Estimation of Strong Ground Motions on Hard Rock and Soft Sediment Sites in the Ashigara Valley Using the Empirical Green's Function Method

Abstract

We applied the Empirical Green's Function Method to simulate ground motions on hard rock and soft sediment at near distance (8km) for the 1990 Hakone earthquake (M_JMA=5.1) using the records of the foreshock (M_JMA=2.9). We examined the seismic data used in the Ashigara Valley Blind Prediction Test (Kudo, 1992), records from weak and strong motions observed at 3 superficial and 2 borehole sites. The main goal of this study is to investigate the non-linear behavior of soft soil at KS1 and KS2 sites. Our method is based in a linear superposition of ground motions, which allows us to check the non-linear effect in soft sediments due to strong motions (more than 100 gals) by comparing the observed waveforms with the simulated ones. The simulation method proposed by Irikura (1986) is modified in order to correct a difference in stress drop that was found between these two events. We found the stress parameters play an important role in the simulations. The seismic source model is determined by using only rock sites waveforms, which is effected by minimizing the residuals between the synthetics and the observed waveforms. First we used a long time window to obtain the source model. Second, a time window of 3 seconds including the onsets and largest amplitudes of the S-wave part was used to determine the source model, avolding contamination by seismic noises in the small event records. After defining the seismic source model we simulated the ground motions at KD1, KS1 and KS2, and compared the simulated with the observed seismograms. We found, the difference between the acceleration waveforms (synthetic and observed) to be relatively large at KS1 and KS2 compared to the rest of the sites, and we conclude that a non-linear effect during the 1990 Hakone earthquake (M_JMA=5.1) at KS2 and KS1 exists, based on these. From the comparison of their spectra, we could not find systematic changes of the predominant frequencies between the observed and synthetic at KS1 and KS2.