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Imaging the heterogeneous source area of the 2003 M6.4 northern Miyagi earthquake, NE Japan, by double-difference tomography |
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| Authors: | Tomomi Okada Akira Hasegawa Jun'ichi Suganomata Norihito Umino Haijiang Zhang Clifford H Thurber |
| Institution: | aResearch Center for Prediction of Earthquakes and Volcanic Eruptions, Graduate School of Science, Tohoku University, Sendai 980-8578, Japan bDepartment of Geology and Geophysics, University of Wisconsin–Madison, Madison, WI, USA |
| Abstract: | A shallow M6.4 inland earthquake occurred on 26 July 2003 in the northern part of Miyagi Prefecture, northeastern Japan. This earthquake was a typical inland thrust earthquake, a type that is common in NE Japan. We obtained a detailed seismic velocity structure in the focal area of this earthquake by the double-difference tomography method. Arrival-time data came from temporary seismic stations deployed above the mainshock fault plane. Both the P-wave and S-wave velocities in the hanging wall were lower than those in the footwall. Aftershocks were aligned along a zone where the seismic velocity changes rapidly. This is consistent with the interpretation 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 fault under the present compressional stress regime. The large slip area by the main shock rupture (asperity) corresponds to an area with relatively high P- and S-wave velocities. A zone with low Vp/Vs was detected along the aftershock area. One of the possible causes of this low-Vp/Vs zone is the existence of high-aspect-ratio pores that contain water. Hypocenters of the main shock, largest foreshock, and largest aftershock are also located within the low-Vp/Vs zone. |
| Keywords: | Fault Co-seismic slip distribution Aftershock Asperity Tomography The 2003 northern Miyagi earthquake |
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