Fig. 4

A conceptual model of Strombolian explosion processes. a and b Just before an explosion, the magma surface swells due to the arrival of a gas pocket, generating the low-frequency (2 Hz) infrasound signal. c In the next moment, the magma film at the surface is ruptured, and gas and magma fragments start to be ejected. High-frequency seismic and infrasound signals are generated at this time. d Afterward, ejection of the gas and magma fragments continues for several seconds. e Model parameterization geometry for a Strombolian explosion and its seismo-acoustic signals propagating to the stations. The gas phase rises with a velocity of u_asc in the conduit from a depth of d_VLP where the VLP signal is produced. When the gas phase (gas pocket) reaches the magma surface at a depth of d_exp, low-frequency infrasound is produced. Soon afterward, a Strombolian explosion occurs, generating both an eruption earthquake and high-frequency infrasound signals. The VLP and eruption earthquake signals propagate through the ground with velocities of v_sL and v_sH, respectively. In contrast, low- and high-frequency infrasound signals propagate through the air with sound velocities of c_con in the conduit and c_air above the crater floor