Spatial and temporal variations of volcanic earthquakes at Sakurajima Volcano, Japan

We conducted broadband seismic observations at three sites of the Sakurajima Volcano, Japan, from December 1992 to March 1993 in order to clarify the spatial and temporal variations of spectral properties of volcanic earthquakes: B-type earthquakes and volcanic tremor episodes. We 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 volcanic 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 early part of seismograms, spectral peaks of B-type earthquakes and volcanic tremor episodes mainly reflect source characteristics rather than site and path effects. Temporal variations of the dominant peaks are classified into three types for B-type earthquakes and into four types for volcanic tremor episodes. Two types of B-type earthquakes are similar to two of volcanic tremor episodes in terms of temporal variations of spectral peaks and of polarization characteristics with the only difference in amounts of energy. The similarities in the temporal variation of spectral properties indicate that B-type earthquakes and volcanic tremor episodes share common source mechanisms with different energy magnitudes. Since volcanic tremor episodes tend to take place prior to the swarms of B-type earthquakes and explosion earthquakes associated with any summit eruptions, we conclude that volcanic tremor episodes occur at the early stage of low-energy radiation, followed by B-type earthquakes as higher energy is radiated.