桜島の火山性微動について
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概要
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By using the seismometrical method, the volcanic micro-tremors of Sakura-jima were observed to investigate their nature and the positions of their epicentres on August 1960. During the observations the summit crater of the volcano kept the ordinary fumarolic state without any explosions. The results of the observations and their analyses are as follows: 1) The later phases of volcanic earthquakes of shallow foci (B type earthquake, after Minakami et al.) and the volcanic pulsations of continuous trains (or micro-tremors) have the same properties in respect to the propagation direction and the mode of oscillation. 2) The micro-tremors oscillate in the directions of their propagation in the horizontal plane and their wave velocities are in the range of 0.4〜0.8km/sec. As their oscillations in the vertical plane are not evident in this study, it is not clear whether they propagate in a mode of the surface waves or propagate like the body waves. 3) By using the tripartite station method, the propagation directions of the micro-tremors are observed at the five stations surrounding the summit crater. From these results an epicentre of the micro-tremors is determined at the region near Kita-dake and three epicentres are also determined on the line of vents, as shown in Fig. 9. But the relation between the epicentres in each position is not evident in this study. 4) The periods corresponding to the peaks of the amplitude-period relations of both the micro-tremors and the later phases of deep focused volcanic earthquakes (A type earthquake, after Minakami et al.) range from 0.25 to 0.38sec. On the other hand, each one of the frequency-period relations of the micro-tremors has two peaks at the respective periods ranging 0.19〜0.24sec. and 0.28〜0.34sec. and the same relation of the later phases of the A type earthquakes has only a peak, whose period is about 0.3sec. If the later phases of the A type earthquakes arise from an effect of the layer system of the observation stations, the shorter period corresponding to one of the peaks of the frequency-period relation of the micro-tremors (0.19〜0.24sec.) will represent the period of oscillation at the origin of the micro-tremors.
- 特定非営利活動法人日本火山学会の論文
- 1961-06-30