Earth Planets Space, Vol. 58 (No. 1), pp. 69-74, 2006LETTER
Yoshihiro Ito, Kazushige Obara, Tetsuya Takeda, Katsuhiko Shiomi, Takumi Matsumoto, Shoji Sekiguchi, and Sadaki Hori
National Research Institute for Earth Science and Disaster Prevention, 3-1, Tennodai, Tsukuba, Ibaraki, 305-0006, Japan
(Received August 10, 2005; Revised November 12, 2005; Accepted November 21, 2005; Online published January 27, 2006)
The 2005 west off Fukuoka prefecture earthquake (MJMA = 7.0) occurred on March 20, 2005 in northwest Kyusyu, Japan. The fault geometry and rupture propagation of the main shock are investigated by applying the centroid moment tensor (CMT) inversion method to densely-distributed broadband seismic network data. An accurate distribution of small aftershocks is also examined by a double-difference method. The source fault of the main shock whose moment magnitude was 6.6 had a strike of 123°, and the rupture propagated in the southeast direction. We obtain forty-four CMT solutions of the main shock and aftershocks with a moment magnitude greater than 3.5. For the main shock, the fault strike estimated using P-wave first-motion polarities differs from that of the moment tensor solution by 13° in the counterclockwise direction. Some aftershocks with a moment tensor solution similar to the focal mechanism of the main shock derived from the P-wave first motions are distributed north of the epicenter of the main shock. The largest aftershock with a strike of 8° in the clockwise direction, which differs from that of the CMT solution of the main shock, was located south of the main shock. These changes in the fault strike play an important role in the initiation and termination of the rupture process of the main shock.
Key words: Centroid moment tensor solution, doublet, rupture propagation, fault bend.