Surface deformations associated with the October 2004 Mid-Niigata earthquake: Description and discussion

Abstract

AbstractThe Mid-Niigata earthquakes (mainshock: JMA M6.8, the largest aftershock: JMA M6.4) occurred on 23 October 2004 in the Shinano River fold and thrust zone, western margin of Northeast Japan. Two major Quaternary reverse fault systems, fault related to Higashiyama—Tamguiyama fold in western side and Muikamachi fault in eastern side, had already been mapped in aftershock area from the geological structure and tectonic morphostructure. These active fault and fold strike almost north-northeast—south-southwest. We investigated destructions of man-made structures and surface deformation using the releveling data by Geography Survey Institute. In turn, we examined about the origin of observed surface destructions from the compare with aftershock distribution. Our leveling and G.S.I leveling results indicate the deformation concordant with the Higashiyama-Tamugiyama fold and the offset along the a part of the northern half of Muikamachi fault. The vertical displacements are estimated to be ca. 70 cm and less than 20 cm, respectively. The surface destructions were identified almost only along the Suwatoge flexure (correlated to the eastern limb of the Higashiyama-Tamugiyama fold), Tagawa anticline, Obiro fault and Muikamachi fault. The shortening amounts were estimated to be ca. 16 cm at the Suwatoge flexure, less than 2 cm at the Tagawa anticline and ca. 8 cm at the Muikamachi fault from the compressional destruction of man-made structure. The maximum vertical displacement and shortening was appeared at the bottom of Suwatoge flexure. The Suwatoge flexure is located onto the shallower extension of the row of aftershocks which was correlated to main shock earthquake. The Muikamachi fault is located near the shallower extension of the source fault of the largest aftershock. The stress regime inferred from the character of the observed surface destructions is concordant with the trend of geological structure and focal mechanism solutions. The small compressional destructions of man-made structures are byproducts created by buried faulting and folding. Thus, the Higashiyama-Tamugiyama folding structure grew in association with the buried faulting in the 2004 earthquake.