Fig. 6

Schematic model of the shallow conduit of Aso volcano. Inside the active vent, a lava pond exists. The space above its magma–air interface is the shape of a frustum pipe and acts as a resonant pipe. The radius of the closed end of the frustum pipe \(a_\text {c}\) is identical to that of the lava pond. Strombolian explosions occur at the surface of the lava pond. At the time of an explosion, seismic and infrasound signals are generated simultaneously at the source of explosion (SRC). The seismic signal propagates through the lava pond at a velocity of \(v_\text {p0}\) and then propagates toward the station STA from the margin of the lava pond at P-wave velocity. The infrasound signal propagates through the air. The source position is expressed by a distance r from the center of the lava pond. We assumed that the sources are distributed across the entire surface of the lava pond ((\(0\le r \le a_\text {c}\))