Earthquake Motions Observed on Rock and Ground

Abstract

Earthquake motions were observed using an automated seismic recording system with short-period velocity seismometers at rock site and ground site, which are adjacent, on the eastern side of Kyoto basin. The influence of topographic features on the motions at a rock site was studied from examining the nature of particle motions of P and S waves. Amplification factors of the motions at a ground were studied from comparison of spectral characteristics between motions at the rock and those at the ground. The ratios of the amplitudes of P and S motions at the ground to those at the rock vary appreciably from 0.8 to 8.0 from event to event. The spectral ratios between the ground motions and rock motions show that the frequencies and amplitudes of the peaks are different in dependence of the incident direction of the events. The causes of the variation of the amplitude ratios is found to be that the amplitudes of the motions at the rock site on the hill, vary with the incident angles and the directions of seismic arrivals. And the peak frequencies of the responses of seismic motions at the ground site, underlied by dipped base rock, are considered to vary with the directions of seismic arrivals.