桜島の「C 型」火山性微動

抄録

Spindle-shaped volcanic tremors being similar to sinusoidal waves are frequently observed at the Sakurazima Volcano. They are called tremors of C-type by the Kagoshima District Meteorological Observatory (KDMO). It has hitherto been presumed that they have some connection with the intrusion of lavas into the base of crater. We have, however, no reliable evidences as yet. The authors reexamined the tremors marked as C-type by KDMO, using particle motions and periods of waves. The results are as follows. The mean periods of tremors range from 0.4 to 2.0 sec, but most of them from 0.5 to 1.3 sec. The peaks of frequency distributions of periods are at 0.7, 1.0 and 1.3 sec and they seem to vary from time to time. The investigations of particle motions show, however, that the tremors of different type of waves are mixed into the same group as C-type and that the predominant part of one tremor is composed of P or S type of waves and the other is surface waves. Nevertheless, the possibility of variance of predominant period with time is not denied because tremors of the same type of waves have different periods one another. The tremor of C-type is composed of superposition of waves having slightly different periods though it is very similar to sinusoidal waves. The mean period still has some meanings because the periods of waves composing the tremor are confined to a narrow range.