HR: 1330h
AN: S52B-0135    [PDF]
TI: Characteristics of Seismic Activities in and around the Boso-Peninsula, in Central Japan, with Dense Seismographic Array Observation
AU: * Igarashi, T
EM: igarashi@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 1130032 Japan
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 1130032 Japan
AB: We investigated seismic activities in and around the Boso Peninsula, which is located east of Tokyo, Japan. A velocity structure beneath the Boso area is very complex, consisting of three layers: an overriding plate in land, northward subducting Philippine Sea plate, and westward subducting Pacific plate. The area includes a part of the source region of the 1923 Kanto Earthquake (M=7.9) and other older large earthquakes. Several slow-slip events were also detected by continuous GPS observations. First, we investigated whether or not exist repeating earthquakes, which was found in Tohoku district, in northeastern Japan (Igarashi et al., 2003), also beneath the Boso Peninsula. It is suggested that the repeating earthquakes occurred when a small asperity was periodically broken due to stable sliding of the plate boundary surrounding the asperity. If this is the case, the repeating event can be a kind of sensors to detect an aseismic slip in the area. We used the same algorithm for detecting them as Igarashi et al. [2003] did in Tohoku area. We found that many repeating earthquakes occurred on the subducting Pacific plate boundary and some of them around the subducting Philippine Sea plate. The repeating earthquakes in the Philippine Sea plate occurred on the edge of source regions of large interplate earthquakes or slow events. This suggests that the repeating earthquake is related with the degree of the plate coupling. Furthermore, we expect the repeating earthquake to be a clue to understand the location of asperities and also next large earthquake. Next, we investigated other small earthquakes. Seismicity beneath the Boso Peninsula is reported to be lower than that around the area, and large events did not occur in recent 50 years. The entire Japans Island is covered with a highly dense seismographic network. However, an interval of seismographs in the Boso Peninsula is relatively wide for populous area. And a thick sediment also prevents to detect a small event there. An accurate crustal structure and relationships between seismicity and configuration of boundary have not been well understood. Therefore, we need a dense seismographic array in the Boso Peninsula to clarify the seismic velocity structure of deeper part of the crust and subducting plate, detect micro earthquakes, and image the seismic fault of earthquakes that may destroy the metropolitan area. These seismographs are installed along the line of the northeast-southwest direction. Station intervals are about 2 km to 10 km. We install 30 stations during this fiscal year, data from which are continuously transmitted to the Earthquake Research Institute, the University of Tokyo, in Tokyo, by using the Internet and a satellite communication system. The new array enables us to detect smaller events beneath the array than the current networks, to clarify whether the lower seismic activity is real or not beneath the peninsular. Observed waveforms have many converted phase, which seem to appear on a common boundary. We expect to estimate the location of the upper plate boundary of the subducting plate by the conversion point, resulting in characterization of seismic activities in this region.
DE: 7200 SEISMOLOGY
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2003 Fall Meeting