Local site effects in Istanbul: Microtremor study in Ataköy area Mathilde Bøttger Sørensen 1...
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Transcript of Local site effects in Istanbul: Microtremor study in Ataköy area Mathilde Bøttger Sørensen 1...
Local site effects in Istanbul: Microtremor study in Ataköy area
Mathilde Bøttger Sørensen1
Caglar Yalciner2 and
Kuvvet Atakan1
1Department of Earth Science, University of Bergen, Norway2Osman Gazi University, Eskisehir, Turkey
Why are we interested in local site effects in Istanbul?
• Important factor for actual ground motion caused by an earthquake
• Necessary in order to move ground motion simulation results from bedrock level to surface of Earth
What did we do:
• Measured microtremors in the Ataköy area• Calculated H/V spectral ratios to obtain
information about frequencies with high amplification
• This information will later be combined with geotechnical data as input in 3D finite-difference modelling in cooperation with ETHZ
Geology of Istanbul
From http://www.geocities.com (Geological map by the Istanbul Greater City Council Directorate of Soil and Seismic Studies)
Data acquisition
• GBV-316 and pocket PC
• 3 x 10 min recorded at each site
• Sites in grid with 150-500 m grid spcaing
• Sites on all formations• Collected a total of 30
sites
Data processing
• H/V spectral ratios calculated using JSesame software (SESAME project)
• Automatic window selection by searching traces for windows with low noise level using STA/LTA
• H/V is calculated for each window or as average for site
Results, Bakirköy FormationBakirkoy formation
0
0,5
1
1,5
2
2,5
0 2 4 6 8 10
Frequency
H/V
sp
ec
tra
l ra
tio
A005
A007
A008
A009
A010
A011
A012
A013
A014
A015
A017
A018
A019
A020
A023
A024
A025
A028
Results, AlluviumAlluvium
0
0,5
1
1,5
2
2,5
3
0 2 4 6 8 10
Frequency
H/V
sp
ectr
al r
atio
A003
A004
A016
A021
A022
A026
A027
A029
A030
Results, Güngören FormationGungoren formation
0
0,2
0,4
0,6
0,8
1
1,2
0 1 2 3 4 5 6 7 8 9 10
Frequency
H/V
sp
ectr
al r
atio
A006
Results, Artificial fillManmade structures
0
0,5
1
1,5
2
2,5
0 1 2 3 4 5 6 7 8 9 10
Frequency
H/V
sp
ectr
al r
atio
A001
A002
Main results at the moment
• Main peak at 1-1.5 Hz, in agreement with ITU results
• Secondary peak at 4-6 Hz at some sites• Peaks at higher frequencies are probably
caused by noise
Other available data
• Borehole data (Japan International Cooperation Agency/Istanbul Metropolitan Municipality)
• Earthquake data (Kandilli observatory, Bogazaci University)
• Geomorphological map (Istanbul Technical University)
• Geological map (Istanbul Technical University)
• Hydrogeological map (Istanbul Technical University)
• Well logs (Istanbul Technical University)
• Resistivity data (Istanbul Technical University)