Imaging the heterogeneous source area of the 2003 M6.4 northern Miyagi earthquake, NE Japan, by double-difference tomography

• Published in: Tectonophysics 2007-02, Vol.430 (1), p.67-81
• Main Authors: Okada, Tomomi, Hasegawa, Akira, Suganomata, Jun'ichi, Umino, Norihito, Zhang, Haijiang, Thurber, Clifford H.
• Format: Article
• Language: English
• Subjects: Aftershock; Asperity; Co-seismic slip distribution; Fault; The 2003 northern Miyagi earthquake; Tomography
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2006.11.001

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 V p/ V s was detected along the aftershock area. One of the possible causes of this low- V p/ V s 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- V p/ V s zone.