A study of residuals for strong ground motions in Adapazari basin, NW Turkey, by ground motion prediction equations (GMPEs)

The city of AdapazarA +/- - located in the Marmara Region of northwest Turkey - is situated on a deep sedimentary basin and was the city most heavily damaged by the strong ground motion of the 17 August 1999 Kocaeli earthquake (moment magnitude M-w = 7.4). This study determines site amplifications of the attenuation relationships for shallow earthquakes in the AdapazarA +/- basin by using the previous ground motion prediction equations (GMPEs) and the traditional spectral ratio method. The site amplifications are determined empirically by averaging the residuals between the observed and predicted peak ground acceleration (PGA) and spectral acceleration (SA) values for various periods. Residuals are significantly correlated with the known characteristics of geological units. A new attenuation model has also been developed for 5% damped spectral acceleration to determine the dependence of strong ground motions on frequency. Site amplification was evaluated at ten stations located in the AdapazarA +/- basin. The PGA site amplification values varied between 1.130 and 3.120 in all stations except two; these exceptions resulted in de-amplifications. Calculated de-amplification values of these stations indicate a consistency for rock medium. The site amplification values of some stations indicate an increase while moving away from the surface rupture and approaching the contact between the alluvial deposits and bedrock. One station, located on the discontinuity between the two different media, has the highest site amplification value (3.12) in the study area. This high value might be a result of the focusing of the seismic waves due to the discontinuities located on both sides of the narrow basin. The traditional spectral ratio method was also used to determine amplification in the AdapazarA +/- basin. One station on bedrock was chosen as a reference station. When the results of this method are evaluated, the amplifications at high periods are attributed to the thick sedimentary deposits in the basin and the apparent de-amplifications at low periods are partly due to the reference site response. When the spectral ratios or spectral residuals of the stations located on alluvial deposits or soft soils are considered, it is observed that these stations have high spectral ratio or residual values, especially at high periods. The source of these high period amplifications may be the geometry of the basin, the presence of soft-loose soil levels and high period basin-transduced surface waves.