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A non-linear Bayesian Monte-Carlo method is presented to estimate a Vsv model beneath stations by jointly interpreting Rayleigh wave dispersion and receiver functions and associated uncertainties. The method is designed for automated application to large arrays of broad-band seismometers. As a testbed for the method, 185 stations from the USArray Transportable Array are used in the IntermountainWest, a region that is geologically diverse and structurally complex. Ambient noise and earthquake tomography are updated by applying eikonal and Helmholtz tomography, respectively, to construct Rayleighwave dispersion maps from 8 to 80 s across the study region with attendant uncertainty estimates.Amethod referred to as ‘harmonic stripping method’ is described and applied as a basis for quality control and to generate backazimuth independent receiver functions for a horizontally layered, isotropic effective medium with uncertainty estimates for each station. A smooth parametrization between (as well as above and below) discontinuities at the base of the sediments and crust suffices to fit most features of both data types jointly across most of the study region. The effect of introducing receiver functions to surface wave dispersion data is quantified through improvements in the posterior marginal distribution of model variables. Assimilation of receiver functions quantitatively improves the accuracy of estimates of Moho depth, improves the determination of the Vsv contrast across Moho, and improves uppermost mantle structure because of the ability to relax a priori constraints. The method presented here is robust and can be applied systematically to construct a 3-D model of the crust and uppermost mantle across the large networks of seismometers that are developing globally, but also provides a framework for further refinements in the method.

SUMMARY Observations of seismic anisotropy in continental crust are of primary importance to understanding deep crustal structure and dynamics. While many crustal minerals are elastically anisotropic, seismic studies commonly assume that only the most anisotropic minerals (e.g. micas) are responsible for the observed anisotropic signal while making the simplifying assumption that potential interaction of multiple mineral phases is insignificant. This study explores the interference effects between mica and quartz on the calculated response of compressional and shear waves in high-strain tectonites (mylonites). Sample texture and mineralogy were evaluated via electron backscatter diffraction and the calculation of seismic response was carried out over a range of mica to quartz ratios. Both natural and synthetic crystallographic orientation data, which mimic quartz and mica orientations under increasing temperature in high, non-coaxial strain settings, were evaluated. Not surprisingly, the results show that adding quartz to micaceous lithologies significantly reduces the magnitude of seismic anisotropy. However, the study also highlights the mutually destructive nature of this relationship and suggests a critical threshold proportion between the two phases across which the symmetry of the end-member induced anisotropies becomes significantly altered, a result that may have significant implications for studies interpreting crustal dynamics from seismic anisotropy.

Upper-mantle seismic anisotropy has been observed using a variety of methods, including S and SKS splitting, P and P n traveltimes, P polarization anomalies and P to S conversions, and surface waves. Care must be taken when comparing the results from different methods because of bias introduced by depth sensitivity, frequency dependence, and simplifying assumptions concerning the form of anisotropy. We examine the differences and show that some apparent contradictions cited in previous studies can be reconciled using simple models. We perform forward modelling on a suite of anisotropic media, progressing from simple elastic symmetries to tensors obtained from laboratory measurements and numerical strain models. The results provide a systematic overview of the effect of a given anisotropy class and geometry on seismic observables. We simulate the full complement of body wave measurements-SKS and S splitting, P n traveltimes, teleseismic P traveltimes and teleseismic P particle motion (P p o l )-to show any apparent differences between the phases. We also investigate depth and frequency sensitivity using reflectivity modelling in layered anisotropic media. Our principal findings are as follows. (1) No models, including low-order symmetries and multiple layers, exhibit a mean fast shear wave splitting direction nearly orthogonal to a consistent fast direction determined from P observables. For P delays, the azimuthal cos(2θ) variation is representative of the fast direction of anisotropy (rather than cos(1θ), which has led to a certain amount of confusion in the literature). (2) P times average linearly over the raypath; SKS weights toward the upper part of the model; and P p o l and P n are even more sensitive to shallow anisotropy. Conclusive evidence in the literature for a disagreement between fast directions from SKS, on one hand, and P n and P p o l , on the other hand, can be explained by layering. (3) The azimuthal dependence of SKS splitting results does not necessarily indicate layered or laterally heterogeneous anisotropy. The azimuthal dependence of SKS splitting is not observed for hexagonal symmetry with horizontal fast or slow axes, but has to be taken into consideration for dipping hexagonal and any orthorhombic and lower symmetry media. Teleseismic S shows a much stronger azimuthal dependence than SKS and SKKS. This makes procedures that stack splitting results over a wide range of incidence angles or azimuths questionable.