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Earth Planets Space, Vol. 58 (No. 12), pp. 1605-1610, 2006

LETTER

Precise aftershock distribution of the 2005 West Off Fukuoka Prefecture Earthquake (Mj=7.0) using a dense onshore and offshore seismic network

Kenji Uehira1, Tomoaki Yamada2, Masanao Shinohara2, Kazuo Nakahigashi2, Hiroki Miyamachi3, Yoshihisa Iio4, Tomomi Okada5, Hiroaki Takahashi6, Norimichi Matsuwo1, Kazunari Uchida1, Toshihiko Kanazawa2, and Hiroshi Shimizu1

1Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
2Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
3Faculty of Science, Kagoshima University, Kagoshima 890-0065, Japan
4Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan
5Graduate School of Science, Tohoku University, Sendai 980-8578 Japan
6Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan

(Received December 7, 2005; Revised July 25, 2006; Accepted August 1, 2006; Online published February 2, 2007)

Abstract: The 2005West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We combined data from these stations and permanent stations located around the aftershock area, and determined the hypocenter of the mainshock and aftershocks. The mainshock was in the northwestern central part of the aftershock region, at a depth of 9.5 km. The mainshock was on a left-lateral strike-slip fault. Aftershocks were located in a depth range of 1-16 km and laterally extend for about 25 km in a NW-SE direction. We found that the aftershocks fell into four groups. This might be due to the heterogeneous structure in the source region. In the group that includes the mainshock, we estimated two fault planes bordering on the depth of the mainshock. There are 10-degree differences in both strike and dip angles between the lower and upper planes. From the aftershock distribution and the focal mechanisms, the rupture first propagated downward, and then propagated upward.
Key words: The 2005 West Off Fukuoka Prefecture Earthquake, aftershock distribution, ocean bottom seismometer (OBS), temporary telemetered and locally recorded observations.


Corresponding author E-mail: uehira@sevo.kyushu-u.ac.jp


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