Inferring the oriented elasticity tensor in the crust across the Western US using surface wave data

Radial and azimuthal anisotropy has long been observed using surface waves and are believed to be controlled by earlier episodes of deformation within the Earth’s crust and uppermost mantle. Although radial and azimuthal anisotropy reflect important aspects of anisotropic media, few studies have tried to interpret them jointly. In this study, we describe a method of inversion that reconciles simultaneous observations of radial and azimuthal anisotropy under the assumption of a hexagonally symmetric medium with a tilted symmetry axis defined by a dip and strike angle. Our inferences occur within the framework of Bayesian Monte Carlo method, which yields a posterior distribution that reflects both the data and prior constraints.

We show that observations of radial anisotropy and the 2ψ component of azimuthal anisotropy for Rayleigh waves obtained using USArray data in the western US can be fit well assuming tilted hexagonally symmetric elastic tensors in the crust and uppermost mantle. Major results include the following. (1) Inherent S-wave anisotropy (γ) is fairly homogeneous vertically across the crust, on average, and spatially across the western US. (2) Averaging over the region of study and in depth, γ in the crust is approximately 4.1%±2%. (3) The crustal strike angle of anisotropy (defined by the intersection of the foliation plane with earth’s surface) in the posterior distribution bifurcates into two disjoint sets of models. (4) γ in the crust is approximately the same in the two groups of models. (5) Dip angles in the two groups of models vary spatially in similar ways and display geological coherence. (6) However, Rayleigh wave fast axis directions are ambiguously related to the strike of anisotropy and are oriented orthogonal to the strike angle in the geologically preferred group of model. (7) The estimated dip angle may be interpreted in two alternative ways. It is either an actual measurement of the dip of the foliation plane of anisotropic material within the crust or it is proxy for another non-geometric variable, most likely a measure of the deviation from hexagonal symmetry of the medium.

Overall, joint analysis of radial and azimuthal anisotropy provides a more comprehensive, although still incomplete, estimation of the inherent crustal elastic moduli that cannot be achieved by either data alone.