Density structure inferred from gravity anomalies in the eastern area of the Itoigawa-Shizuoka Tectonic Line, central Japan

Abstract

A gravity survey was conducted between October 2002 and December 2003 in and around the eastern area of the Itoigawa-Shizuoka Tectonic Line (ISTL). The total number of measurement points amounted to about 436 and 326 stations were taken at the same points as the seismic survey points. The location and altitude were decided by differential GPS and the accuracy is thought to be within several ten-centimeters. The data was compiled from existing data (Geological Survey of Japan, 2000), so the total number of compiled gravity maps is about 3,540 points. All measured gravity data were referred to the International Gravity Standardization Net 1971 (IGSN71) and the normal gravity values were estimated according to the gravity formula of 1980. Terrain corrections were conducted within a range of 60 km by approximating the real topography to an assemblage of annular prisms interpolated by mesh terrain data and random terrain data of the gravity points. The effect of the sinking of the topography due to the Earth’s curvature was taken into consideration. Bouguer corrections within a range of 60 km in arc distances were made using a spherical cap crust formula. The density for both terrain correction and Bouguer corrections was chosen to be 2,670 kg/m3, because the mean surface density of the whole area is estimated to be a slightly large density by the CVUR method. The features of Bouguer anomalies is characterized by a low anomaly in Omachi city, in the northern parts of Matsumoto basin, and a high anomaly in the central highlands, Central Uplift Zone, about 10 km west of Ueda city. The low anomaly shows that the thickness of the sedimentary layer is more than 1 km from 2D/3D automatic analysis.