Interpretation of Aftershock Distribution and Geological Cross Section
Extending the fault plane downward to the northwest at about 40 degrees from the main shock, it connects to the base of the enhanced Bouguer gravity anomaly zone that is distributed in a NNE direction in the area from Izumozaki through Miyagawa. According to the Japan National Oil Corporation (1999), it is estimated that there is a low-angle thrust fault with a detachment in the lower part of thick sedimentary layers, on the lower extension of the Torigoe fault, (Figure 5, "Long-term Evaluation of the Nagaoka-Heiya-Seien Fault Zone", Headquarters for Earthquake Research Promotion, 2004). In this case, extension of the Torigoe fault would cross the extension of the shallow part of the earthquake fault, as determined from the aftershock distribution. Therefore, the relationship between the earthquake fault and the Nagaoka-Heiya-Seien Fault Zone should be investigated, and at present a relationship cannot be dismissed.
Earthquake Research Institute, University of Tokyo
[The Niigataken Chuetsu-oki Earthquake in 2007 (August 8, 2007)]