Inferring the oriented elastic tensor using surface wave observations: Contrasting the western US with eastern Tibet

Abstract:

Radial and azimuthal anisotropy in seismic wave speeds have long been observed using surface waves, which are believed to be controlled by 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. We describe a method of inversion that interprets simultaneous observations of radial and azimuthal anisotropy under the assumption of a hexagonally symmetric elastic tensor with a tilted symmetry axis defined by dip and strike angles. Our inferences occur within the framework of a Bayesian Monte Carlo inversion, which yields a posterior distribution that reflects both variances of and covariances between all model variables. We show how uncertainties in the surface wave travel time measurements and prior hypotheses affect the posterior distribution of models, and compare results from the western US and Tibet. In the western US, the data can be explained well by a models whose anisotropic symmetry axis is oriented at a constant angle with depth in the crust. In contrast, across eastern Tibet, the data require that the symmetry axes are oriented differently between the upper and middle-to-lower crust. In Tibet, the data prefer models with steeply dipping upper crustal and shallowly dipping middle/lower crustal symmetry axes, which may reflect a fractured brittle upper crust above a ductile middle/lower crust dominated by subhorizontal flow.