[S07-P] Poster

[S07-P-07] Generation conditions of long-period ground motions in the Kanto Basin

^○Yurie Mukai, Takashi Furumura (Earthquake Research Institute, The University of Tokyo, Tokyo, Japan)

In the Kanto (Tokyo) Basin in Japan, the long-period ground motion with period between 3 to 10 s are strongly developed when large (>M7) and shallow earthquakes occur nearby Tokyo. It is reported that the level of the long-period ground motion was very large from the earthquakes in Niigata but is several times weaker from the earthquake of Tohoku. In this study, we examined the cause of larger long-period ground motion in the Kanto Basin based on numerical simulation of seismic wave propagations. We first examined the directional amplification properties of the long-period ground motion in the Kanto Basin using a 3D heterogeneous sedimentary structure and a source model of the 2004 Niigata Chuetsu (Mw6.8) Earthquake. It is confirmed that the level of the long-period ground motion in Tokyo is strong when the source in Niigata, and is weak in Tohoku. However, it is insufficient to explain the large differences in the observation. Thus, we examined the source radiation effect of this event by modifying the strike of this source. It is found that the level of the long-period ground motion in central Tokyo is largely fluctuated with change in the strike of the fault, which had a largest when it corresponds to the case of the Niigata Chuetsu Earthquake. To examine the contribution of the surface wave propagation from the source to the Kanto Basin we conducted an another simulation with modified the free surface with a rigid boundary to suppress the surface wave propagation. As a result, the long-period motion in central Tokyo is dramatically weakened to about 1/2, indicating the importance of the surface wave propagation from the outside to the basin. The above results demonstrated that the strong long-period ground motion developed in the Kanto Basin during the Niigata Chuetsu earthquake was due to the fact that the surface wave is strongly radiated from the source to the direction to the Kanto Basin and that it can propagate well in the path to the Kanto Basin.