Archaeoseismology and Palaeoseismology in the Alpine ... - Tierra

1 st INQUA‐IGCP‐567 International Workshop on Earthquake Archaeology and Palaeoseismology)
THE IMPORTANT KEY FACTOR OF PALEOTECTONIC STUDIES ON SEISMIC
HAZARD EVALUATION OF MOSHAMPA DAM AND HYDROPOWER PLANT
125
H. Samari (1) and A. Mobini (2)
(1) Islamic Azad University‐ Mahallat Branch, Geology Group, Mahallat, Markazi province, IRAN. Hsamari@yahoo.com
(2) Tamavan Consulting Engineers, No.1, 2 nd Alley, Kaj St., Fatemi St., Tehran 1414763442, IRAN. mobini@tamavan.com
Abstract: Moshampa dam and hydropower plant project site is located within the northwest of Central Iran zone. Paleotectonics studies
reveals that NE1 fault in northeast of the site, (38.5km) is the most important seismic source. No seismic data of it’s activity has been found,
but based on paleotectonic evidence it is considered as an active fault. This fault, after a gap distance through its two terminals in northwest
and southeast, is connected to two active faults. Tectonics and paleotectonic studies reveal that, the NE1 fault as a middle fault segment
belongs to the large paleobasement fault that marks the borders of Alborz, Azerbaijan and central Iran seismotectonic provinces.
Seismotectonic data show both of two segments of both sides of this basement fault have been active for several times in 20 th century. In spite
of historical and 20 th century seismic gap in the middle segment of this basement fault (NE1), its tectonic setting indicates, that probably some
earthquakes will occur around NE1 fault in the future.
Key words: Paleotectonics, Basement fault, Central Iran, Moshampa
INTRODUCTION
As it was mentioned earlier, the Moshampa dam and
power plant is located at the northwest of the Central
Iran structural zone. Based on seismicity, geology and
topography conditions, the type of dam is earth rock fill
with central clay core. The height of dam is 124m and
reservoir normal water volume is about 700MCM.The
powerhouse is located in the vicinity of dam in the left
abutment which its net head is 88.78m, its total discharge
is 142.24m 3 /s, total power out put is about 110MW
(4*27.5) and total energy is about 170GWH.
The Alborz, Azerbaijan and Zagros structural zones
respectively situated at the northwest, north and
southwest of the site. The closest active fault to the
project site is the NE1 fault segment, 13.7km to the
northeast. Due to the importance of the site, the studied
area encompasses a radius of 300 kilometers. First, the
tectonic unit of the Central Iran is described and in the
next section, its seismic characteristics are briefly
explained.
TECTONICS OF THE CENTRAL IRAN
Central Iran zone, which has a triangular shape, is one of
the main tectonic units in Iran. It is considered as one of
the largest and most complex geological zones. This zone
comprises of the oldest metamorphic rocks (Precambrian)
up to the recent active and semi‐active volcanoes. In fact,
this region could be regarded as the oldest location of the
continent within Iran, which has experienced numerous
geological events. The thickness of the Early Precambrian
rocks outcrop at Central Iran is more than ten thousand
meters, which itself is formed from the erosion of the old
igneous rocks. This series is severely metamorphosed by
the Katangan orogeny movements and has established
the Central Iran platform. In turn, this platform is covered
by the continental sediments or shallow marine Late
Precambrian to Triassic sediments, which is better known
as the platform cover. However, the epirogenic
movement that usually causes vertical earth movements
along the faults also causes the paraconformity and facies
change Moreover, evaporate sediments were sometimes
created in Neo‐Proterozoic and Paleozoic. The crystalline
basement of Central Iran and its platform cover has been
cut by large faults since Paleozoic. Based on stratigraphy
gaps in some regions, the basement has experienced
continuous vertical movements that have had some
influence in the formation of Tertiary volcanoes.
SEISMOTECTONICS OF THE CENTRAL IRAN
The Central Iran is situated between two Zagros & Alborz‐
Kopedagh thrusted and folded mountain belts, which is
trapped between the Eurasian plate in the north and the
Arabian plate in the south due to the Arabian plate
movement to the northeast. Thus, it is under a
compressive regime in direction of N20E, where the old
Central Iran structures during the various orogenic phases
are reactivated and with their subsequent movements are
triggering earthquakes. The seismotectonic studies of the
Central Iran reveal the following interesting and
important facts:
- The distribution of earthquake epicenters are not
homogeneous (mostly in northwest, west and
northeast)
- The earthquakes of the Central Iran have higher
magnitudes in comparison to the other parts of Iran
- Focal depths of earthquakes in Central Iran are
shallower than in other parts of Iran
- The return period of the earthquakes in Central Iran
is longer
- The activity of the thrust faults as well as the
deformation of the earth surface is the influential
factor in seismic activities of the Central Iran.
MAIN ACTIVE FAULTS IN THE STUDIED AREA
According to the small‐scale seismotectonic maps, the
active faults at the periphery of the site are concentrated
in two locations. The first group is within a 50 kilometer