Application of the Monopole Source to Quantify Explosive Flux During Vulcanian Explosions at Sakurajima Volcano (Japan)

A primary goal in volcano seismology is to characterize source motions internal to a volcano in terms of their representative forces. In a similar manner, much volcano infrasound research strives to recover eruptive force time histories corresponding to material accelerations occurring at Earth’s free surface. These motions may correspond to explosive emission of gas and pyroclasts (e.g., Banister, 1984), rapid ground distensions of a volcanic dome (e.g., Johnson and Lees, 2010), and/or gravity driven rock fall or pyroclastic flows (e.g., Yamasato, 1997). When free surface motion is unsteady it imposes stresses upon the surrounding atmosphere, which are propagated as acoustic airwaves. Typically, the sounds produced by volcanic phenomena are recorded with low-frequency infrasound sensitive microphones. The intervening atmosphere is relatively homogeneous and nonattenuating for infrasound propagation of a few kilometers, therefore the recorded excess pressures closely represent volcano source time functions. Such force time histories are equivalent to volumetric accelerations of the atmosphere at or near to the source.