糸魚川‐静岡構造線活断層系・松本盆地東縁断層南部に沿う左横ずれ変位地形

Abstract

The East Matsumoto Basin faults (EMBF) consist of the northern segment of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL), central Japan, and extend for ca. 30 km along the eastern margin of the Matsumoto Basin. The faults have been considered to be eastdipping reverse faults, which were re-activated sometime after the tectonic inversion event during the middle Miocene in central Japan. However, we found geomorphic evidence of strikeslip movement on the EMBF, such as systematic left-lateral offsets of abandoned channels and scissoring vertical offsets on the fan surface, which were probably formed during the Nara era. These offsets were measured as left-lateral displacements of 6 to 7 m with a slight vertical component, and were probably formed in association with the most recent faulting event of EMBF. In this case, the style and the amount of slip per event are comparable with those of the neighboring Gofukuji fault, which has one of the highest probabilities of seismic risk calculated among active faults on land in Japan. These refined geomorphic features indicate that the principal slip component on the southern portion of EMBF is not merely dip-slip but left-lateral slip, taking account of the cumulative vertical displacement inferred from uplifted hills since Miocene to Plio-Pleistocene at the eastern side of the fault traces. These facts and presumptions are consistent with the deformation pattern around the ISTL revealed by recent continuous GPS observations. In addition, these new findings allow us to understand that the subsidence in the Matsumoto urban district can be explained as a pull-apart basin related to a 1.5-km-wide left-step between the EMBF and Gofukuji faults. Su8rface geometry, style, and amount of faulting of both faults suggest that this left-step does not seem to be a significant enough segment boundary of the ISTL to arrest rupture propagation. Thus, the portion between the southern EMBF and Gofukuji fault is appropriately a single fault segment.