「1978年伊豆大島近海地震」の際現れた地震断層
Beneath the sea floor between the Izu Peninsula and Izu-Oshima Island in Japan, an earthquake with a magnitude of 7.0 ("Izu-Oshima-Kinkai Earthquake") occurred at 12:24, January 14, 1978. In the Izu Peninsula, Shizuoka Prefecture, the earthquake caused 27 deaths, 197 woundeds and severe damage on houses, water pipes, roads, railways, etc. Surface tectonic fractures formed at the Inatori section of Higashi-Izu Machi on the east coast of the Izu Peninsula. The authors mapped surface fractures and measured the movement of faults at as many points as possible. Most of surface tectonic fractures were observed discontinuously in a 4 km long zone of north-west trend ("Inatori-Omineyama earthquake fault'') which extend from the Inatori coast through the western part of the town to the east foot of Mt. Omineyama. Small en echelon fracturcs (3-5 m long) forms fissure zones which were some 100 m long and several meters wide. The Inatori-Omineyama earthquake fault is formed by an en echelon arrangement of 11 fissure zones. Thus, surface tectonic fractures show double en echelon arrangements. The orientation of en echelon fractures, the separation of recognizable points on the fracture walls, and the mounds consistently indicate right-lateral strike slip with slight upthrow of the south-west side. The precise amount of slip along fractures were measured based on offset stone walls and other man-made structures. The measured amounts of slip shows the maximum on the central part of each fissure zone and less on the both ends. The maximum measured displacement is the right-lateral slip of 1.2 m and 40 cm upthrow of the south west-side at the foot of Mt.Omineyama. The maximum measured slip on each fissure zone shows a tendency to decrease south-eastward toward the Inatori coast, although the epicenter located farther eastward off the coast.
