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

Abstract

S U M M A R YSince 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 Ataköy 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 Ataköy 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.

Simulated Strong Ground Motions for the GreatM9.3 Sumatra–Andaman Earthquake of 26 December 2004

“…This is a reasonable estimate, considering that we are dealing with strong ground motion in an area where local site effects are expected to be significant. Similar levels of amplification have been estimated for the city of Istanbul, Turkey, based on modeling of strong ground motion (Sørensen et al, 2006), however, much more details about the geological and geotechnical conditions are needed before similar studies can be carried out for Banda Aceh. Another important information given by the simulated waveforms is the duration of the ground shaking that has a significant impact on the resulting damage.…”

Sensitivity of Ground-Motion Simulations to Earthquake Source Parameters: A Case Study for Istanbul, Turkey

“…The peak will be very critical for building damage in these densely populated areas largely occupied by ~ 5 story buildings, which have resonance frequencies around this value (Aydinoglu, 1998). Since we expect significant site effects in this area with amplification also around this frequency (Sørensen et al, 2006), the actual response can be even higher than predicted in this study. Again we also see peaks around 0.2 and 0.5 Hz which can affect high-rise buildings.…”

“…This is important to keep in mind when interpreting the simulation results because local site effects are indeed present and important, especially in the southwestern part of the city (e.g., Birgören et al, 2004;Sørensen et al, 2006). This issue has been addressed by Sørensen et al (2006), aiming to combine ground-motion simulation results with information about local site effects.…”

“…Site KUM (Kumköy) is north of the city at the Black Sea coast and is included for investigating the effect of distance on the spectra. Sites SIT (Avcilar) and ATA (Ataköy) are located in areas known to be affected by local site effects (Tezcan et al, 2002;Sørensen et al, 2006). The remaining two sites, HIS and BUS, are located in the historical center (Sultanahmet) and the business center (Levent), respectively.…”

Estimation of shallow S-wave velocity structure and site response characteristics by microtremor array measurements in Tekirdag region, NW Turkey

“…They found fundamental frequencies of 0.5-1 Hz for Atakoy and Zeytinburnu. On the other hand, Sørensen et al (2006) obtained dominant peak amplifications (3-4) at around 1 Hz from microtremor H/V results at 30 sites in Atakoy and its surroundings.…”