Site amplification at Avcılar, Istanbul |
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| Authors: | M Ergin S
zalaybey M Aktar MN Yalin |
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| Institution: | aTÜBİTAK Marmara Research Center, Earth and Marine Sciences Research Institute, P.O. Box 21, 41470 Gebze, Turkey;bBoğazici University, Kandilli Observatory and Earthquake Research Institute, 81220 Çengelköy, Turkey;cIstanbul University, Faculty of Engineering, Department of Geological Engineering, 34850 Avcılar, Turkey |
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| Abstract: | Avcılar is the suburb of Istanbul that was most heavily damaged during the August 17, 1999 Mw 7.4 Izmit earthquake. Strong ground motion caused fatalities and damage in Avcılar despite being 90 km from the epicenter. We deployed five portable seismograph stations equipped with Reftek 24-bit recorders and L4C-3D seismometers for 2 months, in order to understand why the local site response was different from elsewhere in Istanbul. A reference station was placed on a hard rock site, and the remaining four stations were placed on other geological units, in areas that had experienced varying levels of damage. We calculated frequency-dependent ground amplification curves by taking the ratios of the spectra at soft and hard rock sites. We obtained similar site response curves for most earthquakes at each site in the frequency range of 0.3–1.6 Hz, and observed no significant site amplification beyond 2.0 Hz at any site. The overall characteristics of the recorded S-waveforms and our modeling of the calculated site amplification curves are consistent with amplification as a result of trapping of seismic energy within a 100–150 m thick, low-velocity subsurface layer. We also review the applicability of microtremor measurements to estimate local site effects at Avcılar. For these data, we used ratios of spectra of horizontal to vertical components to obtain each site response. These results are compared with standard spectral ratios. These microtremor measurements provide consistent estimates of the amplification at most sites at the higher end of the frequency band, namely above 1 Hz. The results from both methods indeed agree well in this part of the frequency band. However, the microtremor method fails to detect amplification at lower frequencies, namely <1.0 Hz. |
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| Keywords: | Site effect Ground amplification Microtremor H/V ratios Frequency of resonance |
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