Volcanic Micro-tremor of the Volcano ASO （ I ） - Nature of the Volcanic Micro-tremor of the Second kind and its Source Condition

Abstract

The nature of the volcanic micro-tremor of the second kind and its source condition were investigated. The characteristics of the volcanic micro-tremors at Volcano Aso are closely related to the activity of the volcano. According to K. Sassa, there are four kinds of volcanic micro-tremors when classified from their periods, and these tremors have different characteristics. The volcanic micro-tremor of the “2nd kind” is a long period of 3.5 to 7.0 sec. The period of this tremor is maintained for the duration of unchanged activity of the volcano. The mode of this tremor corresponds to the compressional one and was observed in the form of several groups of regular wave trains during inactive periods, while during active periods it became a continuous train of somewhat irregular waves. The wave forms of this tremor on the seismograms depend on, in general, the characteristics of the active origin of the volcano, the mode in which waves have propagated and observed instruments. The spectrum of waves having a long period and compressional wave forms such as the 2nd kind may be taken to indicate information concerning the nondistorted source conditions. For this reason, an attempt was made to determine the source conditions of the 2nd kind of volcanic micro-tremor from its frequency spectrum.