Excavation Study Across The Anchi Active Fault In The Eastern Marginal Area Of The Echigo Plain, Central Japan -Investigation Into The Nature Of A Short Subsidiary Fault Close To The Master Active Fault-

The eastern margin of Echigo Plain, central Japan, is characterized by NNE-SSW trending active faults. The Anchi fault is made up of two segments: the Anchi-nishi fault and the Anchihigashi fault. Each fault trace is approximately 2-km long. They may be short subsidiary faults on the hanging wall of the master fault in this region(the Tsukioka fault). Trench investigations across the Anchi fault reveal its rupture history as follows: 1) the Anchi-nishi fault and the Anchi-higashi fault are west-dipping reverse faults; 2) the Anchi-nishi fault shows a vertical offset of 1.3-1.9 m after ca. 30 ka; and, 3) the vertical displacement of 3.5 m is associated with two faulting events along the Anchi-higashi fault, one between ca. 30 and 60 ka and the other younger than ca. 30 ka. Geomorphic features indicate that a reverse fault branches into these two faults at a certain depth. Thus, the total vertical offset for the last ca. 30 ky is between 3.1 to 3.7 m. This value is one third of that along the master active fault. If the source fault consists of these active faults, we should evaluate the activity of the master fault taking account of the activity of the Anchi fault. It is very important to know the nature of subsidiary faults, even if they display very short traces.