Relations between long-period ground motions and subsurface structures

It is important to grasp ground motion distributions right after a major earthquake. Ground motion is very sensitive to subsurface structure, but because seismic stations are sparsely distributed, it is necessary to estimate ground motion distributions at sites with no stations from subsurface structure data at those sites and ground motions data recorded only at surrounding stations. In this study, we investigated relationship between ground motion and subsurface structure to estimate ground motion distribution applying corrections according to the subsurface structure differences. Maximum velocity responses with a period of 3 s or longer were correlated with the first natural period of the deep subsurface structure, but maximum velocity responses with a shorter period correlated more strongly with the average S-wave velocity in the upper 30 m (AVS30) than with the first natural period. However, the ratios of maximum velocity responses at one station to those at a nearby station often differed for different earthquakes, indicating that there is limitation in estimating the ratios of maximum velocity responses only from the subsurface structures. Moreover, we did not detect any notable correlations between the subsurface structures and the durations of the velocity responses. Although these results were obtained by using relative velocity responses, similar results were obtained when pseudo-velocity responses were used.