桜島の火山性微動の性質について

抄録

Seismic observations of volcanic micro-tremors were carried with two small nets of three components seismograms, set temporarily on the east and west sides of the Sakura-jima-volcano, making up for seismic monitoring net arround the active crater. In the various type of occurrence of micro-tremors, the tremors of typical case, in which tremors occured succesively, immediately after the occurrence of shallow volcanic earthquake and followed by summit eruption, were mainly studied about the natures as seimic waves. Examining particle motion diagrams, phase velocities and Fourier spectra, we obtained the results as follows: (1) P and SV phases were identified from shallow earthquakes, and the origins were estimated beneath the summit crater of Minamidake. (2) Fourier spectra of micro-tremors showed regular peaks at some fundamental frequencies and at those of integer multiple of the fundamental ones. (3) Fundamental peak frequencies varied with different time of volcanic activity. (4) In the typical case, fundamental peak frequencies increased gradually from O.5 Hz to 1.3 Hz with short time between the occurrence of shallow earthquake and following summit emption. (5) Those observational facts suggest that dimension and/or physical boundary conditions of origins of micro-tremors can change easily. (6) Because of complexity of vibrational modes, neither wave type nor phase velocities of microtremors could be determined, and the location of origins were not obtained directly.