Bouguer Gravity Anomaly around Kirishima Volcanoes, Kyushu

A gravimetric survey was carried out during May to June, 1969, over an area of 750 km2 immediately north of Kirishima volcano, one of the active volcanoes in Kyushu. About 190 stations were occupied and an additional 180 stations occupied by the Geological Survey of Japan just after the destructive earthquake of February, 1968, were included to cover the total area of 1,300 km2. The IGSN71 gravity net was used as reference and the 1967 Geodetic Reference System was adopted for the gravity value computation. Regional average density was obtained according to the method proposed by RIKITAKE et al. (1965) and the terrain and Bouguer corrections were computed to the nearest 40 km2. The contour map of the Bouguer anomaly (Fig. 5) clearly shows two local negative anomalies: one occupying the center of a conspicuous topographic low, called Kakuto basin; and the other lying immediately east of Kakuto basin. The former gravity low (up to ― 15mgal) strongly supports the hypothesis by Arita (1957a, b) and others that Kakuto basin is a caldera of the Crater Lake type formed by collapse following the eruption of large-scale pyroclastic flows. The welded deposits of the pyroclastic flows are found in many places outside Kakuto basin (Fig. 2). The latter gravity low (up to -20mgal) also suggests the presence of another caldera, the Kobayashi caldera, which is apparently older than the Kakuto caldera. Although the data is still insufficient, the Kobayashi caldera is probably of the CraterLake type