火山.第２集

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The author has observed the short period micro-tremors near the crater since 1959. He obtained the following results about them. 1) The micro-tremors have different predominant periods in each time of the volcanic activity. The difference is seemed to be due to the depth of the origin of micro-tremors. 2) The predominant period of micro-tremors which are oscillating on a vertical plane in a direction to the crater is different from that of micro-tremors oscillating on a perpendicular plane at right angle to the crater. Taking consideration of the difference of the predominant period, modes of oscillation and velocities of propagation, the former seems to be a kind of Rayleigh wave and the latter a kind of Love wave. 3) The micro-tremors are propagating from the crater at the velocities from 600m/sec. to 1500m/sec.. Micro-tremors in a wave train are transmitted from nearly the same direction. And they present phenomena like dispersions of surface waves. 4) There are good correspondences between the micro-tremors and the micro-earthquakes originating near the crater. The micro-tremors are seemed to be Rayleigh and Love waves generated by the micro-earthquakes near the crater.

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