Fig. 1

Station locations and observed signals. a Topographic map around the active crater of Aso volcano. The seismic station (KAF) and infrasound station (ACM) are located on the crater rim \(\sim\)300 m from the vent (black cross). During the eruptive period, a surveillance camera was also installed on the rim (white square) and captured an image every 20 s. b Seismic and c infrasound signals were sampled at a rate of 100 Hz. When a Strombolian explosion occurred, the seismic signal arrived prior to the infrasound signal. Two arrows show the arrivals of the seismic signal and high-frequency infrasound component (\(\sim\)15 Hz). The background of the infrasound signals is composed of a low-frequency component (\(\sim\)0.5-Hz) derived from acoustic resonance inside the vent (Yokoo et al. 2019). d, e Cross sections between the vent and stations. \(L_\text {s}\) and \(L_\text {i}\) are the propagation paths of the seismic and infrasound signals from the source of explosion toward the stations (Eqs. (2), (3))