3次元深部地下構造がやや長周期地震動の特性に及ぼす影響

Abstract

The effects of 3-D deep underground structures in and around Yokohama City on the characteristics of rather long-period ground motion are examined. The detailed 3-D deep underground structure was modeled by interpreting the time delay between PS converted waves and the direct P waves (PS-P time) calculated from earthquake records. This PS arrival time was determined on the stacked receiver function waveforms. The results show that the basement depth varies between 2.5 to 4.0km. The propagation characteristics of rather long-period ground motions in the southern part of the Kanto basin were examined by analyzing records pertaining to the east off Izu Peninsula earthquake of May 3rd, 1998. The results show that Love wave packets traveling across the basin turn into the main contributors to the observed rather long-period ground motions. The amplitudes of the Love waves appear well correlated with the basement depth. The observed phase velocities for periods between 5 and 10s show good agreement with those of fundamental mode Love waves calculated for the estimated model. However, in cases where steep dipping of the basement is detected, a simple underground model beneath the sites cannot explain the amplitudes and phase velocities. This is caused by the effect of these irregularities of the deep underground structure on the waves propagation.