A FAULT OUTCROP AT THE ONOSAWA PASS AND RECENT DISPLACEMENTS ALONE THE ATERA FAULT CENTRAL JAPAN

Abstract

The Atera fault, a major active fault in central Japan, extends 80 km from NW to SE and has left-lateral displacement with a relative uplift of the NE block. Evidences of Late Quaternary displacements along a main fault of this fault zone were clearly observed on a road-cutting at the Onosawa Pass, 6 km NW of Sakashita Town, Gifu Prefecture. The observations of the fault outcrop and C14 datings of black soils or buried woods reveal the followings: 1). Several fault planes run parallel to each other within the shatter zone of a few meters in width, which are N20-30° Win strike and 80-85° Emn dip. 2). The most recent faulting is recognized along the fault plane which lies on the most southwestern margin of the shattered zone. The latest displacement occurred after the deposition of Holocene soils dating back to 4, 330 years B P. 3). The Early to Late Quaternary deposits including the deposits of 27, 330 years B. P. are caught in the shattered zone, all these depo-sits being inclined nearly vertically. 4) An alternative accumulation of sand-gravel beds and black soil layers on the downthrown side suggests that the Atera fault has moved every 2, 000-4, 000 years.