Empirical approach for the duration effects in the lower frequency range on strong motions inside large sedimentary basins

Abstract

The quantitative evaluation of site amplification is an essential component of strong ground motion prediction because a site amplification factor (SAF) significantly differs from site to site, reflecting its specific geophysical underground profile. For the large-sized sedimentary basins, we investigated the spatial properties of the SAFs based on the generalized inversion technique using Fourier spectra. We found that the spatial distribution of the SAFs for the Swave portion with 5 to 15 seconds in duration (sSAF) was relatively similar in a small region for a low-frequency range from 0.12 to 0.5 Hz. We have also confirmed that SAF for the whole duration of motion (wSAF) is quite similar in a neighborhood area, although its amplitude is much larger than sSAF inside a basin, which reflects the inclusion of basin-induced surface waves. We proposed the prediction procedure for wSAF at an arbitrary site, using the ratio

between them named WSR. It is interesting to see similar WSRs in both horizontal and vertical components, even though their sSAFs are quite different. We confirmed these observed facts by a numerical simulation of Osaka Basin.