Abstract
Mihara Volcano, lzu-Ooshima Is., erupted violently on 15th November, 1986. At that time, vast erupted lava covered in craters on summit of the Volcano. After one year, the same volcano erupted again on 16th November, 1987. And then, though this eruption was on a small scale, the craters caved-in.
Measurement on distribution of natural environmental gamma ray over lzu-Ooshima ls. was carried out on 4th December in 1986 and on 19th January in 1987, by the airborne gamma ray spectrometer system of AERRS on a helicopter Bell 212. Moreover, thermal imageries on the same island by the airborne thermal infrared video camera of AERMS were recorded at the following three epochs: on 28th November in 1986 after the first eruption, on 12th November in 1987 before the second eruption and on 19 th November in 1987 after the second eruption.
As a result, some new facts on volcanic eruption and cave-in of the crater were obtained, that is,
1) On the occasion of volcanic eruption, spectrum composition of natural environmental gamma ray over the volcano shows some significant changes.
2) Previous to cave-in of the crater, a ringlike thermal anomalous zone appears on the outskirts of crater, and it corresponds to a boundaryline of depressed area.
