SECONDARILY GENERATED SURFACE-WAVES IN THE TOKACHI BASIN, HOKKAIDO, JAPAN

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 c omparison of the seismograms observed at the basin site with those at the rock site reveals that later phases having large amplitude and lon g duration are observed at the basin site even for deep earthquakes. T he predominant period of the later phases is 2 to 5 s. We have also ca rried out the seismic array observations at the basin site, to investi gate the propagation characteristics and wave types of these later pha ses. By application of the semblance and the polarization methods to t he seismograms observed in the array, many Rayleigh waves having a pha se 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 surfa ce waves change quickly with time and are quite different from the bac kazimuth to the epicenters. This suggests that the later phases observ ed in the basin site are mainly composed of the surface waves which ar e generated at the edge surrounding the Tokachi Basin.