A GRAVITY SURVEY ON ASO CALDERA KYUSHU DISTRICT JAPAN (I)

Abstract

On Aso Caldera, gravity surveys have been made by using a Worden gravimeter from 1964 to 1965, in order to investigate the problem, whether there exsists or not a cecular change in gravity value connected with volcanic activities, and to get clues for figuring out the subterranean structure of this caldera. In this paper, the gravity values observed at each station and the distribution of Bouguer anomalies are reported. The distribution of Bouguer anomalies shows that the low gravity anomaly decreasing toward the center of the caldera is very conspicuous. The iso-anomaly contour lines conform to the caldera shape, and the relative anomaly at the center of Aso caldera to the neighbour amounts to about -20 mgal. This fact shows that Aso caldera is characterized by strong negative anomalies in contrast to the outer somma part and it can be classified as "low anomaly type caldera" such as Kutcharo caldera lake or Aira caldera of the so-called Krakatau type. Comparing the gravity values obtained by the present surveys with the ones obtained by the Earthquake Research Institute in 1964 and by the Geographical Survey Institute in 1959, it is concluded that there was no significant secular change in gravity value during the period from 1954 to 1964. And further discussions on atmospheric pressure and temperature effects on the Worden gravimeter are made.