Velocity Structure of the Subducted Yakutat Terrane, Alaska: Insights From Guided Waves

Dispersed $\textit{P}$ wave arrivals from intermediate‐depth earthquakes in the Alaskan subduction zone provide insight into the low‐velocity structure of the subducting oceanic crust. $\textit{P}$ wave arrivals from 41 earthquakes in the eastern section of the arc show significant guided wave dispersion, with high‐frequency (>1 Hz) energy delayed by up to 2–3 s. We simulate this dispersion using a 2‐D finite difference waveform propagation model, systematically varying both $\textit{P}$ wave velocity and low‐velocity layer thickness parameters to find the lowest misfit between the observed and synthetic waveforms. We infer a 6 to 10 km thick low‐velocity layer with a $\textit{P}$ wave velocity contrast of 7–15% with the overriding mantle, velocities which cannot be entirely accounted for by metamorphosed mid‐ocean ridge basalt compositions. We postulate that this structure is the remnant of the subducted Yakutat terrane, significantly thinned at depth by metamorphism or delamination of material during subduction.