Excavation Study of the Kitatake Active Fault at Nagasawa, Yokosuka City, Kanagawa Prefecture, Japan

Abstract

The Kitatake fault is the major fault of Kitatake fault zone, WNW-ESE striking across the Miura Peninsula, south of Tokyo. The Kitatake fault zone is almost parallel to Kinugasa fault zone (north) and to Takeyama fault zone (south). All of these fault zones are active and show right-lateral slips inferred from geomorphic features. The eastern half of the Kitatake fault delineates the boundary between Zushi Formation (Late Miocene) in the north and Hayama Group (EarlyMiddle Miocene) in the south and shows south-facing scarps probably owing to the resistant Zushi Formation.
We carried out observations in 1994 and dating in 1995 with three trenches, one being 9 meters deep, on the unconsolidated Holocene sediments along the Kitatake fault. We obtained 36 dates from 33 samples.
The latest episodic slip along the fault seems to have ocurred at about 1, 440 yBP (14C), that is A.D. 600650. The ratio of the horizontal component with the vertical one may be about 2.04.2. The amount of the latest episodic slip is not certain, but might be in the order of meters.
Another episodic slip prior to the latest one probably occurred at least before 3, 070 yBP ( ¹⁴C).