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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.

Precise aftershock distribution of the 2007 Chuetsu-oki Earthquake obtained by using an ocean bottom seismometer network