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Summary For a weakly anisotropic medium, Rayleigh and Love wave phase speeds at angular frequency ω and propagation azimuth ψ are given approximately by V(ω, ψ) = A0 + A2ccos 2ψ + A2ssin 2ψ + A4ccos 4ψ + A4ssin 4ψ. Earlier theories of the propagation of surface waves in anisotropic media based on non-degenerate perturbation theory predict that the dominant components are expected to be 2ψ for Rayleigh waves and 4ψ for Love waves. This paper is motivated by recent observations of the the 2ψ component for Love waves and 4ψ for Rayleigh waves, referred to here as “unexpected anisotropy”. To explain these observations, we present a quasi-degenerate theory of Rayleigh-Love coupling in a weakly anisotropic medium based on Hamilton’s Principle in Cartesian coordinates, benchmarking this theory with numerical results based on SPECFEM3D. We show that unexpected anisotropy is expected to be present when Rayleigh-Love coupling is strong and recent observations of Rayleigh and Love wave 2ψ and 4ψ anisotropy can be fit successfully with physically plausible models of a depth-dependent tilted transversely isotropic (TTI) medium. In addition, when observations of the 2ψ and 4ψ components of Rayleigh and Love anisotropy are used in the inversion, the ellipticity parameter ηX, introduced here, is better constrained, we can constrain the absolute dip direction based on polarization measurements, and we provide evidence that the mantle should be modeled as a tilted orthorhombic medium rather than a TTI medium. Ignoring observations of unexpected anisotropy may bias the estimated seismic model significantly. We also provide information about the polarization of the quasi-Love waves and coupling between fundamental mode Love and overtone Rayleigh waves in both continental and oceanic settings. The theory of SV-SH coupling for horizontally propagating body waves is presented for comparison with the surface wave theory, with emphasis on results for a TTI medium. 

Abstract A shallow sub‐seafloor seismic model that includes well‐determined seismic velocities and clarifies sediment‐crust discontinuities is needed to characterize the physical properties of marine sediments and the oceanic crust and to serve as a reference for deeper seismic modeling endeavors. This study estimates the seismic structure of marine sediments and the shallow oceanic crust of the Alaska‐Aleutian subduction zone at the Alaska Peninsula, using data from the Alaska Amphibious Community Seismic Experiment (AACSE). We measure seafloor compliance and Ps converted wave delays from AACSE ocean‐bottom seismometers (OBS) and seafloor pressure data and interpret these measurements using a joint Bayesian Monte Carlo inversion to produce a sub‐seafloor S‐wave velocity model beneath each available OBS station. The sediment thickness across the array varies considerably, ranging from about 50 m to 2.80 km, with the thickest sediment located in the continental slope. Lithological composition plays an important role in shaping the seismic properties of seafloor sediment. Deep‐sea deposits on the incoming plate, which contain biogenic materials, tend to have reduced S‐wave velocities, contrasting with the clay‐rich sediments in the shallow continental shelf and continental slope. A difference in S‐wave velocities is observed for upper oceanic crust formed at fast‐rate (Shumagin) and intermediate‐rate (Semidi) spreading centers. The reduced S‐wave velocities in the Semidi crust may be caused by increased faulting and possible lithological variations, related to a previous period of intermediate‐rate spreading. 

Abstract We estimate depth‐dependent azimuthal anisotropy and shear wave velocity structure beneath the Alaska subduction zone by the inversion of a new Rayleigh wave dispersion dataset from 8 to 85 s period. We present a layered azimuthal anisotropy model from the forearc region offshore to the subduction zone onshore. In the forearc crust, we find a trench‐parallel pattern in the Semidi and Kodiak segments, while a trench‐oblique pattern is observed in the Shumagins segment. These fast directions agree well with the orientations of local faults. Within the subducted slab, a dichotomous pattern of anisotropy fast axes is observed along the trench, which is consistent with the orientation of fossil anisotropy generated at the mid‐ocean ridges of the Pacific‐Vancouver and Kula‐Pacific plates that is preserved during subduction. Beneath the subducted slab, a trench‐parallel pattern is observed near the trench, which may indicate the direction of mantle flow. 

Abstract Comprehensive observations of surface wave anisotropy across Alaska and the Aleutian subduction zone would help to improve understanding of its tectonics, mantle dynamics, and earthquake risk. We produce such observations, using stations from the USArray Transportable Array, regional networks across Alaska, and the Alaska Amphibious Community Seismic Experiment in the Alaska‐Aleutian subduction zone both onshore and offshore. Our data include Rayleigh and Love wave phase dispersion from earthquakes (28–85 s) and ambient noise two‐ and three‐station interferometry (8–50 s). Compared with using two‐station interferometry alone, three‐station interferometry significantly improves the signal‐to‐noise ratio and approximately doubles the number of measurements retained. Average differences between both isotropic and anisotropic tomographic maps constructed from different methods lie within their uncertainties, which is justification for combining the measurements. The composite tomographic maps include Rayleigh wave isotropy and azimuthal anisotropy from 8 to 85 s both onshore and offshore, and onshore Love wave isotropy from 8 to 80 s. In the Alaska‐Aleutian subduction zone, Rayleigh wave fast directions vary from trench parallel to perpendicular and back to parallel with increasing periods, apparently reflecting the effect of the subducted Pacific Plate. The tomographic maps provide a basis for inferring the 3‐D anisotropic crustal and uppermost mantle structure across Alaska and the Aleutian subduction zone.