2003 年 56 巻 2 号 p. 199-211
The 1918 Omachi Earthquake occurred near Omachi city, along the northern Itoigawa-Shizuoka Tectonic Line Fault Zone. Leveling data associated with the earthquake were re-examined and corrected for artificial alteration. Corrected leveling data revealed about 200mm of uplift in the central Omachi and a slight subsidence in the southern part. However, the leveling data are not enough to resolve a fault model of the Omachi earthquake by themselves. Therefore a new fault model is developed based on the leveling, triangulation, and structural data. Estimated fault plane is 4.4km×10.0km wide, shallowly inclined to ENE at the base of the Matsumoto Basin. Estimated fault slip amounts to 1.2m and the seismic moment is estimated to be 1.8×1018Nm (Mw=6.1). The fault model is consistent with the observed surface rupture and horizontal crustal strain, which have been hardly explained by previous models. The 1918 Omachi earthquake is considered to have released WNW-ESE compressional stress by rupturing a shallow part of the crust. Although the source fault is discriminated from the East Matsumoto Basin Fault, these faults are interpreted to be the members of the same fault system. An integrated understanding of the whole fault system as well as inelastic deformation in the surrounding area is necessary to refine estimates on earthquake probabilities and strong ground motions.