Abstract
We conducted topographical and geological investigations in the areas along the West Yauneyama and the Tsumo faults and their extension areas, in order to review in detail characteristics and distributions of the faults in southwestern Shimane Prefecture. By combining the results with an existing gravity anomaly map and data of seismicity in the studied area, we then discuss the connectivity and slip styles of the faults.
Based on new lineament interpretation and the field investigation, we showed that the West Yauneyama and the Tsumo faults extend 10 km to the southwest and 1 km to the east in length, respectively, compared to those indicated by previous studies. A strike-slip duplex can be recognized in the Itaigawa area, Mito town, Masuda City in which the Tsumo fault meets the northeastern part of the West Yauneyama fault. Because the area surrounded by the strike-slip duplex is lower than in elevation that surrounding the strike-slip duplex, the surrounded area is regarded as a depression. Deformation structures found in some fault outcrops show that the slip on the Tsumo fault inverted from sinistral to dextral motion. The slip sense suggests that the depression surrounded by the strike-slip duplex continues to extend to the NE-SW direction as a pull-apart, which is referred to the Itaigawa Pull-Apart in this paper, and subsequently the depression may subside.
