Analysis of ambient noise in Yalova, Turkey: discrimination between artificial and natural excitations

Ambient noise measurements acquired in Yalova, which was highly damaged during the 1999 Izmit earthquake, are analyzed to explore the site characteristics. The region of Yalova is governed by complex geological and geomorphological structures consisting of river beds extending from the mountains to the sea, ridges between them, plains in front of them with different size, and the sea coast. As a result of these shallow geological features, the H/V curves exhibit complex patterns. Clear peaks in the H/V curves, which can be interpreted as reliable site resonance frequency, are observed only at about half of the measurement sites. At the remaining sites industrial peaks, broad peaks, or flat responses dominate the spectral ratio graphs. We observed that man-made noises generated by marble cutting machines in Hersek delta mask the site resonance frequencies or can be misinterpreted as a resonance frequency. In total, we identified three anthropogenic noise sources at fundamental frequencies of 1.3, 1.5, and 1.7 Hz along with their two- and threefold harmonics. The parts of H/V curves showing unusual low scattering can be a clue to identify anthropogenic effects. In the assessment of H/V curves, the site location and the similarity of the near surface geology were taken into account. The Laledere plain with thick and soft sediment sequence surprisingly displays flat responses due to a possible low impedance contrast. The Ciftlikkoy and Hacimehmet plains exhibit clear resonance peaks at nearly 1 Hz possessing the largest amplitudes. These sites experienced the highest damage in Yalova during the Izmit earthquake. In contrast, the Cinarcik region which was also exposed to high damage, do not show any obvious amplifications on the H/V curves. Generally, the H/V curves for valley and ridge sites in Yalova reveals a resonance peak at approximately 1 Hz and almost flat curves, respectively. However, several sites on the ridges and valleys portray different patterns

Selecting the most suitable rupture model for the stochastic simulation of the 1999 Izmit earthquake and prediction of peak ground motions

In this study, we use a stochastic finite-fault technique based on a dynamic corner frequency to investigate how the fault and slip models affect the high frequency simulations of the 1999 Izmit (Turkey) earthquake. Seven different rupture models, one of them generated using common fault parameters and random slip distribution are tried to obtain the best matching with the observations. The synthetic seismograms computed in the frequency band 0.1-25 Hz are compared with the observations both in time and frequency domain. Six accelerometric stations located close to the observed surface rupture are chosen for the comparisons considering the fact that the slip contributions are visible at these station records better than the other stations. We also estimated average H/V spectral ratios using the available accelerometric recordings to take into account site amplification at each site. We also acquired ambient noise data at some stations that lack sufficient earthquake records. The results show that none of the rupture models fully simulate the observations at all the stations. Most of the rupture models underestimate the Fourier amplitudes at frequencies lower than 0.4 Hz, whereas overestimate them at higher frequencies. The underestimation may result from a directivity effect which likely causes higher amplitudes on the observed ground motions in low frequency band. While all the rupture models display similar average bias functions, the minimum average error for spectral amplitudes is obtained for the rupture model of Bouchon et al. (2002). The achievement of the random slip distribution model also yields satisfactory results. After we have optimized the rupture model, we have simulated the strong ground motions on a regular grid (0.2 degrees x 0.2 degrees) covering the study area for bedrock conditions. In total, we have estimated peak accelerations and peak velocities at 135 points. The results show that the maximum acceleration values during the lzmit earthquake reached 785 cm/s(2), and the largest velocity values were around 75 cm/s. The peak ground accelerations are still smaller than those predicted by the empirical relations but are well above the observed ones. The low accelerations might be attributed to the low stress drop that might be due to the large rupture length. (c) 2012 Elsevier Ltd. All rights reserved