Changes to the volcanic outgassing mechanism and very-long-period seismicity from 2007 to 2011 at Mt. Asama, Japan

- Volcanic gas emission from gas bursts following a very-long-period seismic event was estimated at Mt. Asama.
- The very-long-period seismic events were modeled by sudden outgassing induced by propagation of gas phase expansion.
- Variability in volcanic gas emission and seismic activities revealed the change in the outgassing mechanism of Mt. Asama.

Volcanic gases can provide significant information on the state of magma, which is important for interpreting and predicting volcanic activity. We present a study of the seismicity and volcanic gases from Mt. Asama, Japan from 2007 to 2011 to elucidate the change in the mechanism of volcanic gas supply for very-long-period (VLP) seismic events. Two types of degassing occur at Mt. Asama: persistent degassing and a gas burst (ash-free eruption) following a VLP seismic event. We used the linear relationship between the seismic moment and sulfur dioxide (SO2) emissions, and seismic records to estimate the quantity of SO2 gas in the ash-free eruption, which causes the VLP seismic events. The amount of SO2 and volcanic gas composition data were used to calculate the amount of volcanic gases, including H2O, CO2, and other species. We compared the volcanic gas emission rates of ash-free eruptions (eruptive gas emission rates) and total volcanic gas emission rates including the persistent degassing. The ratio of the eruptive gas emission rate to the total gas emission rate (eruptive/total gas ratios) showed a drastic change with eruption. An increase occurred immediately before the 2008 and after the 2009 eruptions. The VLP seismic events are likely caused by sudden outgassing at a shallow depth that is induced by the propagation of the gas phase expansion and rupture. We suggest that the rates of sudden outgassing are controlled by the distribution of gas phases in the conduit. The variability of the eruptive gas emission rates suggests changes in the gas phase distribution in the conduit.