HR: 0800h
AN: S31A-0197    [Abstracts]
TI: Crustal heterogeneities on electrical resistivity in the source regions of the 2004 Niigata earthquake and the 2007 Noto earthquake
AU: * Uyeshima, M
EM: uyeshima@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, the University of Tokyo, 1-1-1, Yayoi, Bunkyo, Tokyo, 113- 0032, Japan
AU: Yoshimura, R
AF: Disaster Prevention Research Institute, Kyoto University, Gokasyo, Uji, Kyoto, 611-0011, Japan
AU: Ogawa, Y
AF: Volcanic Fluid Research Center, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro, Tokyo, 152-8551, Japan
AU: Oshiman, N
AF: Disaster Prevention Research Institute, Kyoto University, Gokasyo, Uji, Kyoto, 611-0011, Japan
AU: Siripunvaraporn, W
AF: Department of Physics, Faculty of Science, Mahidol University, 272 Rama VI Road, Bangkok, 10400, Thailand
AU: Earthquake Source Regions, R o
AB: The 2004 Niigata Earthquake of Mj6.8 occurred on 23 Oct. 2004 in the backarc area of the central main Japan island. Focal mechanism of this earthquake is the reverse fault type with a strike of approximately N35E and a dip of approximately 60 degree. In long-term velocity field revealed by an analysis of Japanese nationwide GPS array (GEONET) (Heki and Miyazaki, 2001), North Eastern Japan(NEJ) and South Western Japan(SWJ) respectively acts almost rigidly and large strain rate is restricted in rather narrow zones. Niigata-Kobe Tectonic Zone (NKTZ: e.g. Sagiya et al., 2000) forms a motional boundary between NEJ and SWJ running along the back-arc side of the central Japan, where the present earthquake focal area is located. Across its source region, wideband magnetotelluric (MT) survey was performed in Nov. and Dec., 2004 in southern and northern part of the focal area, respectively. Since phase tensor analysis indicates that 3-D features are dominant in the longer periods for both of the profiles, we tried to determine a 3-D structure by inverting full-tensor data for both profiles with the aid of a DASOCC 3-D code. Results from the 3-D interpretation are as follows: (1) the main shock together with aftershocks occurred in relatively resistive portion beneath the surface conductive thick sedimentary layer. (2) The north profile is found to be located on EW trended ridge structure. (3) Lower crustal moderately high conductivity area exists beneath eastern side of main- and aftershock zones. 3 years after the Niigata Earthquake, a damaging earthquake (Mj6.9) occurred on 25 March 2007 near the west coast of the Noto Peninsula which is located in the north of the NKTZ. We also performed wideband MT survey for the onshore area of the source region immediately after the mainshock. We first constructed 2-D resistivity models along five profiles using only the TM mode responses. Significant characteristics of the resistivity models are: (1) Beneath the mainshock hypocenter, there is a conductive body which spreads to the eastern edge of the active aftershock region. (2) A resistive zone is located in the gap of the aftershock distribution between the mainshock hypocenter and the largest eastern aftershock. (3) One of the largest aftershock occurred at the boundary of the resistive zone described above. These results suggest that the deep conductors represent fluid- filled zones and the lateral heterogeneity could have controlled the slip distribution on the fault plane. In addition to these 2-D results, we will present a preliminary 3-D interpretation.
DE: 1515 Geomagnetic induction
DE: 5109 Magnetic and electrical properties (0925)
DE: 7215 Earthquake source observations (1240)
DE: 8012 High strain deformation zones
DE: 8015 Local crustal structure
SC: Seismology [S]
MN: 2007 Fall Meeting