Temporal change of upper-crustal VP/VS ratios with volcanic evolution in Redoubt Volcano

Volcanic activities accompany changes in the physical and chemical properties of the medium. The ratio between P- and S-wave velocities, VP/VS, is sensitive to the medium property changes. We investigate temporal variations of the VP/VS ratios in the upper crust around Redoubt Volcano in the 1989-2014 time period using the traveltimes of local earthquakes. The VP/VS ratios reached ∼2.0 immediately prior to the 1989-1990 and 2009 eruptions. These high VP/VS ratios may be associated with partial melts in the edifice. The VP/VS ratios rapidly decreased to ∼1.7 during the 1989-1990 and 2009 eruptions due to decreasing temperatures, fluids, and pore pressures and increasing effective stresses after melt eruptions and gas emission. The change rates of the VP/VS ratios showed evident spatial distributions. The VP/VS ratios increased at a rate of 0.025 (±0.003) yr−1 below the 2009 vent during the inter-eruption period between the 1989-1990 and 2009 eruptions due to an increase in the temperature, which may be associated with the recharge of the magma in the lower crust. The increase rate of the VP/VS ratios was as low as 0.005 (±0.001) yr−1 in the regions located away from the eruption site. The VP/VS ratios appear to have increased consistently at a rate of 0.017 (±0.004) yr−1 since the 2009 eruption.