Precise aftershock distribution of the 2004 Mid-Niigata prefecture earthquake-Implication for a very weak region in the lower crust

The 2004 Mid-Niigata prefecture Earthquake (Mjma 6.8) occurred in the region of large strain rates (>0.1 ppm/y contraction) in the intraplate region in Japan. The mainshock was followed by four major aftershocks with Mjma >= 6.0. The hypocenters of the mainshock and two large aftershocks that occurred in the central part of the aftershock region were located near the lower limit of the earthquake distribution, while hypocenters of the other two aftershocks near both ends, are located near its upper limit. Furthermore, the fault planes of the latter two aftershocks were confined within the upper half of the upper crust. Also, the lower limit of the aftershock distribution is deepest in the central part and becomes shallower toward the NNE and SSW ends. These data can be explained by the hypothesis that a localized stress concentration occurred near the bottom of the seismogenic region only in the central part. The stress concentration may be generated by the deformation in the very weak region of low strength in the lower crust beneath the central part of the aftershock region.