重力異常からみた三瓶カルデラの基盤形態

抄録

Sanbe Volcano is a large lava dome, 2 km in diameter, emplaced at the center of an elliptical caldera with a long axis of 7 km and a short axis of 5 km. One crater is exposed in the center of this dome, and four peaks surround this crater. However, the basement depth of the caldera has not been determined. We have made gravity measurements around Mt. Sanbe in order to investigate the caldera morphology. The residual gravity anomaly (which is obtained after the regional gravity trend is taken away from the Bouguer anomaly, assumed to be 2.67 g/cm³ in density) is low at the center of Sanbe Caldera. Mass deficiency, estimated by Gauss's theorem, indicates that Sanbe Caldera is one of the low anomaly type calderas, as large as the Mashu caldera. Basement depression under Sanbe Caldera has been determined by three dimensional analysis, when the density contrast between the basement rocks and Sanbe volcanic rocks is 0.43 g/cm³. The depression is square shaped, with a steep rim and a flat floor. The square outline suggests that the depression part may be subsided along normal faults ; accordingly this caldera may have originated in a cauldron. The western and eastern rims of the depression correspond to the somma, but the northern and southern rims are discordant. The poor topographic expression in these areas may be due to collapse of the caldera walls. The deepest part of the basement is centered on the east side of the present crater. This deep center may be the only vent of Sanbe Volcano, because other depressions are not observed beneath the lava dome.