桜島火山の火口近傍に於ける高感度地震観測 (I) : B 型火山性微小地震の初動について

抄録

In the vicinity of the active crater of the vulcanian type volcano, there occur B type (Minakami, 1960) volcanic micro-earthquakes. Many investigations about the nature of B type earthquake have been done, but the mechanism of which is left unsolved. To study the mechanism of B type earthquake, it is necessary to know the geographical distribution of initial motion. And judging from the nature of B type earthquake, high sensitive seismometric observation near the crater is indispensable so as to record its initial motion. From this view point, the author has begun a preliminary observation at the two stations near the crater of volcano Sakurajima. Maximum magnification of seismograph system is about 2×10⁵ times at 15 c/s. The results obtained through the observation are as follows. At No.1 station which is 1, 600 m away from the active crater, some B type earthquake inital moitions were recorded clearly, but at No.2 station which is 2, 700m away from the crater, the initial motions were observed vaguely. The direction of initial motion at No.1 station is not uniform, but some are upward and others are downward. This fact seems to suggest that the mechanism of B type earthquake and that of the explosion one are not of the same nature, because at Asama or Sakurajima volcano, when initial motion is clear, explosion earthquakes always begin with upward phase.