FOCAL MECHANISM OF VOLCANIC EARTHQUAKE OF THE VALCANO ASO

Abstract

During the recent active period, from April 1965 to March 1966, of the Volcano Aso, a swarm of volcanic earthquakes was observed at the Hondo observation room of Kyoto University. By analysing the data, the locations of foci of volcanic earthquakes are determined and the focal mechanism is discussed. The main results obtained are as follows: (1) the distribution of foci of volcanic earthquakes is varied with the volcanic activity, that is, the foci are located in comparatively narrow region before the eruption, but scattered after the eruption; (2) the characteristic earthquakes regarded as a rarefaction type are found, that is, the initial phases are all "pull" around the epicenter. These rarefaction type earthquakes are observed in the earlier time and the active time, but not in the later time.