Spectral ratio analyses of explosion earthquakes at Sakurajima Volcano, Japan

Abstract

It's quite important to clarify what physical parameters control the magnitude of volcanic explosion to understand magma processes in the conduit. Analyses of explosion earthquakes, which are associated with vulcanian eruptions, enable us to quantitatively evaluate dynamic process of the explosive eruptions. In the present study, we analyze hundreds of explosion earthquakes recorded at Sakurajima volcano, Japan, during 2012–2013. Applying a spectral ratio method that can retrieve the source spectral information without being disturbed by heterogeneous structure to three-component seismograms recorded at three stations operated by Japan Meteorological Agency, we examine scaling relations in explosion earthquakes. To obtain an averaged feature, we group the observed explosion earthquakes into five classes (I, II, III, IV, V) according to their maximum amplitudes and calculate the spectral ratios between the five classes. Because vulcanian eruptions continue to effuse volcanic ash for several to tens of minutes, spectral amplitude ratios are calculated by shifting a time window of 10 s for five successive 10s-long time windows from the onset to coda wave. The results show that the spectral amplitude ratios are characterized by the low and high corner frequencies: about 2 Hz and 3.5 Hz, respectively. With increasing the magnitude of explosion earthquakes, amplitude ratios at low and high frequency ranges, which are 0.5–2 Hz and 3.5–10 Hz, respectively, increase, while the corner frequencies do not significantly change. Since the explosion earthquakes at Sakurajima volcano occur at a same crater without changing the vent radius, the observed spectral amplitude ratio change may be attributed to pressure changes in the conduit. Another finding is that the amplitudes of spectral ratio gradually decrease with lapse time keeping the corner frequencies. Such a lapse time dependence of spectral ratios is completely different from those of tectonic earthquakes in which direct and coda waves show a same spectral amplitude ratio. We infer that the early part of the seismic waves of explosion earthquakes is generated by an initial rapid pressure release in the conduit while coda waves are by successive oscillations of the magma system during continuous ash emission.