Earth Planets Space, Vol. 57 (No. 6), pp. 545-549, 2005LETTER
Takuo Shibutani1, Yoshihisa Iio1, Satoshi Matsumoto3, Hiroshi Katao1, Takeshi Matsushima3, Shiro Ohmi1, Fumiaki Takeuchi1, Kenji Uehira3, Kin'ya Nishigami1, Bogdan Enescu1, Issei Hirose1, Yasuyuki Kano2, Yuhki Kohno4, Masahiro Korenaga4, Yutaka Mamada1, Masatoshi Miyazawa1, Ken'ichi Tatsumi1, Tomotake Ueno2, Hiroo Wada1, and Yohei Yukutake 2
1Disaster Prevention Research Institute, Kyoto University, Uji 611-0011, Japan
2Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
3Institute of Seismology and Volcanology, Faculty of Sciences, Kyushu University, Shimabara 855-0843, Japan
4Graduate School of Sciences, Kyushu University, Fukuoka 812-8581, Japan
(Received February 16, 2005; Revised May 6, 2005; Accepted May 15, 2005)
The 2004 Mid Niigata Prefecture Earthquake (Mj = 6.8) occurred on 23 October 2004 in the northeastern part of the Niigata-Kobe Tectonic Zone where large contraction rates were observed. The mainshock was followed by an anomalously intense aftershock activity that included nine Mj 5.5 aftershocks. We deployed three temporary online seismic stations in the aftershock area from 27 October, combined data from the temporary stations with those from permanent stations located around the aftershock area, and determined the hypocenters of the mainshock and aftershocks with a joint hypocenter determination (JHD) technique. The resulting aftershock distribution showed that major events such as the mainshock, the largest aftershock (Mj = 6.5), the aftershock on 27 October (Mj = 6.1), etc. occurred on different fault planes that were located nearly parallel or perpendicular to each other. This might be due to heterogeneous structure in the source region. The strain energy was considered to have been enough accumulated on the individual fault planes. These features are probably a cause of the anomalous intensity of the aftershock activity.
Key words: The 2004 Mid Niigata Prefecture Earthquake, aftershock distribution, complexity of earthquake faults, temporary online aftershock observations.