やや深発地震による強震動記録を使った Site Effects の研究: 九州地方

Abstract

In view of the recent increase of tall or big structures with a natural period of 1 to 10sec, it is one of the most urgent and indispensable subjects to establish the effects of surface geological conditions around a site, i. e. site effects, on long-period strong ground motion. In order to elucidate the site effects at various sites in Japan, we have analyzed strong ground motions during intermediate-depth earthquakes, on which source and path effects may be simple. In this paper, we analyze JMA strong-motion seismograms from four intermediate-depth earthquakes in Kyushu district with a depth of 116 to 130km and a magnitude of 6.0 to 6.8 (JMA magnitude). These seismograms represent the site response to nearly vertical incident plane S waves; they are simply composed of the primary S wave on a rock site, while they show the long-period scattered waves after the amplified S wave on a basin site. These scattered waves cannot be explained by one-dimensional structure model. These facts suggest that scattered waves are excited by subsurface irregularity of structures. We define thee indices Is, which represent the strength of scattered wave excitation due to incident S waves. Generally, Is values are relatively large at stations on alluvium or diluvium, while they are small at stations located on a rock site. There exists, however, a wide range of Is values at stations on alluvium or diluvium. This indicates the variety of the deep subsurface velocity structures around these stations. Unfortunately detailed velocity structures in Kyushu district are not available. Then, we examine relation between Is values and Bouguer anomalies around the stations, which may reflect the velocity structures near the surface. The Bouguer anomalies around stations having large Is values show a deep trough with a short wavelength of about 50km.