Temporal Changes of Vp/Vs Ratios in the Volcano-Tectonic Seismic Swarm Zones of Redoubt Volcano, Alaska

It is known that the P and S wave velocity ratios, Vp/Vs, reflect the physical and chemical properties of medium including temperature, density, and rock composition. Temporal variations of Vp/Vs ratios in volcanic regions may allow us to infer the changes in medium and magma properties beneath volcanoes. Redoubt volcano is an active volcano that is located at 175 km southwest from Anchorage. The size of volcano is about 10 km in diameter, and the volume is around 30-35 km². The volcano has erupted several times since 1902, and most recently in 2009. The eruptions were generally explosive, and produced lava and pyroclastic flows. Seismic events in the volcano-tectonic (VT) seismic swarm zones of Redoubt volcano are well monitored by Alaska Volcanic Observatory (AVO). We investigate the Vp/Vs ratios in the VT seismic swarm zones from observed P and S arrival times. The hypocentral information is collected from the AVO seismic catalogue. Seismic data with high signal-to-noise ratios for earthquakes with epicentral distance less than 10 km are selected for analysis. A total of 6425 P and S travel-time pairs is collected. The Vp/Vs ratios are estimated using a modified Wadati method that is based on the S-P differential travel times versus P travel times. The VT seismic swarm zones are discretized by 0.1°-by-0.1° cells. Tomographic Vp/Vs ratio models are calculated before and after the recent eruption. The average Vp/Vs ratio of the study region is determined to be 1.90, which is significantly higher than that of Poisson solids. Also, systematic temporal changes in Vp/Vs ratios are observed around the volcano before and after the eruption.