神戸市長田区における地下構造の不整形性を考慮した1995年兵庫県南部地震 (M_J=7.2) の余震アレイ観測記録のシミュレーション解析

抄録

Waveform records obtained from the temporal aftershock observation in Nagata Ward, Kobe, were simulated taking into account the irregularity of the underground structure. The strong motion observation of the aftershocks was carried out by the linear array which consists of six stations from the Rokko Mountains to the coast of Osaka Bay, shortly after the 1995 Hyogo-ken Nanbu earthquake. The underground structure under these stations was determined from the results of geophysical surveys, the spectral ratio of observed horizontal components at the sediment stations to those at KWC on granite, and the difference between the travel times of the SP-converted waves and the direct S-waves at the sediment stations. The obtained 2-D structure model shows that the granite basement is steeply deepened along the observation line between the stations KWC and NGH forming a basin edge, and the thickness of sediments increases from 1.0km (NGH) to 1.2km (KMG). The velocity seismograms at the five stations on sediments were synthesized from the observed records at the KWC station on granite for the three aftershocks, using the 2-D structure in-plane and anti-plane models by the finite element method. The synthesized waveforms explain well not only the direct S-wave part, but also the conspicuous later phase part that were found in the observed records at the SYP and KMG stations near the coast of Osaka Bay. Next, we input the Ricker wavelet with a characteristic period of 0.8s from the basement to the same 2-D structure model. The synthetic waveforms were decomposed into two wave types to elucidate the origin of the later phases at the SYP and KMG stations. The one was the response of the flatly layered model at each point and the other was the diffracted wave from the basin edge. It was concluded from the results of these analyses that the later phases were due to constructive interference between the reflected S-waves from the basement and the diffracted waves from the basin edge. It was also found that the Rayleigh and Love waves of the higher modes were predominant in the diffracted waves at the KMG station.