A numerical model for precursory time sequences of the phreatic eruptions of Mt. Ontake, central Japan

Abstract The 2014 phreatic eruption of Mt. Ontake was preceded by increased volcano-tectonic (VT) seismicity, but all other types of precursors were obscure. A subsurface process leading to eruption with few precursors has yet to be identified. This study performed numerical simulations to reproduce the precursory time sequence of the eruption. High-temperature water from a cooling magma reservoir was injected into a modeled porous medium filled with cold water. This resulted in an immediate pressure increase and a delayed temperature increase in the shallow parts. The pressure increased immediately because the injected fluid pushed the existing fluid away from the porous medium. A sharp boundary between low (initial)- and high (injected)-temperature regions, known as a thermal front, developed and migrated upward slowly, leading to a delayed temperature increase. The lag time between the pressure and temperature changes is likely responsible for the precursory VT seismicity. If the injected water temperature was less than the critical point, the water was liquid throughout the model region until the thermal front reached a shallow part, where the pressure was low enough for the heated water to vaporize. This vaporization was sudden and large, likely leading to a large eruption with few precursors, similar to the time sequence in 2014. If the injected water temperature was higher, the water volume increased gradually from liquid to supercritical and then to the gas state. This scenario likely led to a small eruption with numerous precursors, similar to the previous eruption of Mt. Ontake that occurred in 2007.