SPATIAL AND TEMPORAL VARIATIONS OF VOLCANIC EARTHQUAKES AT SAKURAJIMAVOLCANO, JAPAN

We conducted broadband seismic observations at three sites of the Saku rajima Volcano, Japan, from December 1992 to March 1993 in order to cl arify the spatial and temporal variations of spectral properties of vo lcanic earthquakes: B-type earthquakes and volcanic tremor episodes. W e used three STS-2 seismometers recording in the frequency range from 0.03 to 6 Hz. Major spectral peaks of both B-type earthquakes and volc anic tremor episodes are located in the frequency range of 1.1-1.3, 2. 3-2.5 and 3.4-3.6 Hz. From the similarities of temporal variations in spectra among all the stations and of body-wave propagation in an earl y part of seismograms, spectral peaks of B-type earthquakes and volcan ic tremor episodes mainly reflect source characteristics rather than s ite and path effects. Temporal variations of the dominant peaks are cl assified into three types for B-type earthquakes and into four types f or volcanic tremor episodes. Two types of B-type earthquakes are simil ar to two of volcanic tremor episodes in terms of temporal variations of spectral peaks and of polarization characteristics with the only di fference in amounts of energy. The similarities in the temporal variat ion of spectral properties indicate that B-type earthquakes and volcan ic tremor episodes share common source mechanisms with different energ y magnitudes. Since volcanic tremor episodes tend to take place prior to the swarms of B-type earthquakes and explosion earthquakes associat ed with any summit eruptions, we conclude that volcanic tremor episode s occur at the early stage of low-energy radiation, followed by B-type earthquakes as higher energy is radiated.