糸魚川－静岡構造線活断層系北部大町地域の浅層反射法地震探査

The Itoigawa-Shizuoka tectonic line (ISTL) active fault system poses the highest seismic risk, and shows one of the largest slip rates among active faults on Honshu Island. In the Omachi area, the ISTL active fault system comprises the eastern Matsumoto basin (EMB) fault to the west and the Otari-Nakayama fault to the east. To understand the structural relation between these two faults, we conducted a seismic reflection survey across them. The length of the seismic line is 6.4km, and the receiver and shot point intervals are 10m. We used a mini-vibrator as a seismic source with a 180-channel seismic recording system. We can image the structure to less than about 1km in depth. The seismic depth section displays the EMB fault as a boundary between east-dipping reflectors to the east and horizontal reflectors in the Masumoto basin. The EMB fault is interpreted as an east vergent, emergent thrust, and its deeper extension merges with the Otari-Nakayama fault. The Omine Formation, which was laid down on an alluvial fan in the late Pliocene and the early Pleistocene, is distributed between EMB and Otari-Nakayama fault. It is evident that the thrust front migrated from the Otari-Nakaya fault to the EMB fault in Quaternary, and then the Omine Formation located at the foreland basin was uplifted

Seismic Reflection Profiling Across the North of the Itoigawa-Shizuoka Tectonic Line in Omachi, Central Japan

The Itoigawa-Shizuoka tectonic line (ISTL) active fault system poses the highest seismic risk, and shows one of the largest slip rates among active faults on Honshu Island. In the Omachi area, the ISTL active fault system comprises the eastern Matsumoto basin (EMB) fault to the west and the Otari-Nakayama fault to the east. To understand the structural relation between these two faults, we conducted a seismic reflection survey across them. The length of the seismic line is 6.4km, and the receiver and shot point intervals are 10m. We used a mini-vibrator as a seismic source with a 180-channel seismic recording system. We can image the structure to less than about 1km in depth. The seismic depth section displays the EMB fault as a boundary between east-dipping reflectors to the east and horizontal reflectors in the Masumoto basin. The EMB fault is interpreted as an east vergent, emergent thrust, and its deeper extension merges with the Otari-Nakayama fault. The Omine Formation, which was laid down on an alluvial fan in the late Pliocene and the early Pleistocene, is distributed between EMB and Otari-Nakayama fault. It is evident that the thrust front migrated from the Otari-Nakaya fault to the EMB fault in Quaternary, and then the Omine Formation located at the foreland basin was uplifted