Fault plane Determination for the Niigataken Chuetsu-oki Earthquake Using Tsunami Simulations

On July 16th 2007, the Niigataken chuetsu-oki earthquake occurred along the Japan Sea coast of central Japan. This strike of the fault was estimated to be approximately northeast-southwest but from the preliminary aftershock distribution and crustal deformations, it was difficult to determine if the fault dipped toward the northwest or toward the southeast. A tsunami occurred due to the sea bottom deformation caused by this earthquake. This tsunami was not so large, so it did not cause significant damage along the Japan Sea coast, however, it was clearly recorded by a number of tide-gauge stations along the Japan Sea coast. In this study, we use the focal mechanisms from F-net and estimate the location of the fault plane from arrival times of the tsunami waves at Kashiwazaki station in the southern direction and Sado station in the northern direction. We calculated the sea floor deformation assuming both northwest and southeast dipping fault planes, and then calculated the resultant tsunami waveforms for tide-gauge stations that clearly recorded the tsunami. Using comparisons of the model and observed tsunami waveforms at 6 stations, we try to estimate whether the major slip was on a westward or eastward dipping fault plane. The results show that a fault that dips to the south-east fits both the arrival times and the shape of the tsunami waveforms, better than for a fault that dips toward the northwest.