A nationwide GPS array with more than 1,000 permanent stations operated by the Geographical Survey Institute of Japan, has provided many invaluable deformation data such as co-and post-seismic, volcanic, and ongoing secular deformations in the Japanese islands. Based on daily coordinate data of the GEONET stations together with results from a regional network operated by Tohoku University, we derived a map of the strain rate distribution in NE Japan showing that there exists a notable strain concentration zone of EW contraction along the Volcanic Front. The area demonstrates active seismicity including some disastrous earthquakes. Recent seismic tomography studies have revealed the existence of inclined seismic low-velocity zones (LVZ) at depths shallower than ∼150 km in the mantle wedge sub-parallel to the subducted slab. The inclined LVZ reaches the Moho right beneath the Volcanic Front, indicating that the formation of the strain concentration zone is closely related to the rheological structure of the island-arc system.

A destructive large earthquake (the 2004 mid Niigata prefecture earthquake) sequence occurred in the central part (Chuetsu district) of Niigata prefecture, central Japan on October 23, 2004. We have deployed a temporary seismic network composed of 54 stations for aftershock observation just above and around the focal area of the earthquake for about a month. Using travel time data from the temporary seismic network and surrounding routine stations, we obtained precise aftershock distribution and 3D seismic velocity structure in and around the fault planes of the earthquake and four major (M ≥ 6) aftershocks by double-difference tomography. The results clearly show three major aftershock alignments. Two of them are almost parallel and dipping toward the WNW. The shallow and deep aftershock alignments correspond to the fault plane of the mainshock and that of the largest aftershock (M6.4), respectively. The third alignment is almost perpendicular to the WNW-ward dipping planes and perhaps corresponds to the fault plane of the M6 aftershock on October 27. General feature of the obtained velocity structure is that the hanging wall (western part of the focal area) has lower velocity and the footwall (eastern part of the focal area) has higher velocity. Major velocity boundary seems to shift westward in comparison to in northern and southern parts at a location near the central part of the focal area, where the main shock rupture started. Some parts of the fault planes were imaged as low velocity zones. This complex crustal structure would be one of possible causes of the multi-fault rupture of the 2004 mid Niigata prefecture earthquake sequence.

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