島根県における2001年以降の地震活動と地形および地質との比較

This study demonstrates the detailed correlation of recent seismicity (2001-2010) with geomorphological analyses, surface geology and gravity anomaly in Shimane Prefecture, southwestern Honshu, Japan. The epicenter distribution of small earthquakes is characterized by the NE-SW zonal concentration extending from the northern part of Yamaguchi Prefecture to around Mt. Sambe in Oda City (Yamaguchi-Izumo Intra-Plate Seismic Zone: YISZ). With the conjugate sense of the seismic zone, relatively small zone of NNW-SSE trending are observed from Mt. Sambe to Miyoshi City, Hiroshima Prefecture (Sambe-Miyoshi Seismic Zone: SMSZ).
The YISZ can be correlated with the geomorphological boundary between the Tsunozu and Kibi Low-relief Surfaces in the Iwami Highland. With respect to the steep gradient of gravity anomaly, a concealed tectonic line related to the deposition of the Pliocene Tsunozu Formation is implied just beneath the seismic zone. It suggests that the recent activity of the seismic zone occurs at the concealed tectonic line under the present ENE-WSW compression field.
Around the SMSZ, many lineaments with N30°W and N60°W trending are recognized. These trending directions are generally concordant with the focal mechanisms of earthquakes in this region. According to the detailed epicenter distribution in the seismic zone, NNW-trending small segments of seismicity are recognized with a right-stepping arrangement. It indicates that the right-stepping pattern in the seismic zone might be originated by the left-lateral slip of a concealed NW-SE tectonic line.