Abstract
In order to investigate the site effects of the Tokachi Basin, we have carried out seismic observation at a rock and basin station pair. A comparison of the seismograms observed at the basin site with those at the rock site reveals that later phases having large amplitude and long duration are observed at the basin site even for deep earthquakes. The predominant period of the later phases is 2 to 5 s. We have also carried out the seismic array observations at the basin site, to investigate the propagation characteristics and wave types of these later phases. By application of the semblance and the polarization methods to the seismograms observed in the array, many Rayleigh waves having a phase velocity of about 1.3 km/s and a few pure Love waves of about 1 km/s are found in the later phases. The arrival directions of these surface waves change quickly with time and are quite different from the backazimuth to the epicenters. This suggests that the later phases observed in the basin site are mainly composed of the surface waves which are generated at the edge surrounding the Tokachi Basin.
