Earth Planets Space, Vol. 58 (No. 12), pp. 1599-1604, 2006LETTER
Hiroshi Shimizu1, Hiroaki Takahashi2, Tomomi Okada3, Toshihiko Kanazawa4, Yoshihisa Iio5, Hiroki Miyamachi6, Takeshi Matsushima1, Masayoshi Ichiyanagi2, Naoki Uchida3, Takaya Iwasaki4, Hiroshi Katao5, Kazuhiko Goto6, Satoshi Matsumoto1, Naoshi Hirata4, Shigeru Nakao6, Kenji Uehira1, Masanao Shinohara4, Hiroshi Yakiwara6, Nobuki Kame7, Taku Urabe4, Norimichi Matsuwo1, Tomoaki Yamada4, Atsushi Watanabe1, Kazuo Nakahigashi4, Bogdan Enescu5,Kazunari Uchida1, Shin'ichi Hashimoto4, Syuichiro Hirano6, Takeo Yagi4, Yuhki Kohno1,Tomotake Ueno5, Masaki Saito1, and Mio Hori1
1Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara 855-0843, Japan
2Institute of Seismology and Volcanology, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
3Research Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
4Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
5Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan
6Nansei-Toko Observatory for Earthquakes and Volcanoes, Faculty of Science, Kagoshima University, Kagoshima 892-0871, Japan
7Faculty of Sciences, Kyushu University, Fukuoka 812-8581, Japan
(Received April 19, 2006; Revised September 19, 2006; Accepted September 22, 2006; Online published February 2, 2007)
On March 20, 2005, a large MJMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were installed on land around the aftershock area immediately followed the occurrence of the mainshock. Because aftershocks were located mainly in offshore regions, we also installed 11 ocean bottom seismometers (OBSs) just above the aftershock region and its vicinity in order to obtain accurate locations of hypocenters. The OBS observation was carried out from March 27 to April 13, 2005. We further conducted temporary GPS observations in which ten GPS receivers were deployed around the aftershock region. The aftershocks were mainly aligned along an approximately 25-km-long NW-SE trend, and the hypocenters of the main aftershock region were distributed on a nearly vertical plane at depths of 2-16 km. The mainshock was located near the central part of the main aftershock region at a depth of approximately 10 km. The largest aftershock of MJMA5.8 occurred near the southeastern edge of the main aftershock region, and the aftershock region subsequently extended about 5 km in the SE direction as defined by secondary aftershock activity. Enlargement of the aftershock region did not occur after the peak in aftershock activity, and the aftershock activity gradually declined. The distribution of hypocenters and seismogenic stress as defined by aftershocks suggest that the 2005 West Off Fukuoka Prefecture Earthquake occurred on the fault that is the NW extension of the Kego fault, which extends NW-SE through the Fukuoka metropolitan area, and that the largest aftershock occurred at the northwestern tip of the Kego fault.
Key words: The 2005 West Off Fukuoka Prefecture Earthquake, intraplate earthquake, mainshock, aftershock, seismic observation, hypocenter distribution, active fault, Kego fault.