1. Local site effects in Istanbul: Microtremor study in Ataköy area Mathilde Bøttger Sørensen 1 Caglar Yalciner 2 and Kuvvet Atakan 1 1 Department of Earth Science, University of Bergen, Norway 2 Osman Gazi University, Eskisehir, Turkey

2. 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

3. 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

4. Geology of Istanbul From http://www.geocities.com (Geological map by the Istanbul Greater City Council Directorate of Soil and Seismic Studies)

5. Ataköy area, geology Ataköy From I.T.Ü. Gelistirme vakfi iktisadi isletmesi

6. Ataköy area, accessibility

7. 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

8. Microtremor sites, ground coupling

9. Microtremor sites

10. 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

11. JSesame, browsing module

12. JSesame, window selection

13. JSesame, H/V parameters

14. JSesame, display module

15. JSesame, single window display

16. Results, Bakirköy Formation

17. Results, Alluvium

18. Results, Güngören Formation

19. Results, Artificial fill

20. Artificial fill

21. 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

22. Secondary peak

23. 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)

24. 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