Two-dimensional nonlinear site response analysis of Adapazarı plain and predictions inferred from aftershocks of the Kocaeli earthquake of 17 August 1999

Abstract

The objective of this study is to investigate the effects of local site conditions in the Adapazarı plain crossing the severely damaged central part of Sakarya during the 17th August earthquake. A two-dimensional model has been used to obtain the free-field motions in the valley and on surface formations. A preliminary geotechnical profile model developed from available but limited soil data was checked using recordings of some specific locations where a strong ground motion accelerometer array was in operation after the main earthquake. The range of validity of the model was assessed and modifications were made to compare with the actual recorded motions. The purpose of building such a model, which was used to study the aftershocks in Adapazarı, is to provide an improved database to be used for the design of structures in the city. This study also attempts to provide greater insight into the local site response phenomena through the use of a two-dimensional nonlinear analysis. Simulated site responses are in global agreement with the recorded data. In general agreement between the amplification functions of the computed and recorded data is satisfactory for the frequencies of engineering interest. Alluvial sites show amplification factors in the range of 4–6 in the frequencies between 0.6 and 2 Hz. A relatively shallow alluvial station, HASTAHANE, experienced considerable amplification for small aftershocks and deamplification for the strongest aftershock in frequencies between 0.5 and 2 Hz. Another station, TOYOTA, with significant deamplification characteristics and a clearly observed peak response frequency shift at the soil resonant frequency may have experienced soil shear failure during the strongest aftershock. Other alluvial stations, SEKER and GENC, show deamplification characteristics at 0.55 and at around 1 Hz. with the peak frequency shifts during the magnitude 5.8 aftershock. In general, site responses are larger in the alluvial basin for all aftershocks except the magnitude 5.8 event.