瀬戸内海東部の島々における重力測定

－瀬戸内海高重力異常帯－

抄録

The Seto Inland Sea (Seto-naikai) was one of the largest areas of poor distribution of gravity stations in the Japanese Islands. In June of 1991, we visited 65 islands in the central to eastern part of this area for gravity measurements by employing a small boat (6 .5 ton). More than 300 gravity measurements were carried out in this area. The measured data revealed an existence of a high Bouguer anomaly zone along the central part of the Seto Inland Sea. The length and width of the zone are about 180 km and 30 km, respectively. Bouguer anomaly is higher about 20 mgal than the surrounding areas. We named the zone as the “High Gravity Anomaly Belt in the Seto Inland Sea”. It is difficult to interpret this high anomaly belt as a consequence only due to density differences of surface geology, because the studied area is widely covered by granitic rocks. We suggested a possibility that Sanbagawa metamorphic rocks, or similar high-density rocks, presumably exist at shallow depth beneath this area.