On the Second Volcanic Micro-Tremor at the Volcano Aso

Abstract

At Volcano Aso, four kinds of volcanic micro-tremors have been observed by K. Sassa. And each of them has been named the micro-tremor of the first kind, of the second, of the third and of the fourth, of which the second one is of a long period, 3.5 ~ 7 sec. and its origin seems to be considerably different from the rest. So it is supposed that this micro-tremor is generated with the explosive motion of gas-riched magma in a reservoir and that its period, 3.5 ~ 7 sec. is the free one of magmatic reservoir, and then the dimension of the reservoir is calculated. In case that the reservoir begins to vibrate by the explosion of gas-riched magma, the mode which vibrates only to the propagating direction of wave appears chiefly, and therefore the equations of the wave motion of such a mode are solved and the radius of the reservoir, 696 ~ 1392 m., is gotten, being applied the velocity of seismic wave calculated from the records of volcanic earthquakes. This result suggests that the above mechanism may be expected for the micro-tremor of the second kind.