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)

Ataköy area, geology

Ataköy

From I.T.Ü. Gelistirme vakfi iktisadi isletmesi

Ataköy area, accessibility

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

Microtremor sites, ground coupling

Microtremor 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

JSesame, browsing module

JSesame, window selection

JSesame, H/V parameters

JSesame, display module

JSesame, single window display

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

Artificial fill

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

Secondary peak

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)

Future plans

• Process and interpret data...• Check against 1D model• Produce input model for 3D finite difference

model by combining with other data• Run 3D finite difference model and interpret result• Use ground motion simulation results as input in

3D model to obtain ground motion at surface