Earth Planets Space, Vol. 56 (No. 12), pp. 1369-1374, 2004
Naoko Kato1, Hiroshi Sato1, Toshifumi Imaizumi2, Yasutaka Ikeda3, Shinsuke Okada3, Kyoko Kagohara4, Taku Kawanaka5, and Keiji Kasahara6
1Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-0032, Japan
2Department of GeoEnvironmental Science, Tohoku University, Sendai 980-8578, Japan
3Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan
4Graduate School of Medical and Engineering Science, Department of Education, Yamanashi University, Yamanashi 400-8510, Japan
5JGI Inc., 1-5-21 Otsuka, Tokyo 112-0012, Japan
6National Research Institute for Earth Science and Disaster Prevention, Tsukuba 305-0006, Japan
(Received May 27, 2004; Revised August 15, 2004; Accepted September 1, 2004)
The Northern Miyagi earthquake (Mj 6.4) on 26 July, 2003, was a shallow crustal earthquake produced by high-angle reverse faulting. To construct a realistic geologic model for this fault system from depth to the surface, seismic reflection profiling was carried out across the northern part of the source fault of this earthquake. The common mid-point seismic reflection data were acquired using a vibrator truck along a 12 km-long seismic line. The obtained seismic profile portrays a Miocene half-graben bounded by a west-dipping fault. Consistent with gravity anomaly data, the maximum thickness of the basin fill probably reaches 3 km. From the regional geology, this basin-bounding normal fault forms the eastern edge of the northern Honshu rift system and was produced by rapid extension during 17-15 Ma. The deeper extension of the fault revealed by seismic profiling coincides with the planar distribution of aftershocks. The hypocentral distribution of the aftershocks shows a concentration on a plane dipping 55 degrees to the west with listric geometry. Thus, the basin inversion has been performed using the same fault; the 2003 Northern Miyagi earthquake was generated by fault reactivation of a Miocene normal fault.
Key words: Seismic reflection profiling, 2003 Northern Miyagi earthquake, basin inversion, Miocene back-arc rift, fault reactivation, NE Japan.