After the 1984 Western Nagano Earthquake (M=6.8), the airborne surveys on gamma ray and thermal infrared radiation were carried out with a helicopter in the southern Ontake area. The purposes of the survey are to catch the behaviors of bed gases (radon and thoron) around the epicentre and in the collapsed area and to clarify the mechanism of debris avalanche flow from the collapsed area. From the airborne research, the following relations were confirmed.
(1) In the epicentre area, a great quantity of thoron radiates (thoron/potassium and thoron/radon were mor than 0.20 and 0.60, respectively) .
(2) In the collapsed area, a high degree of thoron-radiation is counted (thoron/radon was more than 0.50) .
(3) In the spacious collapsed area, the non-volcanic fissures are distributed densely, and forming of their fissures in this area triggered the spacious collapse (thoron/radon was 0.507) .
(4) In Denj ogawa area, there are the network ground water veins here and there. The ground water smashed out from these veins pushed up the debris flows and flowed down them.