Tuning the deep velocity structure model by 1-D simulation of long-period S-waves

Abstract

Deep sedimentary layers above the seismic basement (Swave velocity ~ 3 km/s) greatly control the amplitude and duration of long-period strong ground motions in large basin areas. Recently, National Research Institute for Earth Science and Disaster Prevention (NIED) has developed velocity structure models in several regions of Japan as part of seismic hazard mapping projects in Japan. The NIED velocity structures in large basin areas need to be adjusted based on the observed features of earthquake records for the prediction of strong ground motions. In general, direct S-wave at a basin site from 3D simulation for a deep basin structure is essentially the same as that from 1-D simulation for a flat layer structure just beneath the site. According to this fact, we propose an improvement in tuning the deep velocity structure with 1-D simulation of long-period S-waves. The observed long-period S-waveform is compared with the synthetic long-period S-waveform from 1-D simulation assuming a tentative flat layer structure just beneath the site. In this comparison, it is important to select an appropriate band-width of the bandpass filter based on the S-wave amplification factors. This process is repeated until both S-waveforms fit reasonably. We apply this tuning method to the NIED model of the Tokachi basin located in the eastern Hokkaido, Japan.