Earth Planets Space, Vol. 57 (No. 4), pp. 351-356, 2005LETTER
Yoshihiro Ito, Takumi Matsumoto, Hisanori Kimura, Hirotoshi Matsubayashi, Kazushige Obara, and Shoji Sekiguchi
National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki 305-0006, Japan
(Received November 30, 2004; Revised March 9, 2005; Accepted March 22, 2005)
Two large intraplate earthquakes (MW 7.0 and 7.3) occurred off the southeastern Kii Peninsula on September 5, 2004. We determined the centroid moment tensors of these earthquakes and their aftershocks. Earthquakes can be divided into two categories: (1) the reverse-fault type with a range in focal depth from 5 km to 20 km; and (2)
earthquakes with a near-vertical nodal plane and a focal depth shallower than 5 km. Earthquakes of type (1) are distributed inside the Philippine Sea plate beneath the Nankai trough, while normal-fault events predominate in the other regions along the trough. The distribution of type (2) events shows two clear lineaments with strikes of NNW-SSE to the north and NE-SW to the west of the focal area. These lineament strikes coincide approximately with strikes at the nodal planes of such events.
Key words: Centroid moment tensor, broadband seismometer, tiltmeter, seismicity.