Activities of the Hanawa higashi fault zone during the last 15 kys in Akita Prefecture, Northeast Japan

概要

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We intend to investigate the activities of the Hanawa higashi fault zone (eastern marginal faults of the Hanawa basin) at Northeast Japan. We obtained the displacement of the faults on terrace surfaces by the interpretations of aerial photographs, field survey and arrayed borings. Fluvial terrace surfaces are divided into higher (H), and lower (L1-L8) surfaces in descending order of ages. Ages of the surfaces are estimated from tephra stratigraphy and14C dating. The L2 surface is the depositional surface of Towada - hachinohe pyroclastic flow deposits which include 15 ka carbonized woods. The L6 and the L7 surfaces are estimated to be 10 ka and 4 ka respectively, from14C age of terrace deposits. Thus the L3 - L5 surfaces are estimated to be 10-15 ka. The L8 surface is younger than 4 ka. Two major faults, Fe 1 and Fe 2, comprise the Hanawa higashi fault zone. These faults dislocate the L2 surface (15 ka) and the L3 surface (10-15 ka). The Fe 2 does not show apparent dislocation on the L7 surface (4 ka). These show that the Fe 2 dislocated after 10-15 ka and the Fe 2 is estimated not to have dislocated since 4 ka. Vertical slip rate of the Fe 1 is calculated to be ca.0.1 m/ky from the offset of the L2 surface. Vertical slip rate of the Fe 2 is calculated to be 0.3-0.5 m/ky from the offsets of the L2 and the L3 surfaces.
Japanese Society for Active Fault Studiesの論文