HR: 1340h
AN: T33B-0539    [Abstracts]
TI: High-V_p/V_S Zone Accompanying Non-volcanic Tremors and Slow Slip Events Beneath the Tokai Region, Central Japan.
AU: * Matsubara, M
EM: mkmatsu@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Obara, K
EM: obara@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AU: Kasahara, K
EM: kasa@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006 Japan
AB: In the Tokai region about 200 km to the southwest of Tokyo, central Japan, the Philippine Sea (PHS) plate subducts beneath the Eurasian (EUR) plate, from the Suruga trough, and this is the only area where the convergent plate boundary runs across the seismic network. With regard to the PHS plate beneath southwestern Japan, deep non-volcanic tremors are observed at depths of about 30 km along the strike of the PHS plate (Obara, 2002). Beneath the Tokai region, Ozawa et al. (2002) detected a long-term slow slip event that started during late 2000 and is still in progress. In this study, seismic tomography based on the data of the dense high-sensitivity seismograph network of Japan (Hi-net) operated by National Research Institute for Earth Science and Disaster Prevention (NIED) revealed the fine-scale P- and S-wave velocity structures from the trough to the deep zone where the non-volcanic tremors occur in contrast with the Nankai trough, SW-Japan. We apply the seismic tomography (Zhao et al., 1992) with spatial correlation of velocities (Matsubara et al., 2004; Matsubara et al., 2005) to 1,387,948 P- and 1,129,771 S-wave arrival times from 36,168 natural sources recorded by 367 stations of the NIED Hi-net with a grid spacing of 0.1° to obtain the crustal and upper mantle structure beneath the whole Japan. No velocity discontinuity is assumed. A low-V layer with a dip angle of 15° extends to depths of about 45 km and a thickness of 20 km and is considered as the oceanic crust of the PHS plate. Microearthquakes accompanying the subducting PHS plate are located in both the low-V and high-V layers of the PHS plate; however, in the high-V_p/V_S zone, microseismicity is low. The non-volcanic tremors are located just above the high-microseismicity zone and in and around the high-V_p/V_S zone with a thickness of about 10 km. According to Seno and Yamasaki (2003) the water dehydrated from the subducting oceanic crust causes the serpentinization of the mantle wedge and hydrofracturing is a possible mechanism of the non-volcanic tremors. Our model indicates that the non-volcanic tremors occur under a certain condition of pressure and temperature; and at depths of 30-40 km, the high-V_p/V_S zone reaches the condition corresponding to above the PHS plate. The southern edge of the high-V_p/V_S zone is consistent with the area where the slow slip event occurs, as estimated by Ozawa et al. (2002). The high-V_p/V_S zone is not brittle and may cause some aseismic slip; however, the zone with the highest V_p/V_S is beneath the inland area. The high-V_p/V_S zone is consistent with the largest back slip distribution (Ohta et al., 2004), and periodic short-term slow slip events occur in this zone (Hirose et al., 2005). Further, the non-volcanic tremors are located at the northern edge of both the high-V_p/V_S zone and large back slip area. This indicates that the plate boundary where the slow slip event was observed is weakly coupled; however, the inland area is decoupled and slips freely.
DE: 7205 Continental crust (1219)
DE: 7220 Oceanic crust
DE: 7270 Tomography (6982, 8180)
DE: 8150 Plate boundary: general (3040)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: Fall Meeting 2005