Multi-planar structures in the aftershock distribution of the Mid Niigata prefecture Earthquake in 2004

A shallow destructive inland earthquake of M6.8 occurred on Oct. 23, 2004 (JST) in the middle part of Niigata prefecture, Japan. The aftershock activity was high as compared to those of the other shallow inland earthquakes in Japan. We relocated the sources of the main shock and aftershock activity by adopting the double-difference earthquake location algorithm. The precisely relocated hypocenter distribution shows that there are a doubleplanar structure with a distance 5 km in parallel dipping in WNW with about 50 degrees and a single-planar structure dipping in ESE with about 15 degrees. The upper and lower planes of the double plane began to be formed with the main shock and the largest aftershock of M6.5, respectively, while the ESE dipping plane began to be formed with the 3rd largest aftershock of M6.1. The three planes represent the fault planes of the large events mentioned above. The aftershock distribution is found to be a superposition of the fault planes of the main shock and the large aftershocks. The high aftershock activity is attributed to the formation of the fault planes.