Basin Structure Effects in the Kobe Area Inferred from the Modeling of Ground Motions from Two Aftershocks of the January 17, 1995, Hyogo-ken Nanbu Earthquake.

Abstract

Ground motion records from aftershocks of the January 17, 1995, Hyogo-ken Nanbu earthquake provide a good opportunity for evaluating the Osaka basin edge effects on wave propagation in the Kobe area. Aftershock ground motion recorded in a small array deployed at sites within and outside the heavily damaged zone, east of Kobe City, show very large amplification at the sedimentary sites located close to the basin edge. The peak velocities at sediment sites were up to 15 times larger than those at the rock site. To model the effect of the basin edge structure and the source radiation in the frequency range 0.1-2.5 Hz, we calculate double-couple point source SH seismograms using two-dimensional finite difference techniques. By comparing the synthetic seismograms with the transverse component of the observed waveforms from two selected aftershocks, we find that the northern edge of the Osaka basin has a fault-like structure, dipping north. The rather complex basin edge structure causes seismic energy to focus at sites close to the edge. The constructive interference of different basin generated waves amplifies the ground motion. The focal area and the amplification due to the basin structure depend on the source location, i.e. the arrival direction of seismic waves. Our results indicate that the basin edge effect is one of the reasons why the zone heavily damaged during the main shock lies along a narrow belt. They also indicate that the thin surface sediments in the Kobe region amplify the ground motion at frequencies around 2 Hz considerably.