Azimuthal Anisotropy of the North American Upper Mantle Based on Full Waveform Inversion

Abstract

AbstractA new azimuthal anisotropy model for the North American and Caribbean Plates, namely, , is constructed based on full waveform inversion and records from the USArray and other temporary/permanent networks deployed in the study region. A total of 180 earthquakes and 4,516 seismographic stations are employed in the inversion to simultaneously constrain radially and azimuthally anisotropic model parameters: , , , and , within the crust and mantle. Thirty‐two preconditioned conjugate gradient iterations have been utilized to minimize frequency‐dependent phase discrepancies between observed and predicted seismograms for three‐component short‐period (15–40 s) body waves and long‐period (25–100 s) surface waves. Model exhibits complicated variations in anisotropic fabrics underneath the western and eastern United States, especially at depths shallower than 100 km. For instance, the fast axis orientations in model suggest the presence of trench‐perpendicular mantle flows underneath the Cascadia Subduction Zone and also follow the strikes of the Snake River Plain, the Ouachita Orogenic Front, and the Grenville and Appalachian Orogenic Belts. The amplitudes of azimuthal anisotropy reduce to around 1% at depths greater than 200 km, and the orientations are subparallel to the global plate motion directions to the east of the Rocky Mountain, except for large discrepancies in central and eastern Canada. At a depth of 700 km, the fast axes change along the trajectory of the Farallon slab underneath the Great Lakes region and Gulf of Mexico, which might indicate the development of 2‐D poloidal‐mode mantle flows perpendicular to the strike of the sinking slab within the uppermost lower mantle. Comparisons between model with a western U.S. model from ambient noise tomography and SKS splitting measurements demonstrate a relatively good agreement for the fast axis orientations, considering the usage of different data sets and imaging techniques. However, the absolute magnitude of azimuthal anisotropy in model might be underestimated, especially at greater depths, given the poor agreement on the amplitudes of predicted and observed SKS splitting times. At the current stage, the agreement among different azimuthal anisotropy models at global and continental scales is still poor even for the United States with a dense station coverage.