Spectra of Volcanic Earthquakes Related to the Eruptions of Sakura-jima Volcano in February, 1975

Abstract

Spectral features of seismograms are investigated for the events closely related to four eruptions on February 26, 27 and 28, 1975, based upon an assumption that variations in spectral compositions can be interpreted as the changes in focal depths of events. A swarm of volcanic earthquakes started about 17 minutes before the eruption of 03h55m (JST) on February 26. The generation of these events is correlated to the explosion mechanism because the highest activity of the swarm occurred at the eruption and because focal depths of the events were as deep as that of the explosion earthquake. Two eruptions occurred at 03h35m and 04h48m on February 27. Events related to the latter eruption are readily distinguished from those following the former by the predominant frequency of power spectrum, spectral composition, running spectrum or seismic activity. Energy sources of the two eruptions were at small depth under the crater. Precursory activities of strong B-type volcanic earthquakes occurred 8 minutes before the violent eruption of 00h16m on February 28. Running spectra of Ct-type, low predominant frequency in the spectrum and little emission of volcanic smoke imply that a great deal of energy might be interrupted at a depth from flowing up.