Secondarily Generated Surface Waves in the Tokachi Basin, Hokkaido, Japan.

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. IntroductionThe seismograms observed in a basin have been investigated to evaluate the site effects of the basin by many researchers (e.g., Tanaka et al., 1979;Minamishima et al., 1986;Frankel et al., 1991;Kinoshita et al., 1992;Yamanaka et al., 1992;Hatayama et al., 1995). These studies have revealed that secondarily generated surface waves originating from body waves or surface waves, which were propagated from epicenters have been observed in the various basins. Zama's review paper (1993) shows that the secondarily generated surface waves have different characteristics in each basin. In the Kanto Basin, dispersing waves having a predominant period of 5 to 10 s with a large amplitude are observed clearly for shallow earthquakes occurring at the west or south-west of the basin. Kinoshita et al. (1992) revealed that those waves were Love waves excited at the Hachioji tectonic line. In the Osaka Basin, a strong later phase having a predominant period of about 3 s with amplitude larger than S wave is seen in the later phases even for deep earthquakes (Hatayama et al., 1995). This phase is interpreted as a Love wave generated at the eastern edge of the basin.In this study, we investigate the site effects of the Tokachi Basin, Hokkaido, Received June 2, 1995; Accepted December 28, 1995 * To whom correspondence should be addressed . Japan. The Tokachi Basin extends about 100 km in the north-south direction and about 50 km in the east-west direction, surrounded by the Hidaka Mountains, the Taisetsu Mountains and the Shiranuka Hills (Fig. 1). It is covered with terrace deposits of Pleistocene, and the maximum thickness of the sedimentary layers is about 3,000 m (Matsushima, 1990). We first compare the seismograms observed at a rock and basin station pair in and around the Tokachi Basin. We show strong excitation of later phases at the basin site for both deep and shallow events. Next, we examine the wave types and propagation characte...

Generation of Basin-Induced Surface Waves Observed in the Tokachi Basin, Hokkaido, Japan.

“…Therefore, we analyze the vertical and horizontal component seismograms separately in this study. Furumura and Sasatani (1996) have confirmed, by array analysis using semblance and polarization techniques, that the later phases observed at OUV with a predominant frequency of 0.2 to 0.6 Hz are mainly composed of the fundamental modes of Rayleigh and Love waves. We hereafter call such low-frequency later phases observed at OUV "basin -induced surface waves ."…”

“…In order to investigate the characteristics of basin-induced surface waves in the Tokachi Basin, we carried out seismic observations at a basin and rock station pair (Sasatani et al, 1990;Furumura and Sasatani, 1996). The basin station, OUV, is located difference between the two stations; the MYR seismograms are very simple with a dominant S-wave pulse, while the OUV seismograms are very complex with later phases of large amplitudes and long duration.…”

“…Hatayama et al (1995) asserted that later phases observed in the Osaka Basin with amplitudes larger than S-waves and a predominant period of 2 to 3 s were Love waves excited at the eastern edge of the basin. Furumura and Sasatani (1996) showed that the later phases observed in the Tokachi Basin with a predominant period of 2 to 5 s were mainly composed of both Rayleigh and Love waves generated at the basin edges. The surface waves generated from body waves are called "basin-induced surface waves" (Kawase, 1993).…”

Seismic vertical array analysis for phase decomposition

“…A horizontal seismic array is used as a tool for estimating the phase velocity and propagating direction of basin surface waves. Many studies based on horizontal array observations of basin surface waves have been carried out (Frankel et al 1991; Hatayama et al 1995; Field 1996; Furumura & Sasatani 1996, etc.). These studies have been concerned with long‐period waves that are isolated from other phases ( S waves and other kinds of surface waves).…”

“…However, an array analysis of the horizontal components, which include S , Love waves and horizontal components of Rayleigh waves, is considerably difficult because it is necessary to retrieve S , Love and Rayleigh waves from the horizontal component records at each array station for the array analysis. This retrieval is, in general, logically difficult (Furumura & Sasatani 1996).…”