Gravity survey in the Central Ranges, Honshu, Japan.

Abstract

An extensive gravity survey was carried out from 1978 through 1980 in the area of the Central Ranges (Japan Alps), Honshu, which consist of the three largest mountain ranges in Japan (Hida, Akaishi, and Kiso). The Central Ranges area of 40, 000km2 has provided about 3, 700 gravity data including 2, 214 newly measured data, from which a Bouguer anomaly map was drawn. Bouguer and terrain corrections were made with a topographic data file of 230×280m2 grid at a distance of up to 80km from any measuring point, taking the earth's sphericity into account. A value of 2.64g/cm3 was used for the average density of surface features, and this was derived from the observed gravity data using a newly proposed least-squares method. The possible errors involved in the analysis were critically evaluated. The newly drawn anomaly map has an overall accuracy of 5mgal (=5×10-5m·sec-2). The Central Ranges area is known as a broad zone of strong negative anomaly, which has now been resolved into two parts, one along the axis of the Hida mountain range and the other along the northern Fossa Magna. Bouguer anomaly and topography are correlated negatively in the Hida range, marginally in the Akaishi range and positively in the Kiso range, suggesting that the correlation becomes progressively negative with increasing massiveness of mountain structure. The Central Ranges are bounded by the Itoigawa-Shizuoka Tectonic Line to the east and by the Atera fault to the southwest across which the regional Bouguer anomaly pattern changes sharply. The Atera fault is a firstclass active fault along which a narrow but very clear Bouguer low is observed that may be attributable to fault gouge material.