Azimuthal anisotropy retrieved from surface waves is important for constraining depthvarying deformation patterns in the crust and upper mantle. We present a direct inversion technique for the three-dimensional shear wave speed azimuthal anisotropy based on mixed-path surface wave traveltime data. This new method includes two steps: (1) inversion for the 3-D isotropic Vsv model directly from Rayleigh wave traveltimes and (2) joint inversion for both 3-D Vsv azimuthal anisotropy and additional 3-D isotropic Vsv perturbation. The joint inversion can significantly mitigate the trade-off between strong heterogeneity and anisotropy. With frequency-dependent ray tracing based on 2-D isotropic phase speed maps, the new method takes into account the ray-bending effect on surface wave propagation. We apply the new method to a regional array in Yunnan, southwestern China. Using Rayleigh wave phase velocity dispersion data in the period band of 5-40 s extracted from ambient noise interferometry, we obtain a 3-D model of shear wave speed and azimuthal anisotropy in the crust and uppermost mantle in Yunnan. This model reveals that two midcrust low-velocity zones are possible weak channels, and the azimuthal anisotropy at a depth of 5 to 30 km is mainly controlled by nearby strike-slip faults, some of which also approximately coincide with the lateral boundaries of the crustal low-velocity zones. Approximately south of 26°N, the upper crustal azimuthal anisotropy from our model is significantly different from the upper mantle anisotropy inferred by shear wave splitting, indicating different deformation styles between the crust and upper mantle in southern Yunnan.

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.

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The North American plate in Alaska is an amalgamation of many tectonic terranes (e.g., Colpron et al., 2007) (Figure 1a). The present terranes result from multiple long-term episodes of accretion, accumulation, and deformation due to the dynamic and complex plate boundary interactions of the Pacific, Siberian, and North American Plates (e.g., Shephard et al., 2013). Northern Alaska is a passive margin and was accreted in the late Jurassic (e.g., Moore & Box, 2016;Till, 2016). The Alaskan Interior experienced complex collisional and extensional deformation in the Permian and Cretaceous periods (e.g., Johnston, 2001;Plafker & Berg, 1994). In southern Alaska, accreted terranes docked in the late Mesozoic to Cenozoic, and strike-slip zones developed due to the subduction of the Pacific Plate and Yakutat microplate (e.g., Shephard et al., 2013). Offshore southern Alaska, the Alaska-Aleutian subduction zone is one of the most tectonically and seismically active convergent margins in the world. Historically, a series of great earthquakes have occurred along the megathrust boundary (Figure 1a).With the recent deployment of the USArray Transportable Array (TA) in continental Alaska (Busby & Aderhold, 2020) and the Alaska Amphibious Community Seismic Experiment (AACSE) in the Alaska Peninsula and offshore (Barcheck et al., 2020), as well as other regional networks, new seismic data have become available to investigate the tectonic architecture and history of deformation of Alaska and the subduction zone both onshore and offshore (Figure 1b). Previous seismic studies in Alaska have focused both on the isotropic and anisotropic structure of the crust and mantle. Isotropic seismic models have been presented based on seismic reflection and