Characteristics of observed peak amplitude for strong ground motion from the 1995 Hyogoken Nanbu (Kobe) earthquake

Over 200 peak amplitudes of strong motion were observed at distances of les s than 250 km from the fault during the 1995 Hyogo-ken Nanbu (Kobe) earthqu ake. We analyzed the attenuation of the peak-ground acceleration and veloci ty as a function of distance and geological site conditions. The observed p eak amplitudes agree well with those predicted by an empirical attenuation relation that was developed for Japanese earthquakes. This demonstrates tha t on average the peak amplitude of the ground motion generated by this dama ging earthquake did not exceed the level predicted by the empirical attenua tion relation. We found a significant effect of the surface geology on the observed ground-motion peak amplitude. In particular for soft-soil sites, l ocated near the fault, the peak-horizontal acceleration decreases rapidly w ith distance as a result of the nonlinear response of soils. In order to ta ke into account the effect of the site conditions we introduced correction factors to the existing attenuation relation. This resulted in a significan t reduction of the residuals between the predicted and observed peak amplit udes. Based on the attenuation relation corrected for the site condition ef fect we generated a map of horizontal peak-ground acceleration in the Kobe and Osaka area for the Kobe earthquake. The area of simulated large ground motion agrees well with the severe damage zone of intensity VII, JMA scale.