Temporal changes of medium properties during explosive volcanic eruption

Volcanic evolutions typically accompany significant changes in medium properties. Monitoring the seismic properties may be useful in identifying the state of volcanic evolution. Temporal variation of shallow crustal V P / V S ratios before and during the 2009 eruption of Redoubt Volcano, Alaska, is investigated using the P and S traveltimes of local earthquakes. High V P / V S ratios of ∼1.9 are observed over a wide region in the precursory phase, suggesting the presence of partial melts in the medium at least several months before explosive eruptions. The high V P / V S ratios decrease until the early effusive phase with rates of −0.179year −1 in the precursory phase (65days) and −2.147year −1 in the explosive phase (40days), which is equivalent to a decrease in the bulk modulus of fluids by ∼1.1 and 7.9GPa, respectively. The decreasing V P / V S ratios in the precursory phase may be due to increasing amounts of overpressurized gas and water vapor in the medium. The rapid decrease in V P / V S ratios during the explosive phase may be caused by melt eruption and gas emission. The V P / V S ratios were observed to be nearly stationary since the early effusive phase, suggesting rare presence of melts and low levels of pore pressures with degassing. These observations suggest that monitoring V P / V S ratios may be useful to identify the eruption state.