The source fault of the 1983 Nihonkai-Chubu earthquake revealed by seismic imaging

- We conducted seismic surveys in the hypocentral region of the 1983 earthquake.
- The crustal structure around the hypocentral region is divided into three types.
- The 1983 earthquake occurred near the boundary of the crustal structure.

From 2011 to 2012, we conducted marine seismic surveys using a multichannel seismic reflection (MCS) system and ocean bottom seismometers (OBSs) in and around the hypocentral region of the 1983 Nihonkai-Chubu earthquake. The survey covered the area from the Japan coast of the Japan Sea to the Yamato Basin and the Japan Basin. We investigated the relationship between the Nihonkai-Chubu earthquake and the crustal structure obtained from seismic reflection imaging and the P-wave velocity structure near the hypocentral region. The focus in the present paper is on seismic data that were acquired using both the MCS system and OBSs. Based on the results, the crustal structure of the Japan Sea around the hypocentral region of the 1983 Nihonkai-Chubu earthquake is divided into three types: island arc crust, thick oceanic crust, and oceanic crust. The Nihonkai-Chubu earthquake occurred near the boundary between the thick oceanic crust and the island arc crust. There is a strong possibility that the east-dipping reflector detected in the crust was the source fault of the Nihonkai-Chubu earthquake. In addition, the location of anticlines formed by the east-dipping reverse fault occurring in the boundary of the crustal structure associated with the source fault of the Nihonkai-Chubu earthquake was confirmed in the range of latitude from 39°20′ to 41°15′N, although they were not found south of this segment. There is also a concentrated area of reverse faults and folds approximately 100 km west of the source region of the Nihonkai-Chubu earthquake. This area is located near the boundary between the thick oceanic crust and the oceanic crust.