Structure of shallow magma sources beneath Augustine Volcano (Alaska) inferred from local earthquake tomography

Abstract

Augustine is an active stratovolcano occupying almost entirely a small island in the Cook Inlet in Alaska. In this study, we use the arrival time data from local seismicity recorded by 15 permanent seismic stations deployed on the flanks of Augustine Volcano by the Alaska Volcano Observatory and invert them by the body wave local earthquake tomography. The resulting model includes the 3D distributions of the P and S wave velocities (Vp and Vs), Vp/Vs ratio and the relocated seismic events. The resolved area extends down to the depth of 2 km below sea level (b.s.l). At shallow depths, within the edifice, we observe a contrasted anomaly with very high Vp, low Vs and very high Vp/Vs, which represents the rigid volcano core composed of igneous rocks (high Vp), but strongly fractured and saturated with fluids and melts (low Vs). At larger depths, below sea level, we retrieve a columnar anomaly of high Vp/Vs ratio representing the magma conduit. On the top of this anomaly, there is a zone with low Vp/Vs coinciding with the strong seismicity cluster, which is associated with the degassing of deep fluids.