阿蘇カルデラの重力異常(低密度堆積層の効果を除去した)とその構造

抄録

Gravtiy survey over the Aso volcanic region was made by author et al from 1965 to 1966. Bouguer gravity anomalies over this area are characterizd by a strong negative anomaly on the Aso caldera. The gravity low of the Aso caldera shows characteristic feature of the “low anomaly type caldera” or "crater lake type caldera" as is pointed out by Yokoyama. Aso caldera is outlined by a circular somma and a range of more than a dozen central cones divides the caldera into two wide plains (atrio), the northern Asodani atrio and southern Nangodani atrio. Contour lines of Bouguer anomaly are ring-shaped on the middle of the caldera, which they are peas-shaped on the Asodani atrio area (Fig.1). The Asodani atrio is a vast horizontal plain rimmed by a caldera wall on the north and the frank of the central cones on the south. Geological evidences show that thick sedimentation of volcanic ashes and gravels occurred in this area when it once constituted the bottom of a huge caldera-lake. It is apparent that this thick volcanic sediment disturbes the structural interpretations of gravity data on this area. Fortunately the thickness of volcanic deposit can roughly be estimated by the observed period of ever-existing microtremor. Thus, Bouguer anomalies over Aso caldera without the effect of the low density deposit are obtained (Fig.8) This gravity map shows that there are ring-shaped contour lines of the low anomaly at the middle of the caldera, however, no abnormal feature is recognized elsewhere. This result shows that the mass deficiency due to the caldera formation which has been proposed by Yokoyama exists only on the middle part of the caldera. On the other hand, gravity anomaly at the area where no peculiar feature is recognized is due to the topography of the caldera having been formed by the collapse and enlargement of the caldera wall.