有馬・高槻構造線周辺における重力測定

抄録

Gravity measurement was carried out at more than 200 stations with LaCoste-Romberg gravimeters in the vicinity of the Arima-Takatsuki Tectonic Line, a prominent fault system in the northern Kinki district, central Japan. The areas with low Bouguer anomalies derived from the measurement correspond to sedimentary planes and basins, while areas of high Bouguer anomalies to the mountainous areas. The areas of steep horizontal gradients of the Bouguer anomaly lie along active faults developed in the district. In particular a line of steep gradient of the gravity anomaly is seen along the Arima-Takatsuki tectonic line, which also correspond to the boundary between the sedimentary plane by the Yodo River and the outcropped area of the Tanba zone. Most of changes in the Bouguer anomalies are explained by that of the thickness of the unconsolidated sedimentary layer of about 1km. The seismic activity in the area however is clearly bounded by the Arima-Takatsuki tectonic line; in its northern part the seismicity is very high, while in the southern part a few earthquakes occur. This suggests that the fault movement at the depth of the earthquake occurrence, 5 to 15km is concerned to the variation of the thicknesses of the sedimentary layer. A line of a weak horizontal gradient of the gravity anomaly is detected along a clear boundary of the seismicity which is located along the Inagawa river striking northwest to southeast. This implies a deep embedded fault in the area, though the boundary does not corresponds to any confirmed active faults.