HR: 1340h
AN: T43B-1361    [Abstracts]
TI: Three-dimensional Seismic Structure of the Locked-Sliding Transition on the Subducting Plate Boundary beneath the Southern Part of Kii Peninsula, Southwestern Japan
AU: * Kurashimo, E
EM: ekura@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, 113-0032, Japan
AU: Kato, A
EM: akato@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, 113-0032, Japan
AU: Hirata, N
EM: hirata@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, 113-0032, Japan
AU: Iwasaki, T
EM: iwasaki@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, 113-0032, Japan
AU: Iidaka, T
EM: iidaka@eri.u-tokyo.ac.jp
AF: The University of Tokyo, 1-1-1, Yayoi, Bunkyo-ku, 113-0032, Japan
AU: Ito, K
EM: ito@rcep.dpri.kyoto-u.ac.jp
AF: Kyoto University, Gokasho, Uji, 611-0011, Japan
AU: Yamazaki, F
EM: yamazaki@nagoya-u.jp
AF: Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602, Japan
AU: Miyashita, K
EM: miyas@mito.ipc.ibaraki.ac.jp
AF: Ibaraki University, 2-1-1 Bunkyo, Mito, 310-8512, Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: NIED, 3-1, Tennoudai, Tsukuba, 305-0006, Japan
AB: The Nankai trough region, where the Philippine Sea Plate is subducted beneath the SW Japan arc, is a well- known zone of interplate earthquakes. The most recent great earthquakes occurred in 1944 (Tonankai Earthquake, M=7.9) and 1946 (Nankai Earthquake, M=8.0). Detailed crustal and upper mantle structure of the subducting Philippine Sea Plate and the overlying SW Japan arc are important to constrain the process of earthquake occurrence. Recent seismic experiments reveal the relation between the crustal structure and the seismogenic zone. However, little is known about the deeper part of the plate boundary, especially the transition zone on the subducting plate. To reveal the detailed structure of the transition zone on the subducting plate, we conducted seismic array observations in the southern part of Kii Peninsula, southwestern Japan. We deployed a temporary seismic array in the southern part of Kii Peninsula. Thirteen 3-component portable seismographs were deployed from January to February 2004. Waveforms were continuously recorded. We also conducted a deep seismic reflection profile to image the deeper part of the plate boundary in the southern Kii Peninsula. Two hundred eighty seismometers were deployed on a 60-km-long line in the east-west direction with about 200 m spacing, and waveforms were continuously recorded during a four-day period in November 2004. Three explosive shots were fired on EW-line. Our seismic experiment was designed in conjunction with the 2004 Daidai-toku seismic experiment (NS-line) across Kii Peninsula (Ito et al., 2006). We recorded the explosive shots fired on the NS-line as well as local earthquakes. Arrival times of local earthquakes and explosive shots were used in a joint inversion for earthquake locations and 3-D Vp and Vp/Vs structures. To obtain the detailed structure image of the transition zone on the subducting plate, the data recorded on the EW-line and NS-line were processed using the seismic reflection technique. Seismic reflection image shows the lateral variation of the reflectivity along the top of the subducting Philippine Sea plate. A broad reflection band is present where the clustered tremors occurred. Clustered tremors are located in and around the low Vp and high Vp/Vs zone. The low Vp and high Vp/Vs generally suggests the existence of fluid (e.g., Zhao et al., 1996), and indicates the occurrence of the tremors may be associated with fluids. At the northern Cascadia margin, there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to be a broad reflection where aseismic slip is thought to be occurring (Nedimovic et al., 2003). These studies suggest that the lateral variation of the reflectivity along the top of the Philippine Sea plate indicates the change of the degree of plate coupling caused by fluid.
DE: 0900 EXPLORATION GEOPHYSICS
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8123 Dynamics: seismotectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 9320 Asia
SC: Tectonophysics [T]
MN: 2007 Fall Meeting