A GRAVITY SURVEY ON ASO CALDERA KYUSHU DISTRICT, JAPAN (II)

A gravity survey at 292 points in an area of about 1700 km2 over the Aso volcanic region was made by using Worden and La Cost & Lamberg geodetic gravimeters from 1964 to 1966. Bouguer gravity anomalies over this area are characterized by a strong negative anomaly on the Aso caldera and positive anomalies on the south-western flank of somma of the Aso caldera. The gravity low of the Aso caldera shows characteristic feature of the "low anomaly type caldera" as is pointed out by Yokoyama. On the south-western flank of the somma of the Aso caldera, the oldest formation consists of semi-shist (Sanbagawa System) and outcrop of basalt is also being found by geological surveys. The gravity high which reaches to 22 mgal at the center coincides with the distribution of basaltic formation. It reveals, therefore, the high density of material such as basaltic or metamorphic rocks. The underground structure of the Aso caldera was computed from the observed gravity data by using the sin x/x method. For the computations, models of basement layer and overlying layer, reasonable density contrast between two layers and the depth of datum-plane were assumed. Thus, several models were obtained which are, however, the first approximated ones