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Earth Planets Space, Vol. 54 (No. 11), pp. 1065-1070, 2002

Subsurface structure and faulting of the Median Tectonic Line, southwest Japan inferred from GPS velocity field

Takao Tabei1, Manabu Hashimoto2, Shin'ichi Miyazaki3, Kazuro Hirahara4, Fumiaki Kimata4, Takeshi Matsushima5, Torao Tanaka6, Yasuhide Eguchi1, Takashi Takaya1, Yoshinobu Hoso2, Fumio Ohya2, and Teruyuki Kato3

1Faculty of Science, Kochi University, Kochi 780-8520, Japan
2Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan
3Earthquake Research Institute, The University of Tokyo, Tokyo 113-0032, Japan
4Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8602, Japan
5Graduate School of Science, Kyushu University, Shimabara 855-0843, Japan
6Faculty of Science and Technology, Meijo University, Nagoya 464-8502, Japan

(Received December 30, 2001; Revised July 30, 2002; Accepted August 19, 2002)

Abstract: The Median Tectonic Line (MTL) is the longest arc-parallel fault system in southwest Japan whose right-lateral strike-slip is related to oblique subduction of the Philippine Sea plate (PH). We constructed a dense Global Positioning System network along a 200 km-long traverse line across the MTL in 1998 to estimate deep fault structure and slip distribution. Horizontal velocities were determined at 65 sites through campaign measurements and show crustal shortening in the direction of the plate convergence. Using multi-rectangular segments and depth-dependent coupling at the plate interface, we calculate and remove elastic deformation caused by the PH subduction. The residual velocity field shows right-lateral strike-slip block motion of about 5 mm/yr across the MTL, consistent with geological estimates. However, the block boundary does not coincide with the surface trace of the MTL, being displaced 20-30 km to the north. The residual velocity field is reproduced by a model with a 35-45° northward-dipping fault plane, full locking of the upper portion to a depth of 15 km, and steady slip of 5 mm/yr below. GPS results are supported by imaging of an inclined fault plane revealed by seismic profiling and currently low activity of shallow earthquakes.


Corresponding author E-mail: tabei@cc.kochi-u.ac.jp


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