Abstract
In this study, geomorphologic and geologic features of the West Yauneyama fault in southwestern Shimane Prefecture, are clarified from geomorphological analysis, geological mapping, andi nvestigations of fault outcrops. Regional relationship between the fault and a seismic activity zone is also examined by the catalog of the Japan Meteorological Agency. Crustal structure of the West Yauneyama fault is discussed by using a compiled gravity anomaly map. Geologic and topographic investigations indicate that the ENE-to NE-trending West Yauneyama fault is approximately 30 km long, extending from Kanagi Town to Hikimi Town, Shimane Prefecture. Based on the certainty of lineaments and the occurrence of tectonic landforms, the West Yauneyama fault is tentatively divided into northern and southern segments. The geomorphologic feature suggests that the northern segment has probably moved right-laterally during the Quaternary. Systematically right-lateral deflection or bend can be detected in a number of channels across this segment. In contrast, the southern segment has no movement during the Quaternary because of a lack of tectonic landforms. A seismic activity of magnitudes of greater than 2 occurs along the northern segment of the West Yauneyama fault, and a steep gradient of gravity anomalies ranging from 2 to 3 mgal/km can also be detected along it, while around the southern segment, no seismic activity has been observed in the period from October 1 in 1997 to July 31 in 2001, and a shallow gradient of gravity anomalies ranging from 0.5 to 1.5 mgal/km can be found. This suggests that the northern segment of the West Yauneyma fault extends down a deep depth in the Earth's crust.
