桜島火山における地震観測について(第4報) (火山性微小地震の分類および頻度分布について)

Abstract

Micro-earthquakes appered to have their origins near the crater of Sakurajima are classifiedinto 4 types by their dominant period or their duration of the vibration.SP type is one whose dominant period is about 0.1 sec. Or less, BS type about 0.2 sec.And B type about 0.3 sec. Which is observed high frequently. D' type has a long duration ofvibration, which is accompanied with the gas emission.Examining the relation between the frequency distributions of each type and the eruptionin the active stage from the latter part of Oct. to the beginning of Nov. in 19(3, the followingrelations are found.SP type goes on increasing of its frequency till the most violent explosion and after thendecreased.The number of BS type suddenly increased in two or three days before the explosion, anddecreased in the time of being eruptionB type is numerous in all stage, and D' type increased to some extent after the violentexplosion.