Fig. 2
Snapshot of a Strombolian explosion at Aso volcano and corresponding seismo-acoustic waveforms. a Ash-free Strombolian explosions (such as type 1 explosions; Patrick et al. 2007; Gaudin et al. 2017) captured by the CAM surveillance camera at 19:36 on 21 April 2015. Incandescent lapilli were ejected from the active vent at the crater center. b Seismo-acoustic waveforms of a Strombolian explosion on 21 April 2015 recorded at UMA, KAF and ACM stations. From bottom to top, the waveforms are the very-long-period (VLP) seismic signal (< 0.1 Hz), eruption earthquake signal, and infrasound signal. Seismic signals at UMA and KAF are the vertical component. The VLP signal typically started with a downward motion at tsL (the pink-colored zone shows the reading error). The eruption earthquake signal emerged with a high-frequency and high-amplitude phase at tsH. The infrasound signal was composed of low- and high-frequency signals arrived at tiL and tiH. c Power spectrogram of the infrasound signal computed for a 5-s-long sliding window with a 0.1-s overlap