Abstract

The 1995 Hyogo-Ken Nanbu earthquake struck the heavily populated Kobe and adjacent cities in western Japan. More than 6400 people were killed, and more than 150,000 buildings were destroyed. The characteristics of mainshock ground motions in the heavily damaged area are needed to understand how buildings and bridges performed and why they reached failure. Unfortunately, very few strong ground motions were recorded in the heavily damaged area during the mainshock. In this study, we attempt to estimate mainshock ground motions by using the empirical Green's function method (EGF method). First, we assume an initial source model with the asperities based on the rupture process obtained by inversion of strong-ground-motion records. For simplicity, we consider each asperity as a subevent with uniform stress drop in a finite extent. Then, the initial model was improved by matching the synthetic and observed ground motions using a trial-and-error procedure. The final model consists of three subevents: subevent 1 with stress drop of 163 bars, under the Akashi Strait around the rupture starting point; subevent 2 with stress drop of 86 bars, under the Nojima Fault in Awaji Island; and subevent 3 with stress drop of 86 bars, under Kobe. Finally, we estimate strong ground motions using aftershock records at sites where the mainshock was not recorded. The near-source motions in Kobe synthesized with the best-fit model are characterized by two large pulses with a duration of 1 to 3 sec. The pulses are caused by forward rupture directivity effects from subevents 1 and 3. Peak horizontal acceleration and velocity of the synthesized motions at the heavily damaged sites are about 1000 cm/sec2 and 130 cm/sec, respectively, while those at a rock site in the near-source region are about 300 cm/sec2 and 60 cm/sec.

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