Earth Planets Space, Vol. 60 (No. 11), pp. 1121-1126, 2008LETTER
Masanao Shinohara1, Toshihiko Kanazawa1, Tomoaki Yamada1, Kazuo Nakahigashi1, Shin'ichi Sakai1, Ryota Hino2, Yoshio Murai3, Akira Yamazaki4, Koichiro Obana5, Yoshihiro Ito2, Kazuhiro Iwakiri4, Ryo Miura3, Yuya Machida3, Kimihiro Mochizuki1, Kenji Uehira6, Michitaka Tahara6, Asako Kuwano1, Shin'ichiro Amamiya3, Shuichi Kodaira5, Tetsuo Takanami3, Yoshiyuki Kaneda5, and Takaya Iwasaki1
1Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
2Graduate School of Science, Tohoku University, Sendai 980-8578, Japan
3Institute of Seismology and Volcanology, Hokkaido University, Sapporo 060-0810, Japan
4Meteorological Research Institute, Japan Meteorological Agency, Tsukuba 305-0052, Japan
5Japan Agency for Marine-Earth Science and Technology, Yokosuka 237-0061, Japan
6Institute of Seismology and Volcanology, Kyushu University, Shimabara 855-0843, Japan
(Received December 6, 2007; Revised March 25, 2008; Accepted March 28, 2008; Online published November 18, 2008)
The Chuetsu-Oki Earthquake occurred on July 16, 2007. To understand the mechanism of earthquake generation, it is important to obtain a detailed seismic activity. Since the source region of the 2007 Chuetsu-oki Earthquake lies mainly offshore of Chuetsu region, a central part of Niigata Prefecture, it is difficult to estimate the geometry of faults using only the land seismic network data. A precise aftershock distribution is essential to determine the fault geometry of the mainshock. To obtain the detailed aftershock distribution of the 2007 Chuetsu-oki Earthquake, 32 Ocean Bottom Seismometers (OBSs) were deployed from July 25 to August 28 in and around the source region of the mainshock. In addition, a seismic survey using airguns and OBSs was carried out during the observation to obtain a seismic velocity structure below the observation area for precise hypocenter determination. Seven hundred and four aftershocks were recorded with high spatial resolution during the observation period using OBSs, temporally installed land seismic stations, and telemetered seismic land stations and were located using the double-difference method. Most of the aftershocks occurred in a depth range of 6-15 km, which corresponds to the 6-km/s layer. From the depth distribution of the hypocenters, the aftershocks occurred along a plane dipping to the southeast in the whole aftershock region. The dip angle of this plane is approximately 40°. This single plane with a dip to the southeast is considered to represent the fault plane of the mainshock. The regions where few aftershocks occurred are related to the asperities where large slip is estimated from the data of the mainshock. The OBS observation is indispensable to determine the precise depths of events which occur in offshore regions even close to a coast.
Key words: 2007 Chuetsu-oki Earthquake, aftershock distribution, fault geometry, ocean bottom seismometer (OBS).