Earth Planets Space, Vol. 60 (No. 8), pp. 883-888, 2008LETTER
Hiroshi Ichihara1, Ryo Honda1, Toru Mogi1, Hideaki Hase1, Hiroyuki Kamiyama2, Yusuke Yamaya1, and Yasuo Ogawa3
1Institute of Seismology and Volcanology, Hokkaido University, N10W8 Kita-ku, Sapporo, Japan
2Ueyama-Shisui Co. Ltd., 1-7, N2E13, Higashi-ku, Sapporo, Japan
3Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan
(Received March 2, 2007; Revised April 10, 2008; Accepted May 14, 2008; Online published September 8, 2008)
The Rumoi-Nanbu earthquake (M 6.1) occurred in northern Hokkaido, Japan, on December 14, 2004. We conducted MT surveys along three profiles in and around the focal area to delineate and decipher the structural features of the seismogenic zone. The inverted 2-D resistivity images of the three sections comprised two layers: an upper conductive layer and a lower resistive layer. The boundary of these layers lay at a depth of approximately 3-5 km. A comparison with the surface geology and drilling data revealed that the upper conductive layer and the lower resistive layer corresponded to the Cretaceous-Tertiary sedimentary rocks and older basement rocks, respectively. A clear upheaval of the layer boundary was found along the profile at the center of the focal area. In addition, borehole data indicated an obvious increase in the Young's modulus toward the lower layer. Therefore, the elastic properties with a complex geometry around the focal zone tended to vary; this probably depicts the zone of stress accumulation that triggered the earthquake.
Key words: Magnetotellurics, stress concentration, intraplate earthquake, northern Hokkaido.