Magma-Fluid Interactions Beneath Akutan Volcano in the Aleutian Arc Based on the Results of Local Earthquake Tomography

Akutan Island hosts a volcano considered as one of most active in the Aleutian Arc. We build a new tomography model including the 3D distributions of the Vp, Vs, and Vp/Vs ratio based on arrival time data from more than 4,000 local earthquakes recorded by 13 seismic stations. In this model, we reveal a columnar anomaly of high Vp, low Vs, and high Vp/Vs ratio with a top boundary at a depth of ∼6 km below sea level, which represents a steady conduit feeding the Akutan volcano. In the upper part of the tomographic model, the highly heterogeneous structures are associated with interactions of shallow magmatic sources, meteoric and magmatic fluids, as well as degassing. Beneath the summit area of Akutan, we observe a prominent anomaly of high Vp/Vs, which may represent a shallow magma reservoir directly responsible for recent eruption activity and ongoing gas emission through the summit fumarole. The other fumarole field at the eastern flank of the volcano appears to be associated with a seismically active zone of low Vp/Vs ratio at depths of 2–4 km below surface. These structures indicate different depths of magma degassing in these two areas, which may explain distinct geochemical features of emitted gases. Besides the prominent anomaly representing the shallow magma reservoir beneath the caldera and the active cone, we observe several areas with high Vp/Vs ratio, some of which are interpreted as shallow magma storage regions, and some as zones of meteoric water penetration.