3. 爆発地震の二, 三の性質(V. 秋田駒ケ岳 1970-71 年の噴火現象)

抄録

Eight seismometers (2 : 1Hz, 6 : 3Hz) were set up during two weeks on the western slope of the volcano. Two seismometers were used for monitoring by smoked paper recording and others were fed to an oscillograph or a data-recorder. Most of the recorded earthquakes were those associated with the eruption, the so called, explosion earthquake. The so called “B-type earthquake” was quite few. The slope of the amplitude-frequency relation of explosion earthquakes was found to be 2.5. The authors would like to stress that this value can not be compare directly with that of other volcanic earthquakes which are caused by the fracture of solid medium. Amplitude decrease of initial motion of explosion earthquakes is shown in Fig.6. Large decrease of amplitude of the initial motion implies that the focal depth being extremely shallow. Acoustic wave was recorded well on the seismogram of No.4 as shown in Fig.7. and Fig.10. Initial onset of these earthquakes is mostly poor which indicates relatively shallower origin than other explosion earthquakes as shown in Fig.9. The authors tried to find chronological relation between the “time of eruption” and the “origin time of explosion earthquake”. “Time of eruption” was accurately determined by an infrared radiation meter set up on the summit of the volcano. Simultaneous observation by both infrared and seismic methods had to give us the solution. Unfortunately, since we used two different crystal clocks for both observations, we were unable to make clear the problem. But the time difference of the both onset scatters to some extent. This result implies the difference in focal depth of explosion earthquake for respective eruption.