Late Holocene faulting of the active faults along the western margin of the Nagano Basin based on the drilling survey at Nakano City, Nagano Prefecture, central Japan

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The Nagano Basin is bounded by northsouth-trending reverse faults on the west. The active faults along the eastern margin of the Nagaoka Hills in the northern part of the Nagano Basin displace alluvial surfaces of Holocene age. The average slip rate and history of late Holocene faulting of these faults have not been well constrained by previous studies. We have obtained subsurface geologic information by core sampler across monoclinal scarp 1.9 m high on a Holocene alluvial surface in Nakano City, Nagano Prefecture. The core samples obtained at 11 sites are 1.1 to 6.3 m long. Correlation of strata and radiocarbon dating show that a peat layer dated at about 4200 years BP is warped. The vertical displacement of the top of the peat layer is measured to be at least 5.1 m. From the age and amount of vertical displacement, the vertical slip rate is estimated to be at least 1.2 mm/yr. We found that surface faulting had occurred at least twice since about 4200 years ago. Moreover, we found the monoclinal scarp along the fault at Angenji, Nakano City had grown up by the 1847Zenkoji earthquake.