Azimuthal Anisotropy of the Crust and Uppermost Mantle Beneath Alaska

Abstract

This study presents an azimuthally anisotropic shear wave velocity model of the crust and uppermost mantle beneath Alaska, based on Rayleigh wave phase speed observations from 10 to 80 s period recorded at more than 500 broadband stations. We test the hypothesis that a model composed of two homogeneous layers of anisotropy can explain these measurements. This "Two-Layer Model" confines azimuthal anisotropy to the brittle upper crust along with the uppermost mantle from the Moho to 200 km depth. This model passes the hypothesis test for most of the region of study, from which we draw two conclusions. (a) The data are consistent with crustal azimuthal anisotropy being dominantly controlled by deformationally aligned cracks and fractures in the upper crust undergoing brittle deformation. (b) The data are also consistent with the uppermost mantle beneath Alaska and surroundings experiencing vertically coherent deformation. The model resolves several prominent features. (1) In the upper crust, fast directions are principally aligned with the orientation of major faults. (2) In the upper mantle, fast directions are aligned with the compressional direction in compressional tectonic domains and with the tensional direction in tensional domains. (3) The mantle fast directions located near the Alaska-Aleutian subduction zone and the surrounding back-arc area form a toroidal pattern that is consistent with mantle flow directions predicted by recent geodynamical models. Finally, the mantle anisotropy is remarkably consistent with SKS fast directions, but to fit SKS split times, anisotropy must extend below 200 km depth across most of the study region.

Amphibious Shear Wave Structure Beneath the Alaska‐Aleutian Subduction Zone From Ambient Noise Tomography

“…Fast orientations of shear wave splitting axis across Alaska constitute a toroidal pattern (e.g., Hanna & Long, 2012;Venereau et al, 2019), consistent with the geodynamical modeling (e.g., Haynie & Jadamec, 2017;Jadamec & Billen, 2010). The toroidal mantle flow pattern is also confirmed by a recent surface wave tomography study (Feng et al, 2020), in which the authors further suggested that the vast majority regions across Alaska undergo vertically coherent mantle deformation from the Moho to a depth deeper than 200 km.…”

“…However, future work is needed to continue to improve resolution of the model, particularly near the Aleutian trench line where the North American plate overriding the incoming Pacific slab. Other fertile directions to pursue in order to improve and extend the model is to infer seismic anisotropy, including azimuthal and radial anisotropy (e.g., Feng et al., 2020; Feng & Ritzwoller, 2019), which could yield more information about the physical properties of the plate and help us better understand the link between plate attributes and seismic behavior.…”

“…There are also seismic studies which investigated subducting zone structures using body wave tomography (e.g., Eberhart‐Phillips, et al., 2006; Gou et al., 2019), surface wave tomography (e.g., Berg et al., 2020; Feng et al., 2020; Feng & Ritzwoller, 2019; Jiang et al., 2018; Nayak et al., 2020; Wang & Tape, 2014; Y. Wang & Tape, 2014; Ward, 2015), receiver functions (e.g., Miller & Moresi, 2018; Zhang et al., 2019) and shear wave splitting (e.g., Hanna & Long, 2012; Venereau et al., 2019). Important isotropic structures, including the geometry of the subducting Pacific slab and the Yakutat microplate, are resolved by numerous seismic tomography studies (e.g., Berg et al., 2020; Feng & Ritzwoller, 2019; Gou et al., 2019; Jiang et al., 2018; Nayak et al., 2020).…”

Subduction of an Oceanic Plateau Across Southcentral Alaska: Scattered‐Wave Imaging

“…From Rayleigh waves, Feng et al. (2020) measure upper mantle anisotropy fast directions and show a toroidal pattern around the subducting Yakutat slab in the Denali segment. This toroidal pattern was interpreted as clockwise flow around the Denali segment subducting slab, as predicted by geodynamic models (e.g., Jadamec & Billen, 2010).…”

Mantle Flow in the Vicinity of the Eastern Edge of the Pacific‐Yakutat Slab: Constraints From Shear Wave Splitting Analyses

“…Fourth, due to the near‐vertical incidence of the XKS phases, the depth of the source of the observed XKS splitting remains ambiguous. Fifth, some aspects of the mantle flow systems in the slab edge region, including the lateral extent of the toroidal flow and the existence or absence of an eastward continuation of the sub‐slab flow after it comes out of the slab edge, are still not well understood (Feng et al., 2020; Hanna & Long, 2012; Jadamec & Billen, 2010; Venereau et al., 2019).…”