Seismic Azimuthal Anisotropy Beneath the Alaska Subduction Zone

• Published in: Geophysical research letters 2024-07, Vol.51 (14), p.n/a
• Main Authors: Liu, Chuanming, Sheehan, Anne F., Ritzwoller, Michael H.
• Format: Article
• Language: English
• Subjects: Alaska subduction zone; aleutian subduction zone; Anisotropy; azimuthal anisotropy; Fault detection; Fault lines; Fossils; Fractures; Oceanic trenches; Offshore; P-waves; Rayleigh waves; Ridges; S waves; Segments; seismic anisotropy; Seismic propagation; Seismic velocities; Seismic wave propagation; Seismic waves; Shear wave velocities; Subduction; Subduction (geology); Subduction zones; surface wave; surface wave tomography; Variation; Wave dispersion; Wave propagation; Wave velocity
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2024GL109758

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. Plain Language Summary The azimuthal anisotropy of seismic waves refers to the directional dependence of the seismic wave propagation speed. We present a comprehensive azimuthal anisotropy model of the Alaska subduction zone to a depth of 200 km, revealing anisotropy caused by local faults and fractures, fossil anisotropy inherited from the oceanic plate within the subducted slab, and sub‐slab mantle flow. The along‐strike variation of crustal anisotropy indicates variations in the stress regime in the forearc region. The along‐strike variation of anisotropy within the subducted slab identifies different origins of the subducted slab. Our model contributes to the understanding of the anisotropic structure and the sources of anisotropy in subduction zones. Key Points A new model of depth‐dependent azimuthal anisotropy of the Alaska subduction zone is built based on a new surface wave dataset The along‐strike variation in the azimuthal anisotropy of the forearc crust is caused by faults and fractures Azimuthal anisotropy within the subducted slab is controlled by fossil anisotropy produced at different mid‐ocean ridges