Local site effects in Ataköy, Istanbul, Turkey, due to a future large earthquake in the Marmara Sea

Mathilde B. Sørensen, 1∗,  Ivo Oprsal, 2†,  Sylvette Bonnefoy-Claudet, 3‡,  Kuvvet Atakan,1, P. Martin Mai, 2, Nelson Pulido, 4, and Caglar Yalciner, 5§,
1 Department of Earth Science, University of Bergen, Allegt. 41, 5007 Bergen, Norway. E-mail: sorensen@gfz-potsdam.de
2 Institute of Geophysics, Swiss Federal Institute of Technology ETH, Zurich, Switzerland
3 LGIT, Universit´e Joseph Fourier, Grenoble, France
4 Earthquake Disaster Mitigation Research Center EDM, NIED, Kobe, Japan
5 Osmangazi University, Eskisehir, Turkey

∗ Now at: GeoForschungsZentrum Potsdam, Section 5.3, Potsdam, Germany.
† Now at: Charles University, Faculty of Mathematics and Physics, Department
of Geop00hysics, Prague, Czech Republic.
‡ Now at: Department of Astronomy, Physics of the Earth and Meteorology,
Comenius University, Bratislava, Slovak Republic.
§ Now at: EOST-IPG, Strasbourg, France.

E-mail: , http://geo.mff.cuni.cz

Abstract
Since the 1999 Izmit and Düzce earthquakes in northwest Turkey, many seismic hazard studies have focused on the city of Istanbul. An important issue in this respect is local site effects: strong amplifications are expected at a number of locations due to the local geological conditions. In this study we estimate the local site effects in the Atak¨oy area (southwestern Istanbul) by applying several techniques using synthetic data (hybrid 3-D modelling and 1-D modelling) and comparing to empirical data.We apply a hybrid 3-D finite-difference method that combines a complex source and wave propagation for a regional 1-D velocity model with site effects calculated for a local 3-D velocity structure. The local velocity model is built from geological, geotechnical and geomorphological data. The results indicate that strongest spectral amplifications (SA) in the Atak¨oy area occur around 1 Hz and that amplification levels are largest for alluvial sites where SA reaching a factor of 1.5–2 can be expected in the case of a large earthquake. We also compare our results to H/V (horizontal to vertical component of the recorded
signal) spectral ratios calculated for microtremor data recorded at 30 sites as well as to ambient noise synthetics simulated using a 1-D approach. Because the applied methods complement each other, they provide comprehensive and reliable information about the local site effects in Ataköy. Added to that, our results have significant implications for the southwestern parts of Istanbul built on similar geological formations, for which, therefore, similar SA levels are expected.

Key words: earthquakes, fault model, finite-difference methods, hybrid method, Istanbul, strong ground motion.

Sorensen, M., B., Oprsal, I., Bonnefoy-Claudet, S., Atakan, K., Mai, P., M., Pulido, N., Yalciner, C. 2006. Local site effects in Ataköy, Istanbul, Turkey, due to a future large earthquake in the Marmara Sea. Geophysical Journal International, 167, doi: 10.1111/j.1365-246X.2006.03204.x, 1413-1424.

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