抄録
Recent activity of the Chojahara fault of the Shinjo basin fault zone in Northeast Japan, is examined base on interpretation of aerial photographs, field survey, trenching survey and arrayed borings.
At the Horiuchi site near the center of the basin, humus layers in the middle and upper parts of the deposits forming the L4 terrace were dated to 6.8-7.0 ka and 1-1.5 ka respectively by 14C dating. Upper part of the terrace deposits includes the Towada-a tephra (AD915). From these ages, the L4 terrace surface was formed nearly 1 ka. The vertical offset of the surface is about 1.6 m. The middle part of the L4 terrace deposits which was deformed (dragged) by reverse faulting, was overlain by the upper part of the deposits.
At the Chojahara site, the L3 terrace surface was dated to 4.5-5.2 ka from 14C ages of the soil. Vertical offset of the L3 terrace surface is about 2.0 m. The latest event of the Chojahara fault is younger than 1 ka, because the fault dislocates the L4 terrace surface. The penultimate event was 4.5-5.2 ka (the age of L3 terrace surface) or older, and it was probably younger than 6.8-7.0 ka (age of the middle part of the L4 terrace deposits). The slip per event is estimated around 1.6-2.0 m from the vertical offsets of the L4 and L3 terrace surfaces.
