Abstract

The Mid Niigata prefecture Earthquake occurred on October 23, 2004. Suzuki et al. (2004) and Maruyama et al. (2005a, 2005b) found small surface ruptures at Obirou and Aoshima areas. Suzuki et al. (2004) also argued that the earthquake was caused by movements of the Obirou Fault and the northern part of the Muikamachi-Bonchi-Seien fault. The aim of this report is to examine active geological structures and landforms in epicentral region of the earthquake. We carried out aerialphoto interpretation, geological survey, and geomorphometry by airborne laser scanning. As a result of our survey, the Suwatoge Flexure was found to be an active structure in the region, together with Obirou Fault and the Muikamachi-Bonchi-Seien fault. The Suwatoge Flexure is recognized as the fold on Uonuma formation (Plio-Pleistocene) and uplifts terraces in west/northwest direction; this is generally correspond with the earthquake source direction. The amount of vertical displacement in each terrace is as follows; more than 70m on Higher terrace (H2 surface), about 40m on Middle terrace (M1 surface) and about 10m on Lower terrace (L1 surface). The mean rate of vertical displacement is estimated 0.3m/103 year based on M1 Terrace surface covered by In-Kt Tephra [≥130, 000 year: Machida and Arai (2003)]. The Suwatoge Flexure is located at the eastern margin of the distributed area of aftershock epicenters, and its south end corresponds with that of the distribution of aftershock epicenters. The largest vertical movement of 71.5cm uplifts (GSI, 2004) as the co-seismic crustal deformation is located on the west side of the Suwatoge Flexure. Therefore, the formation of the Suwatoge Flexure were probably caused by activities of earthquake source faults.

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