HR: 1340h
AN: T53A-1117    [Abstracts]
TI: Asperities of the 1703 Genroku and 1923 Kanto earthquakes and the purpose of the Kanto Asperity Project
AU: * Kobayashi, R
EM: reiji@sci.kagoshima-u.ac.jp
AF: Kagoshima University, 1-21-35 Korimoto, Kagoshima, 890-0065, Japan
AU: Koketsu, K
EM: koketsu@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AB: Earthquakes along the Sagami trough, where the Philippine Sea slab is subducting, have repeatedly occurred. The 1703 Genroku and 1923 (Taisho) Kanto earthquakes (M 8.2 and M 7.9, respectively) are known as typical ones, and cause severe damages in the metropolitan area. The recurrence periods of Genroku- and Taisho-type earthquakes inferred from studies of wave cut terraces are about 200-400 and 2000 years, respectively(e.g., Earthquake Research Committee, 2004). We have inferred the source process of the 1923 Kanto earthquake from geodetic, teleseismic, and strong motion data (Kobayashi and Koketsu, 2005). Two asperities of the 1923 Kanto earthquake are located around the western part of Kanagawa prefecture (the base of the Izu peninsula) and around the Miura peninsula. The strong motion data provided us with much more information on the source time function than the teleseismic data (Kobayashi and Koketsu, 2005). We adopted an updated fault plane model, which is based on a recent model of the Philippine Sea slab, and the asperity around the Miura peninsula moves to the north (Sato et al., 2005). In this study, we have investigated the slip distribution of the 1703 Genroku earthquake. Since no seismic waveform data were recorded, we cannot infer the time-dependent rupture process. We used crustal uplift and subsidence data investigated by Shishikura (2003), and inferred the slip distribution with the same geometry of the fault as the 1923 Kanto earthquake. An additional asperity is located the southern part of the Boso Peninsula and the maximum slip is over 16 m. In the case of subduction-zone earthquakes, the asperity is considered not to be changed. In an interseismic period, coupling between plates is strong in an asperity region, and is weak in a non-asperity region (e.g., Coordination Committee for Earthquake Prediction Research in Universities, 2006). This framework can be applied to the subduction zone along the Sagami trough. The difference between the Sagami and Nankai troughs, where the Philippine Sea slab is also subducting, is that slow slip events occur at the same depths as the asperity. Many studies on the asperity along the Sagami trough have been carried out. Seismicity during these five years is very low in these asperities (Kobayashi and Koketsu, 2005). This suggests strong seismic coupling, which is also inferred from the geodetic study of the slip deficit distribution (Sagiya, 2005) Around the asperities, small repeating earthquakes are observed as well as slow slip events. Kimura (2005) shows that the small repeating earthquakes occurred during the slow slip events. We have proposed seismic monitoring in the Sagami Bay and off Boso region as a part of the Kanto Asperity Project, which is an IODP drilling plan, to characterize asperity and non-asperity regions. Nine borehole sites and inland ones can cover the asperity and non-asperity regions. Good coverage and very low noise in boreholes can catch smaller earthquakes, improve hypocenter and focal mechanism determinations. If seismic surveys with OBS are performed, we can obtain shallow structures of the Philippine Sea slab and knowledge on the relation between amplitude of reflection and asperity. In future, we can get a chance of deep drilling reaching the plate boundary in the asperity region, which cannot be included in this project, if we successfully show good results.
DE: 7200 SEISMOLOGY
DE: 7209 Earthquake dynamics (1242)
DE: 7215 Earthquake source observations (1240)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting