火山地震

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The characteristics of earthquakes originating from volcanoes which have been obtained by many investigators were summarized. Those earthquakes are classified tentatively into explosion-earthquakes, volcanic-earthquakes and volanic micro-tremors. The explosion-earthquakes occur almost simultaneously with the outburst of the volcanoes of Vulkanian type. Therefore it is generally concluded that such earthquakes are generated from the mechanism of a single positive force. However, there are the examples having a mechanism of tension-cracking type. The volcanic-earthquakes are the small swarmed earthquakes occurring in a limited region in the volcanic district and closely related to the activity of volcanoes. There are different kinds. One is the A-type earthquake. Its focal depth is not extremely shallow (1〜10km) and the wave form on the seismogram is similar to that of a shallow earthquake of tectonic origin. Another one is B-type occurring in the limited region beneath the active crater. The volcanic micro-tremors, customary called, include following various kinds of tremors. In the case of Strombolian or Hawaiian type eruptions, they follow the tremors originating from these frequent eruptions. On the other hand, there are non eruptive kind of tremors originating from the very small earthquakes beneath the active crater. And the tremors generating from the free vibration of magmatic chamber or column of lava and so on, are also found. The B-type earthquake or the volcanic micro-tremor are mainly composed of the seismic surface waves (Rayleigh or Love-type) which have been guided from the layered structure in the volcanic district, hence they have their own characteristic period. Such various kinds of volcanic earthquakes can be classified in the following three categories according their mechanisms of occurrence or origins of vibration, i.e. (1) The deeper earthquakes of volcanic origin (A-type), (2) Non eruptive earthquakes such as B-type one or some of the tremors originated from very small earthquakes, and explosion earthquakes occurring at shallow depth, (3) The tremors generated by the vibrations of magmatic chamber or column of lava and so on.
1965-12-20

火山地震

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