HR: 13:40h
AN: T13F-01    [Abstracts]
TI: High-Vp/Vs Zone Accompanying Low-frequency Tremors, Short-term and Long-term Slow Slip Events Beneath the Southwestern Japan
AU: * Matsubara, M
EM: mkmatsu@bosai.go.jp
AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1 Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006, Japan
AU: Obara, K
AF: National Research Institute for Earth Science and Disaster Prevention, Japan, 3-1 Tennodai, Tsukuba-shi, Ibaraki-ken, 305-0006, Japan
AB: With regard to the Philippine Sea (PHS) plate beneath southwestern Japan, deep low-frequency non-volcanic tremors (LFTs) are observed at depths of approximately 30 km along the strike of the PHS plate from Tokai to the western Shikoku regions (Obara, Science, 2002). Accompanying the LFTs, short-term slow-slip events (S-SSEs) are also observed beneath the Tokai region, eastern Kii peninsula, and Shikoku region. The source region of LFT is consistent with that of S-SSE. Some studies revealed that the LFTs beneath the southwestern Japan are located along a high-Vp/Vs zone by seismic tomography. Beneath the Tokai region, Ozawa et al. (Science, 2002) detected a long-term slow slip event (L-SSE) that started late 2000 and ended in 2005. Beneath the Bungo channel, at the west side of the Shikoku region, L-SSE was also observed during 1999 (Hirose et al., GRL, 1999). The National Research Institute for Earth Science and Disaster Prevention (NIED) has deployed the high- sensitivity seismograph network of Japan (Hi-net). Plenty arrival time data enable us to clarify the fine-scale 3D Vp and Vs structure beneath the southwestern Japan. In this study we estimate the relationship with velocity structures and L-SSE as well as LFT and S-SSE. The target region, 29–37°N and 129–139°E, covers the southwestern Japan. A total of 1,198,906 P-wave and 966,496 S-wave arrival times for 31,751 earthquakes recorded at 466 Hi-net stations are available for use in the tomographic method (Matsubara et al., Tectonophys., 2004; JGR, 2005) introduced spatial velocity correlation to the original code of Zhao et al. (JGR, 1992). We assume that artificial velocity discontinuities are absent since an incorrect configuration of the boundary would lead to an incorrect solution. Horizontal grid spacing is 0.1 degree and resolution is 0.2 degree. Slow earthquakes are distributed along the high-Vp/Vs zone beneath the southwestern Japan and they are considered to be caused by the high pore fluid pressure dehydrated from the oceanic crust of the PHS plate. The souce region of the S-SSEs are located in the high-Vp/Vs zone beneath the Tokai and Bungo channel, at the downdip landward end of the high-Vp/Vs beneath the southern Kii peninsula, at the updip seaward end of the high-Vp/Vs zone beneath the western Shikoku region. The high-Vp/Vs regions extend landward beneath the Tokai and western Shikoku region and Bungo channel. The mantle wedge has high-Vp/Vs zone with 10-30 % serpentinized peridotite exist if there is serpentinized. The S-SSE may occur at the plate boundary between the PHS plate and the serpentinized wedge mantle. High-Vp/Vs zone is extended from the S-SSE zone to the updip seaward beneath the Tokai region and Bungo channel along the plate boundary. L-SSEs were observed beneath both regions. The fluid also exists in the high- Vp/Vs (1.84) zone of the oceanic crust beneath the Tokai and the Bungo channel at depths approximately 25-30 km and L-SSEs occur owing to the high pore pressure. On the contrary beneath the Kii peninsula, L-SSE is not observed since the Vp/Vs is high as 1.80 and this value is lower than that beneath the Tokai region.
DE: 7220 Oceanic crust
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
DE: 8150 Plate boundary: general (3040)
DE: 8159 Rheology: crust and lithosphere (8031)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting