HR: 14:35h
AN: S33D-04    [Abstracts]
TI: Deep structure of the strain concentration zone in northern Miyagi Prefecture, NE Japan
AU: * Okada, T
EM: okada@aob.geophys.tohoku.ac.jp
AF: Research Center for Research Center for Prediction of Earthquake and Volcanic Eruptions, Graduate School of Science, Tohoku University, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Nii, K
EM: nii@aob.geophys.tohoku.ac.jp
AF: Research Center for Research Center for Prediction of Earthquake and Volcanic Eruptions, Graduate School of Science, Tohoku University, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Hasegawa, A
EM: hasegawa@aob.geophys.tohoku.ac.jp
AF: Research Center for Research Center for Prediction of Earthquake and Volcanic Eruptions, Graduate School of Science, Tohoku University, 6-6 Aramaki-Aza-Aoba, Aoba-ku, Sendai, 9808578, Japan
AU: Zhang, H
EM: hjzhang@mit.edu
AF: Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139- 4307, United States
AU: Thurber, C H
EM: clifft@geology.wisc.edu
AF: Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, WI 53706, United States
AB: A moderate-size (M6.4) intraplate earthquake (the 2003 northern Miyagi earthquake) occurred on 26 July 2003 in the northern part of Miyagi prefecture, NE Japan. In this area, high seismic activity forms a belt in which another moderate-size earthquake (M6.2) occurred in 1962. From GPS analysis, large strain concentration was observed along this seismic belt (Sato et al., 2004; Miura et al., 2005; Hasegawa et al., 2005). We call this zone of large strain concentration and high seismic activity the northern Miyagi strain concentration zone. We applied the double-difference tomography method (Zhang and Thurber, 2003) to determine the three- dimensional velocity structure and relocate the hypocenters simultaneously. Travel time data is from the Japanese universities joint seismic observation in the Tohoku Backbone range (1997-1998) and the aftershock observation for the 2003 Northern Miyagi earthquake (Umino et al., 2004; Okada et al., 2004). We also used data from stations of Tohoku University, JMA, and Hi-net during the period from 1997 to 2003. Note that we have included events within the subducting Pacific slab in the tomography inversion to increase the resolution of the lower crust structure. In the focal area of the 2003 northern Miyagi earthquake, we obtain a steeply westward-dipping aftershock alignment, which shows the high dip angle of the reverse faulting (dip ~ 50 degree). We also obtain the low-velocity hanging-wall and the high-velocity footwall, consistent with the previous results of Okada et al. (Tectonophysics, 2007, doi:10.1016/j.tecto.2006.11.001). We infer that the 2003 northern Miyagi earthquake occurred along a fault that acted as a normal fault in the Miocene and has been reactivated as a reverse (unfavorably oriented) fault under the present compressional stress regime. The large slip area by the main shock rupture (asperity) corresponds to an area with relatively high velocity. We also obtain a distinct low-velocity area in the lower crust just beneath the focal area of the 2003 northern Miyagi earthquake. This low-velocity zone appears to continue downward to the upper mantle. The lateral extent of this low-velocity zone in the lower crust roughly corresponds to the spatial distribution of the northern Miyagi strain concentration zone. This low-velocity zone might correspond to the upwelling flow of fluid originating from the mantle wedge. This observation supports the hypothesis by Hasegawa et al. (2005) that anelastic deformation due to fluids forms the strain concentration zone. This observation also supports the hypothesis by Okada et al. (2007) and Sibson (1990) that high fluid pressure reactivates the unfavorably oriented fault of the 2003 northern Miyagi earthquake.
UR: http://www.aob.geophys.tohoku.ac.jp/~okada
DE: 7209 Earthquake dynamics (1242)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 8015 Local crustal structure
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
DE: 8180 Tomography (6982, 7270)
SC: Seismology [S]
MN: 2007 Fall Meeting