火山性地震の発震機構

Abstract

In the eruptions of 1965 and 1979 at the Aso Volcano, situated in Kyushu district of Japan, the characteristic occurrence of minor earthquakes were detected. These events wereconcentrated at shallow depth immediately beneath the active crater bottom. The focalmechanisms of these events are vertically polarized, and the vertically "pull" type is predomi-nant in the period before eruption, and the "push" type is predominant reversely in the theperiod after eruption, respectively. This characteristic feature can be explained by introduc-ing a certain gas chamber with variation of pressure.