We develop a 3-D isotropic shear velocity model for the Alaska
subduction zone using data from seafloor and land-based seismographs to
investigate along-strike variations in structure. By applying ambient
noise and teleseismic Helmholtz tomography, we derive Rayleigh wave
group and phase velocity dispersion maps, then invert them for shear
velocity structure using a Bayesian Monte Carlo algorithm. For
land-based stations, we perform a joint inversion of receiver functions
and dispersion curves. The forearc crust is relatively thick (35-42 km)
and has reduced lower crustal velocities beneath the Kodiak and Semidi
segments, which may promote higher seismic coupling. Bristol Bay Basin
crust is relatively thin and has a high-velocity lower layer, suggesting
a dense mafic lower crust emplaced by the rifting processes. The
incoming plate shows low uppermost mantle velocities, indicating
serpentinization. This hydration is more pronounced in the Shumagin
segment, with greater velocity reduction extending to 18 ± 3 km depth,
compared to the Semidi segment, showing smaller reductions extending to
14 ± 3 km depth. Our estimates of percent serpentinization from
V reduction and V/V
are larger than those determined using V reduction in
prior studies, likely due to water in cracks affecting
V more than V. Revised estimates of
serpentinization show that more water subducts than previous studies,
and that twice as much mantle water is subducted in the Shumagin segment
compared to the Semidi segment. Together with estimates from other
subduction zones, the results indicate a wide variation in subducted
mantle water between different subduction segments.