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Abstract With substantial postglacial rhyolite eruptions and ongoing rapid uplift, the Laguna del Maule volcanic field in the southern Andes provides an exceptional opportunity to study the dynamics of an active silicic magmatic system. Using 4,093Parrivals from 137 distant earthquakes recorded by 44 local stations over∼2.25 years, we conduct teleseismic tomography to image the crustal structure down to 40 km below the volcanic field. A prominent low‐velocity body at depths between∼0 and 12 km below sea level (b.s.l.), characterized by a volume of∼500 km³and a peak anomaly of−400 m/s (∼9%), overlaps the location of the upper‐crustal magma reservoir detected in recent gravity and surface wave tomography studies. Its estimated averagePwave velocity of∼4.6 km/s corresponds to an average melt fraction of about 14% and a melt volume of∼70 km³. Petrologic observations are also consistent with generation and storage of rhyolitic melts at depths corresponding to the anomalous zone. Moreover, the tomographic results support a lower crust zone of MASH (melting, assimilation, storage, and homogenization) from a depth of∼25 km to the base of the model, which likely reflects a deep crustal source of magma that contributes to and incubates the shallow silicic reservoir. 

Abstract The Laguna del Maule (LdM) volcanic field comprises the greatest concentration of postglacial rhyolite in the Andes and includes the products of ~40 km³of explosive and effusive eruptions. Recent observations at LdM by interferometric synthetic aperture radar and global navigation satellite system geodesy have revealed inflation at rates exceeding 20 cm/year since 2007, capturing an ongoing period of growth of a potentially large upper crustal magma reservoir. Moreover, magnetotelluric and gravity studies indicate the presence of fluids and/or partial melt in the upper crust near the center of inflation. Petrologic observations imply repeated, rapid extraction of rhyolitic melt from crystal mush stored at depths of 4–6 km during at least the past 26 ka. We utilize multiple types of surface‐wave observations to constrain the location and geometry of low‐velocity domains beneath LdM. We present a three‐dimensional shear‐wave velocity model that delineates a ~450‐km³shallow magma reservoir ~2 to 8 km below surface with an average melt fraction of ~5%. Interpretation of the seismic tomography in light of existing gravity, magnetotelluric, and geodetic observations supports this model and reveals variations in melt content and a deeper magma system feeding the shallow reservoir in greater detail than any of the geophysical methods alone. Geophysical imaging of the LdM magma system today is consistent with the petrologic inferences of the reservoir structure and growth during the past 20–60 kyr. Taken together with the ongoing unrest, a future rhyolite eruption of at least the scale of those common during the Holocene is a reasonable possibility.