Site Amplification effects based on Teleseismic Wave ... - METNET

Reviewed Article
<strong>Site</strong> <strong>Amplification</strong> <strong>effects</strong> <strong>based</strong> on
Teleseismic Wave Analysis: The case of
the Pellice Valley (Piedmont, Italy)
[Ferretti et al. (2007, BSSA)]
Reviewer:
Dr. Ajeet Prakash Pandey
Meteorologist Gr.-I
EREC, IMD, New Delhi
Review Meeting of ADGM (E) at EREC Conference Room, IMD, New Delhi
(March 29, 2010)

Background
One of the most important problems in Seismic Microzonation
studies is the evaluation of site response characteristics
<strong>Site</strong> <strong>effects</strong> related to Geological and Geomorphological
settings actually represent one of the main factors responsible for
building damage
Local site amplification estimation of an area by local seismic
signal analysis is a fundamental step in defining the seismic
response of the area
Regions characterized by low rate of seismicity, having
potential of large damaging earthquakes, it is very difficult to
assess the site response through experimental methods, simply
because of the lack of earthquake occurrences

Contd…..
This problem could have been overcome by computing the
horizontal to vertical (H/V) response spectra for the region using
Nakamura technique <strong>based</strong> on ambient seismic noise data
Previous studies <strong>based</strong> on experimental data illustrate that the
amplification derived from the H/V ratio of ambient noise
measurements is almost always smaller than the one obtained
from earthquake data. However, fundamental frequencies of sites
are Stable
In this paper, use of teleseismic recordings at varied
geological and geomorphological settings are investigated for
assessing seismic wave amplification in the Pellice Valley
(North-Western Alps, Italy) and the results are compared with
that of Ambient Noise data as well as the Local Earthquake
events

Objectives of the Study
The prime objective of the study is to test whether <strong>Site</strong>
Response may be estimated using Teleseismic Signals
only, recorded at a dense temporary network
In fact, horizontal to vertical spectral ratio of
Teleseismic recordings are computed and the results are
compared with that of local earthquake and ambient noise
data
Secondly, use of Teleseismic signal in establishing
correlation between depth of sedimentary coverage and
computed P-phase arrival time delays & relative P-phase
amplitude with respect to reference site

Geological Setup of Pellice Valley
Pellice Valley is an Alpine Valley, located N-W of
Piedmont region, Italy
Substratum characterized by Gneiss rock of Dora-Maria
Massif superimposed with Quaternary ancient to recent
Lacustrine and/or Palustrine deposits
Four different geological settings of the study area –
• On the edge of the Valley – Gneiss Outcrop
• NW part of the study area – Alluvial fan deposits
• Central part of the study area – Ancient deposits
• Riverbed area – Recent fluvial deposits

Geological Map of Pellice Valley

Sub-surface Exploration Surveys Conducted
Geophysical and Geotechnical field surveys are
conducted for defining Physical – Mechanical Parameters
of sub-surface materials as well as providing information
about the Geometry and depth to the Bedrock
Different field surveys conducted in the study area –
• 4 Boreholes – Geotechnical Investigation
• 4 <strong>Site</strong>s – Down Hole Seismic test up to 50 m depth
• 2 Profiles – Seismic reflection surveys conducted to
constrain Stratigraphy of deep
sediments of more than 200 m &
Topography of the bedrock
• 9 Broadband Stations – Ensuring complete coverage
(120 db) of the study area on all types of
geological settings

Documentation Map of Pellice Valley
Seismic Stations
Seismic Profiles
Seismic Stations
Boreholes
9 temporary broadband seismic stations are used
for teleseismic recording
Sesimic station Pe03 lies on the rock and treated
as Reference Station for the Study
> 50 local events (M 2.0 – 3.5) recordings
18 Teleseismic recordings

Methodology
Adopted

Step-I
Signal
Signal to Noise Ratio for
Stations Pe03, Pe06 &
Pe07
Noise
Pe03
Signal-to-Noise ratio
(S/N) calculated using 20 s
of pre-event noise and 20 s of
seismic signal
Noise
Signal
Pe06
In 0.2 – 3.0 Hz frequency
range, the level of teleseismic
signals are much above the
noise level
Noise
Signal
Pe07

Step-II
Horizontal to Vertical
Spectral Ratio (H/V) for
Teleseism
20 s
Window
EQ: 08/02/2005; 15:05; 14.25
S,167.26 E ; 206.0 km; M 6.8
Teleseismic seismograms
are deconvolved with
Instrument response and
filtered by a 20 Hz low pass
filter
Fourier spectra calculated
for windows of 20 s
including the first and more
energetic portion of teleseism

Step-II
(Contd…..)
Fourier spectra pass through
Hanning Window to enhance
major features in the pass band
Fourier spectra of the NS and
EW components are averaged
(root mean sqaure) to obtain
horizontal component Fourier
spectrum
Fourier spectra of vertical
component obtained
Only 3 stations (Pe03, Pe06
& Pe07) are used for the study,
as other 6 stations could not
record significant number (>3)
of high quality teleseismic
record
20 s
Window

Step-III
Stability of H/V Spectral
Ratios of Teleseismic
recording
Different portions of
teleseismic recording
containing different teleseismic
phases [P, PKP, PcP, pP, sP,
sPKP etc.] are also used for
H/V spectral ratio estimation

Results
H/V of Teleseisms
are reliable in the
frequency range 0.2
– 3.0 Hz, as per S/N
ratio analysis
Rock site
Reference Station
No amplification effect
(Flat Response)

Results
H/V of Teleseisms
are reliable in the
frequency range 0.2
– 3.0 Hz, as per S/N
ratio analysis
Ancient Fluvial
Deposit
Broad Peak between
2.0 – 3.5 Hz
Confirms Shallower
Sedimentary Layer over
Rock

Results
H/V of Teleseisms
are reliable in the
frequency range 0.2
– 3.0 Hz, as per S/N
ratio analysis
Recent Fluvial
Deposit in Riverbed
Peak <strong>Amplification</strong> lies
at 1.3 Hz
Confirms Deeper
Sedimentary Layer over
Rock

Mapping of Valley Geometry
Computation of P-wave arrival time delays and
relative amplitude of the initial P-waves carried
out w.r.t. Pe03 reference station
Considering P-wave only in to account, the
unprocessed teleseismic signal is bandpass filtered
between 0.2 – 1.0 Hz, as per S/N ratio in
relatively shorter window length
P-wave time delays are carefully determined by
cross-correlating the first quarter-cycle of the
initial P-waveform of each station w.r.t. Pe03
reference station
Also the amplitude ratio of the first P-wave
arrivals are determined w.r.t. refrence station

Bed Rock
X = Theoretical delay time derived considering vertical propagation
through 1D velocity structure below each station

Bed Rock
X = Theoretical delay time derived considering vertical propagation
through 1D velocity structure below each station

Major Achievements of the Study
Use of teleseismic recordings, in this study, are investigated for
assessing seismic wave amplification in the Pellice Valley (North-
Western Alps, Italy) and compared with the results obtained
through local earthquakes as well as ambient noise data
The reliability of the H/V results, obtained by the application of
a standard spectral ratio technique, considering teleseismic
recordings is confirmed from both H/V of ambient noise data and
H/V of S-wave of local events methods
Accurate analysis of teleseismic data facilitates correlation
between both P-wave travel time delays and its relative amplitude
w.r.t. the bedrock topography and alluvial deposit thickness
The case of the Pellice Valley shows that, in a region
characterized by a low seismicity rate, a spectral analysis <strong>based</strong> on
teleseismic signals could be an important tool to reliably define the
site response and the main geological structure of the valley