Fault plane of the 1964 Niigata earthquake, Japan, derived from relocation of the mainshock and aftershocks by using the modified joint hypocenter determination and grid search methods

We relocated the mainshock and immediate aftershocks of the 1964 Niigata earthquake (Ms 7.5) using both the modified joint hypocenter determination (MJHD), and the grid search (GS), methods to obtain their hypocenters and identify the fault plane of the mainshock. We used P-wave arrival times at 13 Japanese stations reported by the Japan Meteorological Agency (JMA). Firstly, we performed the MJHD method to select readings with high accuracies and to obtain station corrections that related to the lateral heterogeneity of the Earth’s structure. Secondly, we performed the GS method by using only selected readings and assuming the station corrections. Based on the relocated hypocenters, we found that the mainshock and immediate aftershocks occurred along a plane striking N15°E and dipping 11° eastward. This result strongly suggests that one of the nodal planes of the mainshock, for which the strike and dip are N9°E and 34°, respectively, is preferable as the fault plane. When global stations are also used for relocations, the results confirm that the fault plane of the 1964 Niigata earthquake dips gently eastward. Therefore, we concluded that the fault plane of the 1964 Niigata earthquake is gently eastward-dipping.