長野県大町市街地の活断層分布に関する一検討

抄録

　 The Itoigawa-Shizuoka Tectonic Line (ISTL) fault zone extends for ca. 158 km from Otari Village to Hayakawa Town, central Japan. The northern part of the fault zone (from Otari to Akashina in Azumino) has the potential for an earthquake larger than M 7. A M 6.7 earthquake called the Naganoken-hokubu earthquake occurred in 2014 in the northern section of this part. Most of the coseismic surface ruptures appeared along the pre-identified active fault traces. However, the others emerged at sites where no active fault traces had been revealed by previous studies. We have to continue detailed mapping of fault-related landforms to understand active fault distribution of the northern part of the ISTL fault zone. In this study, we report the existence of a ca. 1 km long, NNW-trending trace of possible active fault in central Omachi City, Nagano Prefecture, revealed by geomorphic analysis using high-resolution digital elevation models from airborne LiDAR data issued by Geospatial Information Authority of Japan. Our evidence for this trace is (1) slope-direction anomaly in the fan of the Kashima River identified by detailed elevation contours, (2) east-side-up vertical displacement indicated by topographic profiles, and (3) bend of an abandoned channel near the fault trace. The drainage system anomaly and irregular aqueduct network probably reflect the existence of this trace. This trace would have emerged prior to the construction age of the aqueducts drawn from the Nogu River to the area of the present central Omachi City, based on previous studies on geography and history of the city.