Imaging of the hypocentral faults of the Niigata-Chuetsu earthquake sequence was performed by double-difference tomography using data obtained by a temporary aftershock observation network densely deployed in the hypocentral region. Two parallel "unfavorly oriented" fault planes steeply inclined to the WNW are revealed, that of the main shock and that of the largest aftershock, as well as a zone of velocity change between the high-velocity footwall and the low-velocity hanging wall for both P- and S-waves. This suggests that the main shock and the largest aftershock are caused by the reactivation of normal faults in the Miocene under the current compressional stress regime. Parts of the fault planes are also found to be located within the low-velocity zone. This low-velocity zone continues to the lower crust, which is imaged by an inversion from the regional data, and would correspond with highly fractured zone around the fault plane with high fluid pressure, which reactivated the "unfavorly oriented" fault and triggered the present earthquake. Large coseismic slip area appears to extend to the northeastern portion with relatively high velocity. This suggests that the asperity (large coseismic slip area) of the main shock possibly correspond with a higher velocity region along the fault plane.
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