Simulation Analysis of Strong Motions in the Ashigara Valley Considering One-and Two-Dimensional Geological Structures

Abstract

Accelerograms observed on the soft sediments of the Ashigara Valley, Japan show clear evidence of surface wave propagating inside the valley. We try to simulate these observed accelerograms by using a two-dimensional geological structure of the valley and accelerograms observed on surrounding rock. First, observed records are examined to confirm that the isolated later phases that emerge 19 or 25 s after the S wave arrival are due to the basin-induced surface wave, the surface wave converted from the incident body wave at the edge of the basin. Then one-dimensional wave propagation theory is applied to check the appropriateness of the assumed soil properties. The first 30 s of the accelerograms recorded on the soft sediments show very good agreement with the one-dimensional simulation results, but the later part does not. Therefore a two-dimensional finite element method is used for a basin model whose boundary configuration and properties are assumed based on available geological and geophysical data. The calculated responses show that this two-dimensional model can generate surface waves which travel across the basin very slowly. The amplitude of the Love wave is greater than that of the Rayleigh wave, which is in accordance with what we observe. However, the relative amplitude of the later phase is smaller and the predominant period shorter than the observed. For more quantitative simulation without trial-and-error adjustment detailed information of the soil properties, especially S wave velocities and quality factors of the soft sediments, are crucial. © 1992, The Seismological Society of Japan, The Volcanological Society of Japan, The Geodetic Society of Japan. All rights reserved.