Time-Varying Crustal Velocities and Changing Radial Anisotropy Coincident with Eruptive Phases at Turrialba Volcano

Abstract Seismic ambient noise interferometry is a prevalent tool for monitoring volcanic and seismic hazards. Most studies rely solely on vertical seismic components to assess time-varying velocity changes, yet velocity information extracted from the horizontal components is a powerful complementary investigation tool because it provides constraints on changes in radial anisotropy. Here, we measure relative fractional seismic velocity changes of both vertical and transverse components at the erupting Turrialba volcano. Velocity changes associated with some eruptive phases were present in all seismic components and had strong frequency dependence. Some eruptive phases also showed significant velocity perturbations at lower frequencies as well as divergent behavior between transverse and vertical components. This indicates a positive change in radial anisotropy surrounding the volcano during the eruptive episodes. From our results, we propose two instances of high volcanic fluid pulses at the bottom of the shallow magma reservoir feeding Turrialba that would have altered numerous microcracks and increased pore fluids. A sudden decrease in velocity, and a surge in seismic displacement, during the second pulse coincides with a large eruption in May 2016.