Strong Ground Motions and Site Effects

The ground motions recorded during the Kocaeli earthquake make a substantial contribution to the ground motion data set at short distances from large-magnitude earthquakes. The intensity of ground motions in the near field (i.e., less than 20 km from the fault) was lower than predicted by current attenuation relationships. Düzce is the only station in the near field that recorded motions close to the median prediction. It is believed that site response, coupled with forward-rupture directivity effects at long periods, led to the larger ground motions in Duzce. It appears that rupture directivity effects in the near-fault recordings were most pronounced at periods longer than 3 seconds. The ground motions at Sakarya and Yarimca displayed a static offset in the displacement time history due to their close proximity to the fault. In the intermediate field, ground motions were bound generally by the plus and minus 2 standard deviation predictions from available attenuation relationships. However, rock sites fell below the median prediction, while most soil sites fell above the median prediction, indicating that ground-motion amplification due to soil conditions was important in these areas. In general, acceleration response spectra from recorded ground motions were exceeded significantly by the design spectra from UBC (ICBO 1997). Soft soils throughout the affected region amplified the intensity of ground shaking and led to more long period energy in the recorded ground motions. Heavy damage was concentrated generally in areas underlain by Quaternary alluvium, with the most heavily damaged areas underlain by Holocene alluvium (i.e., Adapazari, Gölcük, Degirmendere, and the north margin of Izmit Bay). However, the Düzce basin, underlain by Quaternary alluvium, experienced only sparse damage. The short duration of ground shaking in Düzce may have contributed to the less intense damage observed there. Soft soils in the Avcilar section of Istanbul, approximately 80 km away from the fault rupture plane, increased the level of ground shaking and led to enhanced damage in the area.