Source process of the 2007 Niigata-ken Chuetsu-oki earthquake derived from near-fault strong motion data

The 2007 Niigata-ken Chuetsu-oki earthquake generated strong ground motions in Kashiwazaki and Kariwa, where the world largest nuclear power plant was in operation. Due to the complexity of the aftershock distribution, activation of the north west-dipping fault and/or the southeast-dipping fault is proposed. To explore the fault geometry and source process of the earthquake, we performed multi-time window linear waveform inversions for both the fault planes from near-fault strong motion data. A fault plane model of 30 km in length by 24 kill in width was set to cover the region of aftershock distribution within 24 h of the mainshock. Both inverted slip models provided moment magnitudes of 6.7 with a small asperity near the rupture starting point, and a large asperity approximately 10 km southwest of the rupture initiation, which is located in the region of relatively sparse aftershock distribution. Both the small and large asperities are located near the intersection between the two conjugate fault plane models, and the asperities of both models have similar radiation patterns. Therefore, the difference of the residuals between the observed and synthetic waveforms for both models was not significant, indicating that it is difficult to conclude which fault is the rupture.