Volcanic seismicity provides essential insights into the behavior of an active volcano across multiple time scales. However, to understand how magma moves as an eruption evolves, better knowledge of the geometry and physical properties of the magma plumbing system is required. In this study, using full-wave ambient noise tomography, we image the 3-D crustal shear-wave velocity structure below GreatSitkin Volcano in the central Aleutian Arc. The new velocity model reveals two low-velocity anomalies, which correlate with the migration of volcanic seismicity. With a partial melt of up to about 30%, these low-velocity anomalies are characterized as mushy magma reservoirs. We propose a six-stage eruption cycle to explain the migration of seismicity and the alternating eruption of two reservoirs with different recharging histories. The findings in this study have broad implications for the dynamics of magma plumbing systems and the structural control of eruption behaviors.

Volcanic eruptions progress by co-evolving fluid and solid systems. The fluid mechanics can be observed through the plumes and ejecta produced, but how does the solid system evolve? When does the conduit open? When does it close? Seismology can potentially tell us about these processes by measuring the failure of the solid rock. Such inferences require the detection of earthquakes during an explosive eruption. Standard earthquake detection methods often fail during this time as the eruption itself produces seismic waves that obscures the earthquake signals. We address this problem by applying supervised and unsupervised search techniques to the existing catalog of the 2008 Okmok eruption to find brittle failure signals during the continuous eruptive sequence. The interaction between fluid pathways and seismicity is reinforced by high precision earthquake relocations that highlight a ring-fault structure, which may be acting as a conduit for fluids to the surface. The timing of the earthquakes during the eruption reveal that the seismicity gradually increases during the vent-opening stage (July 12-July 24), peaks during the vent-widening stage (July 24-August 1) which culminates in a large burst of earthquakes, and then gradually decrease until the end of the eruptive period. Seismic bursts during the eruption are not synchronized with the exhalation of large ash and steam plumes. In other words, when the system is closed, the rock breaks. We call this scenario clog and crack.