Keywords:Vulcanian Eruptions, Infrasound, Sakurajima Volcano
To investigate mechanism of Vulcanian eruptions, we analyzed infrasound data and eruption movies (30 frames/s) of the Showa crater, Sakurajima volcano, Japan. 153 eruptions during December 2011 to August 2015, which accompanied variable infrasound waves and visible ash clouds, were selected for an analysis. In this study, we focus on the infrasound initial compression phase (i.e., initial pulse). First, we estimated volume flow rate (dV/dt) of the initial compression phase using the simple acoustic source model (Lighthill, 1978; Johnson and Miller, 2014). Furthermore, based on the relation dV/dt and ejection velocity u from the eruption movie; dV/dt=πR2u, the vent radius (R) was calculated on each eruption. The estimated vent radius R shows the time change, which may be an indicator reflecting the activity of the Showa crater. Second, we investigated variations of duration (τ+) and its relationship to peak amplitude (Δp) of the infrasound initial compression phase. Enigmatically, τ+ shows bimodal distributions. The relationship between τ+ and Δp shows characteristic distributions and it may be explained by rupture of the pressure source under the crater bottom (e.g., gas chamber).
Acknowredges: We thank Kyoto University and MILT for using movie and infrasound data. GPS time-synchronized eruption movies were digitally captured and recorded continuously in HDD at Kurokami station, Kyoto University, which is located 3.5 km east of the Showa crater (Aizawa et al., 2016, EPSL). 200 Hz sampling infrasound time series data were obtained at Arimura station, MLIT, which is located 2.3 km south of the Showa crater.