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[1] Moho depth and Vp/Vs estimates from stacking phases of receiver functions along the Aleutian island arc give new constraints on its composition and structure. They expand on the current understanding of island arcs and their relationship to continental crust production. We also present an approach for including constraints from active-source data in receiver function analysis in a region with sparse data coverage to complement this analysis. Moho depth averages 37.5 km with an average uncertainty of 2.5 km along the entire arc. Excluding the westernmost island of Attu yields an average crustal thickness of 38.5 6 2.9 km. The Vp/Vs ratio decreases moving eastward along the arc with an average value of 1.80 in the western and central portion of the arc built on oceanic crust, but 1.63 in the eastern section built on continental crust. This may reflect tectonic and compositional changes along the arc. However, overall the arc appears more mafic than continental crust. Near-constant crustal thickness, despite significant compositional changes, may indicate that nonmagmatic processes such as erosion and isostasy act to regulate arc thickness. Additionally, strong conversions from an upper crustal magma chamber are observed beneath Akutan Island, confirming and clarifying the geometry of the magma body inferred from other techniques. They indicate a volcanic body much larger than the eruptive edifice, a feature that must persist between eruptive cycles.

SUMMARY A new amphibious seismic data set from the Cascadia subduction zone is used to characterize the lithosphere structure from the Juan de Fuca ridge to the Cascades backarc. These seismic data are allowing the imaging of an entire tectonic plate from its creation at the ridge through the onset of the subduction to beyond the volcanic arc, along the entire strike of the Cascadia subduction zone. We develop a tilt and compliance correction procedure for ocean-bottom seismometers that employs automated quality control to calculate robust station noise properties. To elucidate crust and upper-mantle structure, we present shoreline-crossing Rayleigh-wave phase-velocity maps for the Cascadia subduction zone, calculated from earthquake data from 20 to 160 s period and from ambient-noise correlations from 9 to 20 s period. We interpret the phase-velocity maps in terms of the tectonics associated with the Juan de Fuca plate history and the Cascadia subduction system. We find that thermal oceanic plate cooling models cannot explain velocity anomalies observed beneath the Juan de Fuca plate. Instead, they may be explained by a ≤1 per cent partial melt region beneath the ridge and are spatially collocated with patches of hydration and increased faulting in the crust and upper mantle near the deformation front. In the forearc, slow velocities appear to be more prevalent in areas that experienced high slip in past Cascadia megathrust earthquakes and generally occur updip of the highest-density tremor regions and locations of intraplate earthquakes. Beneath the volcanic arc, the slowest phase velocities correlate with regions of highest magma production volume.