Timing and displacement of the most recent faulting on the active reverse fault zone along the western margin of the Kitakami Lowland

概要

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The Kitakami Lowland, running parallel to the volcanic front of Northeast Japan, is typical tectonic depression delineated by active faults. It has been difficult to evaluate the long-term seismic risk on this fault zone, because timing and displacement of the most recent faulting has been poorly constrained. The active faults are located east of and parallel to the eastward thrusting of the Neogene system onto Quaternary sedimentary fill. The most east flexure scarps (FAF3) on alluvial fan are 1-2m high, based on the topographic profiles across scarps. In order to obtain data on paleoseismic activity of this fault zone, we have carried out Geoslicer survey and dug pits on the faulting alluvial fan along topographic profiles. The strata exposed on the Geoslicer sample and pit wall are alluvial fan deposits contained within some lenses of humus. Successive humus named Hu I dated at about 6000 years BP is parallel to the warping ground surface. The age from the most upper humus is about 2600 year BP. This age constrains the timing of most recent faulting event after 2600 year BP with 1-2m vertical displacement. It also suggests that no events occurred between 6000 and 2600 year BP along the FAF3.