Earth Planets Space, Vol. 57 (No. 6), pp. 551-556, 2005LETTER
Aitaro Kato, Shinichi Sakai, Naoshi Hirata, Eiji Kurashimo, Satoru Nagai, Takashi Iidaka, Toshihiro Igarashi, Yoshiko Yamanaka, Satoko Murotani, Tomonori Kawamura, Takaya Iwasaki, Toshihiko Kanazawa
Earthquake Research Institute, University of Tokyo, Tokyo 113-0032, Japan
(Received February 16, 2005; Revised May 13, 2005; Accepted May 27, 2005)
We deployed 56 temporary seismic stations within approximately a month after the occurrence of the 2004 mid-Niigata prefecture earthquake. Using manually-picked arrival data obtained from the temporary and surrounding permanent seismic stations, 1056 aftershocks have been relocated. Based on the spatiotemporal variations in the relocated aftershocks, the cluster activities associated with the mainshock and some large aftershock events are identified. The aftershocks associated with the mainshock, the largest occurred on the two steep west-dipping planes at an angle of 60° and approximately 5 km away. In contrast, the aftershocks following the event on Oct. 27 are aligned on east-dipping plane at a low angle of 25°. It is further observed that the aftershock area extended in both northeastward and southwestward directions at a later stage. The triggered seismicity around the northeast edge was more significant than that around the southwest edge. This difference could be understood by the discrepancy in the shear stress level accumulated at the dynamic shear rupture due to the mainshock.
Key words: Mid-Niigata prefecture earthquake, aftershocks, clusters.