*Earth Planets Space,* Vol. 56 (No. 3), pp. 301-306, 2004

*LETTER*
## Spatial distribution for moment tensor solutions of the 2003 Tokachi-oki earthquake (*M*_{JMA} = 8.0) and aftershocks

Yoshihiro Ito, Hirotoshi Matsubayashi, Hisanori Kimura, Takumi Matsumoto, Youichi Asano, and Shoji Sekiguchi

National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, Japan

(Received November 21, 2003; Revised January 22, 2004; Accepted January 28, 2004)

**Abstract: **
The 2003 Tokachi-oki earthquake with * M*_{w } 7.9 is the largest interplate earthquake occurred ever since the high dense broadband seismometer network, the National Research Institute for Earth Science and Disaster Prevention (NIED) F-net, has been established over Japan. We determine the spatial distribution of moment tensor solutions and centroid depths of the mainshock and aftershocks. All aftershocks are divided to three groups: (1) the thrust fault type whose nodal plane is similar to the main shock; (2) the other thrust type with nodal plane different from the main shock; and (3) the normal fault type. The type (1) shows a depth distribution inclined to NW gently, coincident to the upper boundary of descending Pacific Plate. The active area of the type (1) does not overlap with the co-seismic slip area of the main shock at all. On the other hand, the type (2) shows no characteristic depth distribution with centroid depth scattered above and beneath the upper plate boundary. The type (3) are distributed, mainly, at about 40 km depth above the upper plate boundary. *P* axes of some aftershocks occurred above the plate boundary show the direction from ENE-WSW to ESE-WNW that suggests the effect of the Hidaka collision.

**Key words: **
The 2003 Tokachi-oki Earthquake, moment tensor solution, aftershock, focal mechanism, subduction, collision.

Corresponding author E-mail: yito@bosai.go.jp

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