HR: 0800h
AN: S51B-0161    [Abstracts]
TI: Imaging crust, mantle and slab structure around the Shikoku district, Japan, by Double-Difference Tomography and its relationship to the occurrence of nonvolcanic deep tremors
AU: * Nakajima, J
EM: nakajima@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, Akarami-aza Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: RCPEV, Graduate School of Science, Tohoku University, Akarami-aza Aoba, Aoba-ku, Sendai, 980-8578 Japan
AU: Zhang, H
EM: hjzhang@ice.geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W Dayton St., Madison, WI 53706 United States
AU: Thurber, C H
EM: clifft@ice.geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 W Dayton St., Madison, WI 53706 United States
AB: Obara [2002] detected nonvolcanic deep, low-frequency tremors occurring along the strike of the subducting Philippine Sea plate over a length of 600 km. The depth of the tremors averaged about 30 km, near the Moho, and some tremors migrate at a rate of 13km/day along the strike. Although the occurrence of tremors is hypothesized to be associated with fluids supplied by dehydration reactions of minerals in the slab, there have been no systematic studies of the heterogeneous structure around the source area of tremors. This study is the first attempt to estimate the detailed P- and S-wave velocity structures beneath the Shikoku district and the Kii Peninsula, where deep, low-frequency tremors are frequently found. Double-difference tomography [Zhang and Thurber, 2003] was used to estimate three-dimensional (3D) P- and S-wave velocity models. The inversion started from a one-dimensional velocity model. The total number of earthquakes and seismograph stations are about 4,000 and 120, respectively. The 3D model space is represented by a set of regular grid nodes with grid intervals spaced 20-30 km in the horizontal direction and 5-20 km in the vertical direction. The results show that high-velocity zones are distributed along the deep seismic zone beneath the western part of Shikoku and the Kii Channel, which may correspond to the subducting Philippine Sea plate. On the other hand, low velocity zones are present immediately above them in the western part of Shikoku and the central part of the Kii Peninsula with Vp/Vs ratios generally greater than 1.8. Interestingly, deep, low frequency microearthquakes (tremors) determined by the Japan Meteorological Agency (JMA) occur in and around such low-velocity and high-Vp/Vs zones. Acknowledgments: Arrival-time data and the location of deep, low-frequency microearthquakes are provided by JMA.
DE: 8180 Tomography
DE: 7230 Seismicity and seismotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
SC: Seismology [S]
MN: 2004 AGU Fall Meeting