accepted tool in seismic hazard assessment and risk evaluation and it is

defined as the zonation with respect to ground motion characteristics taking into account source

and site conditions. Making enhancements on the conventional microzonation maps and regional

hazard maps, microzonation of a region generates detailed maps that predict the hazard at much

smaller scales. The role of geological and geotechnical data is becoming very important in the

seismic microzonation in particular the planning of city urban infrastructure, which can recognize,

control and prevent geological hazards (Dai et al., 1994, 2001). The basis of seismic microzonation

is to model the rupture mechanism at the source of an earthquake, evaluate the propagation of

waves through the earth to the top of bed rock, determine the effect of local soil profile and thus

develop a hazard map indicating the vulnerability of the area to potential seismic hazard.

The earthquake damage basically depends on three groups of factors: earthquake source and path

characteristics, local geological and geotechnical site conditions, structural design and construction

features. Seismic microzonation should address the assessment of the first two groups of factors.

In general terms, seismic microzonation is the process of estimating the response of soil layers for

earthquake excitations and thus the variation of earthquake characteristics is represented on the

ground surface. Seismic microzonation is the initial phase of earthquake risk mitigation and

requires multidisciplinary approach with major contributions from geology, seismology and

geotechnical engineering.

Seismic microzonation includes delineation of the zones that are homogenous in seismological

and geological characteristics and a description of zone characteristics by associating dynamic

parameters (peak ground acceleration – PGA, peak ground velocity- PGV, or spectral acceleration-

SA) with the specified probability of occurrence. As such, seismic zonation is the first step for all

further assessments of seismic hazards (Marku and Herak, 1999). These parameters are mapped

at a national scale for a standard ground condition, which are usually rock or stiff soil. Mapping at

this scale is called microzonation (Finn et al., 2004). Building code utilizes national seismic

microzonation maps in specifying the minimum design requirements.

There are two aspects of earthquake hazard safety: i) structural safety against potentially

destructive dynamic forces, and ii) safety of a site related to geotechnical phenomena, such as

amplification, landslides, and liquefaction. Dynamic effects have been considered in building codes

worldwide to ensure the safety of structures under earthquake loading. However, little attention

has been paid to the safety assessment of individual sites in the form of land use regulation.

15.1 Framework for Seismic Microzonation

Earthquake casualties and losses are primarily the result of building and infrastructure failure

induced by earthquake effects. The two principal approaches to reducing these losses are to avoid

high hazard areas for the building and infrastructure sites and to ensure that buildings and

infrastructure are designed and constructed to resist expected earthquake loads. The first

approach relates to land use management and the second approach deals with the design and

construction of individual buildings (DRM, 2004a).

42

Both the seismic microzonation and the building codes must be considered in urban planning and

building design. Although the scientific and engineering basis for these tools is widely available,

application and use must be required and enforced by municipal authorities. The effectiveness of

seismic microzonation and land use management planning is dependent on the effectiveness of

implementation policy and enforcement of zone defined development controls. Microzonation is an

efficient tool to mitigate earthquake risk by hazard-related land use management. However,

microzonation does not replace the existing building and construction codes. Seismic

microzonation maps do not provide detailed hazard parameters at the level of the specific building

site, but they do provide guidance on required site-specific investigations.

15.2 Seismic Micro Zonation for Skardu City 2040

Skardu City is situated at an elevation of 2,230 metres above sea level, though the mountain peaks

surrounding Skardu reach elevations of 4,500–5,800 metres. Upstream from Skardu are some of

the largest glaciers in the world, including the Baltoro Glacier, Biafo Glacier and Chogo Lungma

Glacier. Skardu is located along the Kohistan-Ladakh terrane, formed as a magmatic arch over

a Tethyan subduction zone that was later accreted onto the Eurasian Plate. The region has low

seismic activity compared to surrounding regions, suggesting that Skardu is located in a passive

structural element of the Himalayan thrust. [5] The stone in the Skardu region is Katzara schist, with

a radiometric age of 37 to 105 million years. Numerous complex granitic pegmatites and a few

alpine-cleft metamorphic deposits are found in the Skardu Valley and its tributaries. Skardu Valley

contains the Main Karakorum Thrust separating the metasediments (chlorite to amphibolite grade)

on the Asian plate from the southern volcanoclastic rocks of the Kohistan-Ladakh Island arc.

15.3 Migration of Population in Skardu City 2040

According to the Population Census Organization, (PCO), `2000, the population of Skardu City

during the 1998 census was 26023 with an average annual growth rate of 2.12. The total number

of houses were 3526 with a household size of 7.4. Therefore, during the year 2022 the population

is 1213712 and by the year 2040 the population of Skardu city shall be around 2317087. The city

is surrounded with cluster of villages living in mountains.

Subsequent to the topographic contours, most of the villages expand from bottoms up to steep

slopes. Obliviously, these tough features make living conditions harsh and pitiable on the

inhabitants in all aspects of common life. The central part of the villages is usually congested and

densely populated, however many of the villages are also fairly spread out or consists of more than

one cluster urban area size varies from as small as 1000 households to >3000 households.

Following are some reasons due to which the population is shifting from rural areas to urban areas

of Skardu City. www.Ijeab.com

‣ The education facilities in the mountains area are poor. The literacy ratio in urban areas is

36.8% as compared to 12.6% in rural areas.

‣ The availability of health services in terms of hospitals, health clinics, dispensaries and

medicines are poor in the villages. Diseases like diarrhoea, cholera, chest-infections, goitre,

abdominal problems and seasonal infections (cold, cough etc.) are common among the

43

villagers. The people have difficulty in obtaining the necessary medicines. Serious illnesses

face people to go to Skardu City for treatment and most of them cannot afford this.

‣ The availability of social amenities indicates that villages do not have access to electrical

connections. Moreover, the electrical offices persuade the consumers for limited use of

electricity only to avoid load shedding particularly in winter months.

‣ It has been estimated that about 20% of the population is literate. Out of which some of the

population as reported to have primary, 6% secondary education and 3% higher secondary

education. This percentage is only applicable to male population. The illiteracy rate of 81%

of the population is considerable higher than the national average. The low literacy among

females (4%) is partly due to lack of girl’s schools in the close vicinity.

‣ Water supply is available to most households through spring water. However, the quality of

the service is reportedly not satisfactory.

‣ Sanitation services in terms of sewerage system, wastewater treatment, drainage and solid

waste management are non-existent in the villages. Therefore, the villages area is

characterised by inadequate sanitation conditions. Almost all the households use close

space for excretion where the solid part is collected and used as farm yard manure after

composting. The liquid part is allowed to flow in deep dug wells.

‣ Climate change and forest decline are some physically verifiable factors and phenomenon

and not a simple scientific theory in the lives of rural communities of Baltistan. Research

revealed that forest cover has on decline (either highly decreased or decreased) like

vegetation cover which has also decreased, particularly near villages more as compared to

pastures. Similarly, temperature has increased in winter and spring as compared to summer

and autumn. Snowfall has decreased during spring more as compared to winters. Contrast

to snowfall, rainfall has increased in spring followed by winter, autumn and summer. Glacier

sizes are shrinking and monsoon floods have highly increased flowed by melt water

increase in channels. Due to change in other climatic factors, crop sowing, fruiting and

harvesting periods have prolonged and have a backward shifting trend. It is evident that

there is a significant relationship between the population growth and the deforestation

phenomenon across mountainous areas of Karakoram, Himalaya and Hindukush

(Eckholm, et.al. 1976). In other words, not only there is a relationship between deforestation

and population growth but number of floods and soil erosion as well and environmental

degradation as well. It is therefore, Theory of Himalayan Environmental Degradation

‘continues to influence national environmental policies in the region.

15.4 Seismic Microzonation of Skardu City 2040

After the study of Geological maps, Seismotectonic maps, Borehole Logs of various locations,

Research Papers/Reports, Previous Geophysical Resistivity Survey Reports, Population Growth

trend, Land Growth trend, Built-in trend and previous Master Plan reports, the Skardu City 2040

area has been classified into following three seismic units.

44

1. Skardu Khas seismic micro zone

2. Hussainabad seismic micro zone

3. Shigri Kalan micro zone

Figure-14. Seismic Microzonation of Skardu City Master Plan 2040

15.5 Skardu Khas Seismic Microzone

This area includes the present central city of Skardu (Lat. 35.29 N & Lon. 75.62 E) and towards

south up to Satpara Hydro Power Project (HPP). Mostly soil in the area varies from fine to coarser

material (Silt to clay and sand to gravel). Soil selected in this study is low plastic Lean clay (CL)

and Silty clay (CL-ML). Presence of clay minerals detected by XRD analysis shows that soil is

reactive. Along the deeper portions mostly mafic rocks (basalt) are observed here. Grain size is

fine and foliation is present in the rocks.

As there are no defined rules to follow the building codes, therefore, settlements pattern is mostly

unplanned, scattered, semi-scattered and congested depending upon the availability of land.

Various households formed clusters known as Mohalla’s. Household size varies area to area,

depending upon the prevailing economical, cultural and religious norms of that particular area.

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15.6 Hussainabad Seismic Microzone

The area Hussainabad (Lat. 35.39 N & 75.55 E, Altitude 2777 meters), covers the left bank of Indus

River and for taking in account of the future settlements, also includes the right bank of Indus River.

The area is around 15 km east of present Skardu City. Keeping in view the increasing nature of

human settlements in the area, due to the opening of the Campus of Baltistan University and

increase in the construction of multi-story structures, it is recommended that a comprehensive

subsurface Geophysical survey of the area should be done. Every high-rise Structure may be

constructed after “Site Specific Seismic Hazard Analysis”.

Along this area the site condition is gravelly soil with average shear wave velocity for top 30 meters

(Vs30) are at the level of 200, 450 and 760 m/sec along various locations. The depth to basement

rocks (Z25) is taken as 5 km. Where bedrock is very deep, the soil susceptibility category of the

uppermost 30 meters of soil profile that generally has the greatest influence on amplification is

considered. However, the construction design and material also vary depending upon the

prevailing climatic, economical and environmental situations as well as availability of local material

and absence of basic infrastructure. The soil susceptibility categories are defined based on soil

type, thickness and stiffness, which are used as a basis for defining mapping units.

15.7 Shigri Kalan Seismic Microzone

The area (Lat. 35.27 N and Lon. 75.62 E), includes locations like Hotto-Gamba, Kachura and

Skardu International Airport. The Hotto-Gamba part of this area is situated on the bank of Indus

River. Many seasonal nullas and streams pass through this area due to which the soil of this region

is uneven, it consists of clay, silty clay, silt, sand and gravel. Along the nullah bed some alluvial

material is also deposited. The areas nearby river bank are mostly clays and those near the

mountains are mostly sand and gravel. Due to presence of clay, high water table (due to river) and

the extreme climate conditions soil of this area is found to be problematic, results in development

of cracks in buildings, damage of roads and other infrastructure. This part of the seismic unit is not

suitable for future settlement of people.

The other part of this seismic unit includes areas like Skardu International Airport and Cadet

College Skardu. In addition to silty clay silt and gravel, at some places soft bedrock is present in

samples of boreholes. At some locations olivine rich rock known as unite is observed. It is as

igneous plutonic rock of ultramafic composition. This side of the seismic unit is suitable for future

growth of population and constructions.

15.8 Results of PSHA

The probabilistic seismic hazard analysis was carried out using EZ-FRISK (updated 2022) software

developed by Risk Engineering Inc. USA. All the parameters defined in Table-4 were incorporated

in the model. The most useful way of presenting the result, for the three seismic micro zones of

Skardu City 2040, is in terms of horizontal hazard curves and spectra. Figures 15 through 20, for

different return periods, i.e., for 475, 975 and 2475 years, relating estimated ground motion to

annual exceedance probabilities which are the inverse of return periods in years. These curves

present the annual frequency of exceedance (inverse of return period) of the peak horizontal

ground acceleration expected in the Skardu City.

46

Where bedrock is very deep, the soil susceptibility category of the uppermost 30 meters of soil

profile that generally has the greatest influence on amplification is considered. The soil

susceptibility categories are defined based on soil type, thickness and stiffness, which are used as

a basis for defining mapping units. The Geological/Geotechnical reports and Borehole Logs from

different locations of Skardu City indicates that Vs 30 ranges from 200 to 760 m/s and corresponding

amplification ratios varies from 3.1 to 2.2.

As discussed in previous sections of this report and in accordance with the Table 4.1 contained in

the Building Code of Pakistan (BCP) Seismic Provision (2007), the results of horizontal PSHA were

computed in the form of Total Hazard Curves for four types of subsurface strata as follows:

Type Profile

Vs 30 m/sec

S B Dense Soil/Soft Rock 760

S C Stiff Soil/hard Soil 450

S D Soft Soil 200

However, when special structure such as hydropower project and any multi storey building is to be

constructed then borehole logs in the area are to be studied/analysed. For such types of

constructions site specific seismic hazard analysis are to be carried out and necessary application

of the amplification factors should be used as given in BCP Seismic Provisions (2007 & 2021).

Computed horizontal Peak Ground Acceleration (PGA) in terms of g through the latest state of art

EZ-FRISK (updated 2022) software, along the three seismic micro zones of Skardu City 2040, are

presented in Table-4. The g value in case of Vs 30 = 760 m/sec (dense soil/soft rock) is 0.15g at

seismic micro zones of Skardu Khas and Shigri Kalan (Hotto/Gamba), while it is 0.25g at the

seismic micro zones of Hussainabad and Shigri Kalan (Airport/Surrounding areas). The structures

can be safely constructed after accessing the condition of material present at the desired location

and according to g value given in Table-2 below:

Table-4. Computed g Values for three seismic micro zones of Skardu City 2040

760 m/sec 450 m/sec 200 m/sec

Location

Years g value Years g value Years g value

475 975 2475 475 975 2475 475 975 2475

Skardu Khas 0.12 0.16 0.21 0.15 0.19 0.25 0.17 0.21 0.26

Hussainabad 0.12 0.15 0.20 0.15 0.18 0.24 0.16 0.20 0.26

Shigri Kalan

Hotto & Gamba

Shigri Kalan

Airport & Surrounding Areas

0.12 0.16 0.21 0.15 0.19 0.25 0.17 0.21 0.26

0.12 0.15 0.20 0.15 0.18 0.24 0.16 0.20 0.26

Along the locations where Vs 30 is in the range 200 m/sec or lower, is not suitable for further

constructions purposes. These places are along the nullas or river banks/streams and further

constructions here may be avoided.

47

Figure-15 Total Hazard Curve for Skardu Khas & Hotto/Gamba areas when Vs30 = 760 m/sec

Figure-16 Uniform Hazard Spectra for Skardu Khas & Hotto/Gamba areas when Vs30 = 760 m/sec

48


Figure-18 Uniform Hazard Spectra for Skardu Khas & Hotto/Gamba areas when Vs30 = 450 m/sec

49


Figure-20 Uniform Hazard Spectra for Skardu Khas & Hotto/Gamba

when Vs 30 = 200 m/sec

50

Figure-21 Total Hazard Curve for Hussainabad & Shigri Kalan (Airport & Surrounding areas)


Figure-22 Uniform Hazard Spectra for Hussainabad & Shigri Kalan (Airport & Surrounding

areas) when Vs30 = 760 m/sec

51





52





53

Figure-27 Seismic Microzonation Map Return Period = 475 years, Vs30 = 200 m/sec.

54


55


56


57


58


59


60


61


62

16.0 SOIL LIQUEFACTION AND MITIGATION FOR SKARDU CITY 2040

In the Skardu Region of Northern Pakistan, three mightiest mountain ranges viz: Himalaya,

Karakoram and Hindu Kush, that are prone to seismic related hazard meet. The seismic zone is

vulnerable to earthquakes both from near and far seismogenic sources owing to its unique

geological, geographical and seismotectonic setting. The area has witnessed numerous colossal

earthquakes throughout its prolific geological past causing severe damage to natural and built

environment with intensity levels ranges between VIII–IX. Recent data of instrumental seismicity

shows that very high peak ground accelerations (PGA), can be generated by relatively moderate

(Mw ≥ 6.0) earthquakes in this region, due its geological/geotechnical aspects of the subsurface

stratum. With unplanned and unscientific constructional patterns and tremendous increase in

population since last major earthquake combined with warning level of seismic hazard and high

liquefaction potential index (LPI) leads to the insecure future of the Skardu City 2040, unless proper

mitigation measures are applied.

Kashmir-Hazara earthquake of October 2005 (Mw 7.6), was the deadliest earthquake in the human

history killing >90 thousand people and crumbling of the major infrastructure in the region. This

earthquake brought Northern Pakistan region into the global limelight and related seismic hazard

imposed by such earthquakes in this area. With growing population and rapid

unscientific/unplanned infrastructure development over time the vulnerability has increased

manifold to future earthquakes, as the most of the paleo seismic data in Kashmir Himalaya suggest

the possibility of a major earthquake greater than the 2005 earthquake, due to locked basal

decollement and continuous stress accumulation.

16.1 Soil Liquefaction

Liquefaction is a phenomenon in which deformation of non-cohesive saturated soils happens in

untrained conditions affected by transient, monotonous or repetitive disturbances. In this

phenomenon, saturated soils thoroughly lose their strength and rigidity due to the heavily inflicted

stress. This stress can be owing to rapid changes in the stress status of soils. This issue is

frequently reported in saturated soils, limp soils (low density or not concentrated), and sandy soils.

The above issue is because limp soils tend to compress under loads while compressed soils tend

to increase their volume. If soil is saturated with water, as in soils of sea levels or lower, then water

would fill the space between solid grains (porous spaces). Now, if a pressure is inflicted to soil, it

is inflicted to the water of porous spaces as well, forcing water to exit soil porous spaces and

moving towards less pressured spaces. However, if the pressure is inflicted fast enough and it is

big enough or it is done with enough repetitions (as in earthquakes or inflicted during hurricanes)

so that the water would not be allowed leave the inter grain spaces until the next cycle, a pressure

will be created in the water that is extremely greater than the stress which causes the soil grains

to stick together.

A state of 'soil liquefaction' occurs when the effective stress of soil is reduced to essentially zero,

which corresponds to a complete loss of shear strength. This may be initiated by either monotonic

loading (e.g., single sudden occurrence of a change in stress – examples include an increase in

load on an embankment or sudden loss of toe support) or cyclic loading (e.g., repeated change in

stress condition – examples include wave loading or earthquake shaking). In both cases a soil in

a saturated loose state, and one which may generate significant pore water pressure on a change

in load are the most likely to liquefy. This phenomenon has more destructive power in areas close

to water such as rivers, lakes and mullahs.

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16.2 Soil Liquification and Peak ground acceleration (PGA)

PGA or design basis earthquake ground motion, signifies maximum ground acceleration triggered

during an earthquake shaking at any location. PGA values are used to determine the suitable

earthquake loading for different structures/buildings required to persist the maximum considered

earthquake (MCE) in that region. Higher PGA values indicate higher level of shaking which in turn

can cause higher degree of damage to the buildings.

The PGA values display that high PGA can be generated along the southern margin Skardu Valley.

Historically most of the damaging events have been located and also due to the close proximity of

active seismic sources e.g., MMT, MCT, MBT, IKSZ, and the two syntaxial bends of Northern

Pakistan. Some research analysis also suggests that more the gap between the major

earthquakes, higher degree of hazard is imposed by accumulated strain.

16.3 Liquefaction potential index (LPI)

Liquefaction features can be produced by 5 Mw earthquake (thresh hold energy) although they are

abundantly associated with ~ 5.5 to 6.0 Mw earthquakes, depending upon the lithology,

groundwater saturation, consolidation, depth and source to site distance generally within the

epicentral distance of 40 km.

LPI forecasts the possible liquefaction potential of soil layers during an earthquake which can result

in extensive damage of built environment. LPI is categorized into four classes based on liquefaction

potential of soil layers, i.e.

I. 0 < LPI < 2 (low potential),

II. 2 < LPI < 5 (moderate potential),

III. 5 < LPI < 15 (high potential),

IV. LPI > 15 (very high potential).

Soils with LPI = 0 don't liquefy while as for surface manifestation of liquefaction features LPI ≥ 5 is

typically considered as a threshold value.

16.4 Traditional Architecture of structural units in Skardu

Skardu is the urban centre of Baltistan and is one of the oldest settlements in the Region. Usually

families live in joint families, which means that several generations are living under one roof. A

typical family therefore requires a substantial living area. In addition, they also require space to

keep domestic animals and to store grain and fodder (cattle of livestock) over the winter.

Mostly the houses in Skardu are built on two levels. The lower level is used as an animal pen and

store for grain and fodder. This is usually not more than 1.5 meters high and is usually built of

undressed stone masonry, which may or may not be plastered with mud and often reinforced with

a wooden frame to make it earthquake proof. Alternatively, it may be made of mud-bricks too.

64

Figure-36. Traditional Two-level Dwelling in Skardu

The upper level is the living area and typically comprises of about 4-5 rooms, including a large

kitchen. The width of the rooms is invariably about 3.5 meters, but the length may vary from 3.5 to

6.75 meters. Usually there are an equal number of small and large rooms. One of the larger rooms

is used for entertaining guests and for holding religious gatherings, particularly in the month of

Muharram. Another large room is used as a kitchen. The kitchen has to be large because the

cooking is done on wood stove which takes up a lot of space. Moreover, since this is the warmest

room in the house, it is also used as the primary living space during winter days.

A covered veranda connects the various rooms and it may sometimes incorporate a little courtyard

that is open to the sky. The veranda may also be used for storing firewood and for leaving snow or

mud-covered shoes, when walking into the house.

16.5 An Example Traditional House in Skardu

The plan below shows a small house with just three rooms. Two of these are large and one is

small. One of the larger rooms accommodates the kitchen. The rooms are connected by a

central space that is open to the sky (Figure-37).

Since this is a single storey-house, the animal shelter and grain store are built adjacent to the living

rooms on the same level. Laterally along the inside, the finishes are simple. There is usually a

concrete floor, covered with rugs. The roof structure is often exposed, but may sometimes be

concealed by a fabric false ceiling. The tradition of sitting on the floor can be observed everywhere

in Baltistan.

65

Figure-37. Plan of a traditional house in Skardu with a Central Courtyard

Nowadays, the timber structure used for most dwelling units in Skardu is composed of simple posts

and beams made from the wood of popular trees. Popular trees grow very high, very quickly and

are therefore ideal for long-span beams in addition to being economical (Figure-38).

The house is built on a stone plinth that is typically one meter high. Alternatively, it may be built

upon the animal shelter that is usually 1.5 meter high. In this case, the timber posts are taller,

running from the base of the shelter to the roof of the living quarters. These posts are normally

12.5 centimeter in diameter and are embedded in the mud-brick walls that are one foot thick. The

mud-bricks are simple to make and can be fabricated by the house-owners themselves, using a

simple mould.

The dimensions of a traditional mud-brick are 15 x 15 x 30 (cm). If this brick is purchased in the

village, it costs 15 - 20 Rupees per piece, but in the urban centre it can cost more than twice as

much. The Living rooms are normally 2.5 meters high and covered with a timber roof composed of

popular rafters. The rafters span across the short side of the rooms, resting on 2.5 meters diameter

beams that laid on top of the mud-brick walls, extend about a foot outside on each end. The rafters

have to therefore be about 16' long in order to span across the 30 cm wide rooms. These rafters

are 12.5 cm in diameter like the posts. These rafters are placed at c/c distance of 5.5 meters. They

are then covered over with timber slats or with branches and reeds. This is followed by a layer of

grass and leaves and a final layer of mud to finish the roof.

The windows and doors are also made with the wood of the popular tree and are normally painted.

The mud walls are also plastered with mud and then coated with lime. The floors are finished in

rammed earth and covered with carpet. These houses provide a higher degree of thermal comfort

in summers and winters.

66

Figure-38 Traditional Interior Ceiling of Baltistan

16.6 Chief Materials

The people of this Baltistan build their houses with sun-dried mud-bricks, using timber from popular

trees for structural members like posts, beams and rafters. The mud-brick walls, which area foot

thick, keep the interiors warm in winter and cool in the summer and are an appropriate response

to the climatic context of the region.

Traditional building materials of Skardu are;

a. Mud

b. Stone

c. Timber

67

The saturation of Balti traditional houses is evident in recent shifts in dwelling traditions and

practices. As observed and documented in recent changes in the more urban and accessible

centres of Skardu, the identity and substance of Balti architecture is at a critical juncture. Here,

fast-track concrete buildings often define themselves as regionally distinct using little more than

“Balti-style” construction methods.

16.7 Recent Constructions in Skardu Valley

Earthquakes don't kill, poorly constructed buildings do. As the number of inhabitants in the Skardu

Valley and surroundings has grown to ~3.5 million, conventional structures have offered path to

dilapidated mixture of ineffectively assembled structures, the majority of which are fragile, rigid and

less resistant to seismic ground shaking. Although conventional construction methods of Skardu

Valley fare better and were immune to earthquakes but modern unscientific construction

techniques have replaced such earthquake resistant structures.

In hilly areas and river valleys they are built on terraces with acute slopes made of stone (plinth

and walls), timber framing with mud-straws (in-filled mortar), wood (windows, doors and trusses)

and corrugated galvanized iron sheeting (roofing material). While as in plainer areas extensive use

of concrete as mortar is prevalent. The plinth of the structures directly rests on grounds without

any pier foundation. The foundation is made up of stones with wooden layer or occasionally a down

plinth concrete (DPC) above stone foundation with wall space ratio ranges from 35 to 67%.

Absence of steel increases the stiffness and rigidity in these structures.

For earthquake resistant structures symmetry is the basic standard, because symmetrical

structures possess low torsional effects which in turn help in minor displacements along height and

nearly insignificant floor rotations during earthquakes. Most structures in Skardu City are low-rise

structures with mostly 2–3 storied structures. Occasionally ≥ 4 story structures which mainly consist

commercial and industrial structures. Strength and ductility factors of different structures vary with

number of stories in different parts of the areas as increase in number of stories effects the resonant

frequency of the structure.

16.8 Structural integrity of existing structures

The earthquake resistant construction pattern is an important factor in reducing the loss of life and

property during an earthquake. Although no loss of life and property can be expected during severe

tremors but the loss can be mitigated to some extent if appropriate measures are adopted at proper

time. One of the best techniques to ensure the safety and reduce loss is construction of earthquake

resistant structures in seismically vulnerable areas.

Although no great earthquake has rocked the Skardu Valley, excluding October 8, 2005 Mw=7.6

earthquake having epicenter outside the Skardu Valley, for last two centuries, beyond the average

life span of any person. Hence most of the structures in this region have been never really put to

a real earthquake test. Numerous distant source large earthquakes (>7Mw) magnitude and

frequent near source low magnitude earthquakes occur in Skardu City with moderate damages to

existing structures. With no recent near source large earthquake in this region, ramshackle

hodgepodge of unplanned, weak and unsafe structures has been mostly constructed without

knowledge of future severe seismicity in this region.

68

Structures both residential and commercial present in the Skardu City, would undoubtedly be

condemned instantly as life threatening in developed countries. Although traditional pattern of

construction was safer than the present form of weak, rigid and stiff built structures, which were

more flexible and ductile due to presence of wood runners at each floor level, binding the walls

together with the floors with no abrupt discontinuities or variations in stiffness in the diaphragms

acting as seismic resistant structure.The advancement of modernization and industrialization has

resulted in inevitable destruction of these old conventional structures replaced by modern unsafe

and unscientific structures which ultimately increases the level of hazard.

Recent housing constructions hardly follow the Building Code of Pakistan (BCP) 2007 and now

updated during 2021. Ignorance of abiding the BCP laws has led to construction of ramshackle

hodgepodge of poorly built structures most of which have little resistance to earthquake shaking.

Historical and paleo seismic earthquake data accounts a very high vulnerability to the socioeconomic

and built environment and limelight's the severity of damage in near future if such

catastrophe returns to the area. Nearly half of population reside on seismically active fault systems

that can cause unprecedented damage because the lack of preparedness to deal with such

catastrophes. Furthermore, conversion of susceptible areas for construction purposes without due

consideration of the geotechnical and geological site conditions have doubled the hazard level.

Rapid urbanization has exposed more population to seismic hazard. Densely populated areas have

densely constructed structures while sparse in hilly regions. Building density contribute significantly

in total loss during earthquakes. Dense low rise reinforced and masonry present in most places

increases the trap percentage during earthquakes which might result in more damage. More

emphasis has been laid on architectural design rather than structural integrity. The vulnerability

can be reduced by the implementation of improved building standards, which in turn will decrease

overall risk even though the population exposure to hazard increases. If stringent building

standards can be made compulsory for every single construction seismic risk can be decreased

significantly.

17.0 MICRO SEISMIC MONITORING SYSTEM (MSMS)

The Skardu Region (Figure-13) as a part of Northern Pakistan lies on the seismically active

Himalayan orogenic belt which was created by a slow collision between Eurasian and Indian plates

spanning from the past 30 to 40 million years. The seismicity of Skardu Region is also

characterized by a complex network of active crustal faults spread around the main plate boundary.

This composite seismotectonic environment of the region poses a high level of seismic hazard to

the Skardu Region and its neighbouring areas. In the past, the Skardu Region has been hit by

several destructive earthquakes with Intensities reaching XI and resulting in a huge number of

fatalities (Appendix A). On the other hand, a rapid growth in population and unsustainable

urbanization is also resulting in an increased seismic risk of the Skardu Region. Kashmir-Hazara

earthquake is a recent example where the death toll due to that earthquake exceeded 80,000

people and millions were rendered homeless as a result of houses collapse.

The Building Code of Pakistan (BCP) 2007 & 2021 has placed the Skardu City in Seismic Danger

Zone III and in its close vicinity is Seismic Danger Zone IV. Urban population in the Skardu City is

increasing rapidly with no proper building construction and seismic safety monitoring.

69

In the areas of Skardu City and surroundings, damaging earthquakes occur infrequently. However,

MSMS instruments will continuously record weak ground motions from the more frequent, smaller

earthquakes. The pattern of ground motion amplitudes from these small earthquakes can

potentially provide useful information about the likely distribution of damaging ground motions that

would occur in strong earthquakes. Many earthquake predictions and forecasts have been based

on observations of statistically significant changes in the rates and types of seismic activity over

long, intermediate, or short time scales.

In Gilgit Baltistan areas of proposed Bunji and Diamer Basha Dam Projects, Pakistan WAPDA

have installed MSMS for safety monitoring of seismotectonic activities. The seismic stations are

transmitting seismic signals through satellite link to central recording station (CRS) working at

Tarbela Dam Projects (TDP). WAPDA seismic stations installed at Skardu and Chitral are also

sending its seismic signals to TDP. At the CRS the seismic data is processed and analysed with

the latest state of art ANTELOPE Software. M/S Kinemetrics Inc. USA has supplied and installed

all the MSMS network and ANTELOPE Software.

As part of the attempts to mitigate the seismic risk a Micro Seismic Monitoring System (MSMS)

may be installed in and around the Skardu City. Pakistan WAPDA is operating MSMS for its safety

monitoring of its Large Dams and Hydro Power Projects in many areas of Northern Pakistan.

WAPDA seismic station at Satpara Dam is good example. Such a proposal for installation of

Skardu MSMS is attached at Appendix C.

70

18.0 CONCLUSIONS AND RECOMMENDATIONS

With profound degree of hazard imposed by future earthquakes in the Skardu City and lack of

proper disaster mitigation measures increases the level of hazard. Moreover, geological as well

as environmental conditions highly affect the implementation of mitigation measures at ground

level. For effectiveness of disaster preparedness, mitigation, recovery and response the need of

the hour is to implement the following measures on priority basis. These Conclusions and

Recommendations are provided on the basis of results evaluated from present analysis which

suggests that high PGAs can be generated even by small magnitude earthquakes which can

ultimately lead to high liquefaction chances.

1. Northern sections of Pakistan including Skardu Region are more sensitive to earthquake

activity than the other segments because they are surrounded by the micro plates of

Afghanistan, Iran and India. Main Central Thrust (MCT), Main Karakoram Thrust (MKT),

Main Boundary Thrust (MBT) and Main Mantle Thrust (MMT) are the major faults located

in Skardu Region. The area also includes two Syntaxial Bends, known as Nanga Parbat

Haramosh Massif (NPHM) and Kashmir Hazara, where the rocks strata are folded around

this syntax and are subject to a 90 0 “rotation” from one side to the other side.

2. The most recent strong earthquake that has occurred in the Skardu Region is the Mw 7.6

Kashmir-Hazara October 08, 2005 earthquake, resulting in 72,763 fatalities and 6,8697

injuries alone in Pakistan. Similar earthquakes are likely to occur in the future, and it is thus

important to evaluate the seismic performance of structures based on reliable ground

motion scenarios.

3. During the past, Skardu Region has been hit by several destructive earthquakes with

Intensities reaching XI.). Also, a rapid growth in population and unsustainable urbanization

is also resulting in an increased seismic risk of the Skardu City. Building Code of Pakistan

2007 and 2021 has placed Skardu City in Seismic Danger Zone of 3.

4. Along the Skardu Valley, the seismic activity is mainly associated with the micro

earthquakes and macro earthquakes of Mw ≥ 5.0, and largely coincides with the surface

trace of the Himalayan Main Central Thrust (MCT).

5. As per Deterministic Hazard Analysis the Karakoram strike-slip fault is present about 35 km

NE of the Skardu City. A large amount of seismicity activity having Mw ≥ 4.0 in the City,

have been located from this fault. Maximum magnitude assigned to the fault is Mw 7.7,

while the computed PGA (H) = 0.2 g and PGA (V) = 0.13 g.

6. Due to the presence of three major mountain ranges geology of Skardu Valley is much

diverse consisting of Metamorphic and Igneous rocks. Most of the soil types are found in

the Valley. Mostly the Valley has Mesozoic and Palaeozoic -Precambrian soil types.

7. Based on the Geological Maps, Geotechnical Borehole logs of different locations and study

of research papers/reports, the Skardu City for Master Plan 2040 has been divided into

three seismic microzones with different site characterization (soft rock, hard soil, soft soil).

The seismic microzones are Skardu Khas, Hussainabad and Shigri Kalan.

71

8. Computed g values for Skardu City 2040 seismic microzones, through Probabilistic Hazard

Analysis are given in Table-4. The Total Hazard Curves and Uniform Hazard Spectra for

different Vs30 are given in Figures 15 through 26.

9. Large number of construction activity is under progress in the Skardu City. It should strictly

adhere to existing and state of art Building Code of Pakistan 2021. Constitution of teams

for review and enforcement of these codes for government, semi-government, corporation

and private residential buildings completed or under construction in urban areas.

10. Capacity building of stakeholders, service providers and incident respondents with back up

mechanism under Skardu Development Authority (SDA). Firming up disaster mitigation

management plans for critical departments and to construct the multilayered incident

response teams backed up by emergency response centres for each department.

11. Preparation of Earthquake Drill Manual for Skardu City. Introduction of disaster

management as a subject school, college and university level and conduct mass

awareness programs. Mainstreaming the Disaster Risk Reduction in development

programs by training activities, coordinating mock exercises, training concerned officials for

capacity building for better preparedness and effective response measures.

12. Lack of ground strategies like construction of earthquake resistant structures, flood

management strategies, landslide and avalanche mitigation measures increase the higher

chances of damage by future calamities. The SDA needs immediate establishment of fully

equipped disaster authorities with well-trained disaster rehabilitation force at city as well as

district. The transportation of rescue teams as well as rescue to supply after the disaster

takes lot of time and energy due to rugged and treacherous terrains of the region.

13. More geological, geotechnical investigations should be carried out to document the active

near source seismogenic structure which would help in formulating safer design decisions.

Although natural calamities cannot be stopped but proper strategies and mitigation

measures would substantially decrease the level of hazard and damage.

14. For the seismic safety monitoring purposes, a Micro Seismic Monitoring System (MSMS)

may be installed in and around the Skardu City. Plan for MSMS is given as Appendix C.

72

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