Temporal Changes of Seismic Properties During Inter-Eruption Periods of Redoubt Volcano

Abstract:

Active volcanoes are monitored year-round for mitigation of volcanic hazards. The identification of volcano evolution during inter-eruption periods is crucial for timely preparedness. Volcanic evolutions typically accompany significant changes in medium composition, temperature and stress field, causing changes of properties and structures of the medium including porosity, density, amount of cracks, and fluid saturation that control seismic properties of the medium. The P and S velocities changes with medium properties. Monitoring the seismic properties are useful for identification of volcanic evolution. The ratio between P and S velocities, Vp/Vs, is particularly effective for monitoring the volcanoes. The Vp/Vs ratios are estimated using a modified Wadati analysis based on the travel times of local P and S waves. Redoubt volcano is a glaciated stratovolcano in the northeastern Aleutian volcanic arc. The recent eruptions in 1989-1990 and 2009 are accompanied by lava dome growth and magmatic explosions. The temporal variation of Vp/Vs ratios in the upper crust around Redoubt volcano during 1989-2014 is investigated. The Vp/Vs ratios reach ~1.9 just before the 1989-1990 and 2009 eruptions. The partial melts in the medium may cause such high Vp/Vs ratios. The Vp/Vs ratios rapidly decrease to ~1.7 during explosive eruptions in 1989-1990 and 2009. The decrease of Vp/Vs ratios may be associated with melt eruptions and gas emission. The Vp/Vs ratios increase with a rate of 0.00638 per year during the inter-eruption period after the 1989-1990 eruption before the 2009 eruption, reaching ~1.73 in 1994 and constant during 1994-2008. The mild increase of Vp/Vs ratios may be associated with slow decrease of overpressurized gas and increase of fluid saturation. The Vp/Vs ratios abruptly increase to ~1.9 just before the 2009 eruption, suggesting rapid development of partial melts and pressure field. The Vp/Vs ratios after the 2009 eruption consistently increase with a rate of 0.0116 per year. The relatively high increase rate of Vp/Vs ratios may be due to increasing fluid saturation and decreasing overpressurized gas. The observations suggest that the temporal variation of Vp/Vs ratios may allow us to identify the state of volcanic evolution in inter-eruption periods.