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.