地震 第2輯
Online ISSN : 1883-9029
Print ISSN : 0037-1114
2000年伊豆諸島群発地震における式根島の震度異常と地盤増幅特性
古村 孝志纐纈 一起坂上 実山中 佳子高橋 正義
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2001 年 54 巻 2 号 p. 299-308

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Seismic intensity at Shikine-jima during the earthquake swarm of Izu-Islands in 2000 is usually one or two ranks higher than that at neighbor island, though epicentral distances to these islands are almost similar. In order to investigate the cause of anomalous large seismic intensity, three portable strong motion instruments have been installed at the island.
The spectral ratios of observed S waveforms at sedimentary stations at Shikine-jima demonstrate severe site amplification of 2 to 10 within a wide frequency range between 1 to 10Hz relative to rock site. The experiments based on a GA inversion indicate unusual subsurface structure with very low (VS=31-427m/s) and high attenuation (QS=15-148) superficial layer overlying a bedrock (VS=1000m/s, QS=200) with large contrast at the interface is a main cause of high seismic intensities.
The influence of nonlinearity was also found near a coast of Shikine-jima when the ground acceleration level exceeds 300cm/s2. The nonlinear ground response yielded a considerable reduction of the ground motions, so that the observed peak accelerations during the severe earthquakes were only about 50% of that predicted by linear ground response.

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