Airwaves generated by an underwater explosion: Implications for volcanic infrasound

A shallow explosion in a fluid is one of the fundamental processes of airwave generation in volcanic eruptions. To better understand the mechanism of the airwave generation, underwater explosion experiments were conducted. Although the underwater explosions have been intensely studied over the last century, airwaves have received little attention. In this study, pressure waves were measured in air and under water, and the corresponding motion of the water surface was captured by a high-speed video camera. Airwaves and associated styles of the surface motion show similarities determined by the scaled depth, defined as the depth divided by the cubic root of the explosion energy. The air waveforms are quite different from the pressure waves measured in the water and cannot be completely explained by linear transmission of pressure waves from water to air. The mechanism of airwave generation is a combination of wave transmission and dynamics of the interface boundary. The first mechanism directly reflects the explosion source, and the same source is also observed as the underwater pressure waves. On the other hand, the second is not necessarily visible in the underwater pressure waves but will provide information on the mechanical properties and behavior of the material above the explosion source. Each mechanism is analyzed in the experiments.