Earth Planets Space, Vol. 60 (No. 2), pp. 83-88, 2008LETTER
Shin'ichi Sakai1, Aitaro Kato1, Takashi Iidaka1, Takaya Iwasaki1, Eiji Kurashimo1, Toshihiro Igarashi1, Naoshi Hirata1, Toshihiko Kanazawa1, and the group for the joint aftershock observation of the 2007 Noto Hanto Earthquake
1Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
(Received July 8, 2007; Revised December 6, 2007; Accepted December 8, 2007; Online published February 19, 2008)
The 2007 Noto Hanto Earthquake occurred on March 25, 2007, in the Noto Peninsula, central Japan. A half day after the main shock, we started installing temporary seismic stations in order to determine the precise locations of its aftershocks. Ten universities and two research institutes deployed 88 temporary seismic stations in and around the source area. The observation lasted for about 2 months. We relocated 1318 aftershocks with arrival time corrections at each station. The relocated hypocenters show relatively small errors—less than 0.2 km in the horizontal direction and less than 0.4 km depth. Most of the relocated hypocenters are about 2.0 km shallower than those determined by JMA. The distribution of the aftershocks forms a southeast-dipping plane. The main shock is located at the bottom part of their distribution. The precise aftershock distribution extends into a shallower area than the original, and it coincides with sea floor-ward extension of the active faults previously known from a sonic reflection survey. Heterogeneous distribution of the aftershocks on the fault plane shows low seismicity just above the main shock hypocenter, and middle-size aftershocks are distributed on the periphery of the main shock. A precursory event (M 4.4) that occurred 0.6 s before the main rupture is located close to the M 2.2 foreshock that occurred 12 min before it.
Key words: 2007 Noto Hanto Earthquake, urgent aftershock observation, dense seismic array, precise aftershock distribution.