1996 Volume 44 Issue 5 Pages 553-561
Site amplifications during aftershocks of the 1995 Hyogo-ken Nanbu earthquake were examined in and around the disaster area of Higashinada ward, Kobe City, Japan. Five temporary stations together with the KOB station (CEORKA) were installed just after the main shock to arrange an array for strong ground motion observations across a severe damage band about 1.5 km wide. Several aftershock records were obtained from these stations. The peak values of the observed ground accelerations and velocities at the soil sites were 3-20 times larger than those at the rock site. Spectral ratios between the soil sites and the rock site have peak frequencies characteristic to each station pair. We also estimated a thickness of about 1 km for the sedimentary basin just under the soil sites using SP-converted waves recorded at all soil sites. Waveform simulations using a dislocation point source in a horizontally stratified medium were performed to explain the observed records at both the rock and soil stations. We found that large amplifications at the soil sites were generated by a thick sedimentary layer including low-velocity surface layers. The heavy damage caused by the main shock is therefore strongly connected with the sedimentary structure.