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SUMMARY The sensitivity of Rayleigh wave amplitude to Earth structure has applications to seismic tomography, both in cases where amplitude information is used to supplement phase velocity data to improve images of elastic parameters, and to correct amplitudes for local Earth structure in attenuation tomography. We review the theoretical basis of the ray theoretical approximation, in which the wave amplitudes are controlled by a combination of geometrical spreading and local changes in energy density due to Earth structure. We focus mainly on the latter effect, which we term the constant energy flux approximation. We investigate the ray theoretical basis for this approximation, test it against a full waveform simulation that verifies its accuracy and show how it can be used to compute the sensitivity of amplitude to elastic moduli and density. We investigate how perturbing these parameters in a set of simple Earth models affects Rayleigh wave amplitudes, and demonstrate that a slow velocity heterogeneity can cause either increased or reduced amplitudes, depending upon the depth of the heterogeneity and the observation frequency. Consequently, amplitude sensitivity can be either positive or negative, and its magnitude can vary significantly with frequency. Although an added complication, the very different behaviour of phase velocity and amplitudes to changes in Earth structure implies that the two types of data are complementary and suggest the effectiveness of using both in Rayleigh wave tomography. 

Many researchers have used the birefringence of P‑to‑S converted waves from the Moho discontinuity to constrain the anisotropy of Earth’s crust. However, this practice ignores the substantial influence that anisotropy has on the initial amplitude of the converted wave, which adds to the splitting acquired during its propagation from Moho to the seismometer. We find that large variations in Ps birefringence estimates with back-azimuth occur theoretically in the presence of P‑wave anisotropy, which normally accompanies S‑wave anisotropy. The variations are largest for crustal anisotropy with a tilted axis of symmetry, a geometry that is often neglected in birefringence interpretations, but is commonly found in Earth’s crust. We simulated globally-distributed P‑coda datasets for 36 distinct 4‑layer crustal models with combinations of elliptical shear anisotropy or compressional anisotropy, and also incorporated the higher-order anisotropic Backus parameter C. We tested both horizontal and tilted symmetry-axis geometries and tested the birefringence tradeoff associated with Ps converted phases at the top and bottom of a thin high‑ or low‑velocity basal layer. We computed composite receiver functions (RFs) with harmonic regression over back azimuth, using multipletaper correlation with moveout corrections for the epicentral distances of 471 events, to simulate a realistic data set. We estimate Ps birefringence from the radial and transverse RFs, a strategy that is similar to previous studies. We find that Ps splitting can be a useful indicator of bulk crustal anisotropy only under restricted circumstance, either in media with no compressional anisotropy, or if the symmetry axis is horizontal throughout. In other, more-realistic cases, the inferred fast polarization of Ps birefringence estimated from synthetic RFs tends either to drift with back-azimuth, form weak penalty-function minima, or return splitting times that depend on the thickness of an anisotropic layer, rather than the birefringence accumulated within it.   

Surrounded by subducting slabs and continental keels, the upper mantle of the Pacific is largely prevented from mixing with surrounding areas. One possible outlet is beneath the southern part of the Central American isthmus, where regional observations of seismic anisotropy, temporal changes in isotopic composition of volcanic eruptions, and considerations of dynamic topography all suggest upper mantle flow from the Pacific to the Caribbean. We derive new constraints on the nature of seismic anisotropy in the upper mantle of southern Costa Rica from observations of birefringence in teleseismic shear waves. Fast and slow components separate by ~1 s, with faster waves polarized along the 40°–50° (northeast) direction, near-orthogonally to the Central American convergent margin. Our results are consistent with upper mantle flow from the Pacific to the Caribbean and require an opening in the lithosphere subducting under the region. 

Abstract Abrupt velocity gradients in the upper mantle, detectable by receiver functions (RF) techniques, have been known to exist down to the depths of ∼110 km beneath northeastern North America. Comparisons with the surface wave velocity models have designated some negative velocity gradients (NVGs) as the lithosphere‐asthenosphere boundary (LAB), delineating a relatively thin lithosphere beneath this region. This work presents a systematic survey of upper mantle layering in seismic properties using P‐S RF analysis at 62 long‐running sites with dense lateral sampling. We examine both radial and transverse component RF for indicators of seismic anisotropy and adopt the notion of seismic attributes, utilized in active‐source seismology, to characterize the spatial distribution of directionally variant and invariant signal components. We confirm a widespread presence of NVGs at depths 60–100 km throughout the region, consistent with previous studies using mode‐converted body waves. We also find numerous converting boundaries that reflect changes in directional variation (anisotropy) of seismic velocity, indicating complexity of rock texture in the upper mantle. Some of these boundaries appear as deep as 185 km, implying that the lithosphere extends much deeper than the widespread NVGs would suggest. In this, our results agree with recent estimates of the lithospheric thickness in thermodynamically consistent models combining seismic, gravity, and heat flow constraints. 

Abstract We use splitting in core‐refracted teleseismic shear waves (SKS, PKS, and similar) to investigate anisotropic properties of the upper mantle beneath the Superior craton in eastern North America and the Yilgarn craton in Western Australia. At four sites in each craton, we assemble extensive data sets that emphasize directional coverage, and use three different measurement methods to develop mutually consistent constraints on the nature of splitting and on the likely anisotropic properties that cause it. In both cratons, we see evidence of clear directional variation in both delays and fast polarization directions, as well as lateral differences between sites. Relatively small (0.3–0.8 s) amounts of splitting imply weak anisotropy within 150–220 km thick mantle lithosphere. Anisotropy in the asthenosphere likely contributes to splitting in North America where fast directions align with absolute plate motion, but not in Western Australia where fast polarizations and plate motion are nearly orthogonal. 

Abstract Seismic anisotropy beneath eastern North America likely reflects both the remnant lithospheric fabrics and the present‐day deformation of the asthenosphere. We report new observations of splitting in core‐refracted shear phases observed over 3–5 years at 33 sites in New Jersey, New York, and states in the New England region and also include data from eight previously studied locations. Our data set emphasizes back azimuthal coverage necessary to capture the directional variation of splitting parameters expected from vertically varying anisotropy. We report single‐phase splitting parameters as well as station‐averaged values based on splitting intensity technique that incorporates all observed records regardless of whether they showed evidence of splitting or not. Trends of averaged fast shear wave polarizations appear coherent and are approximately aligned with absolute plate motion direction. The general disparity between the fast axes and the trend of surface tectonic features suggests a dominant asthenosphere contribution for the observed seismic anisotropy. Averaged delay values systematically increase from ~0.5 s in New Jersey to ~1.4 s in Maine. Splitting parameters vary at all sites, and neighboring stations often show similar patterns of directional variation. We developed criteria to group stations based on their splitting patterns and identified four domains with distinct anisotropic properties. Splitting patterns of three domains suggest a layered anisotropic structure that is geographically variable, outlining distinct regions in the continental mantle, for example, the Proterozoic lithosphere of the Adirondack Mountains. A domain coincident with the North Appalachian Anomaly displays virtually no splitting, implying that the lithospheric fabric was locally erased. 

Abstract Layering within the cratonic lithosphere has been explored and reported in different cratons using a range of techniques. However, whether there exists a common feature in the lithosphere for all the cratons is not clear yet. In this study, we carry out a comparison study between the Yilgarn craton in Western Australia and the Superior craton in North America that have never been in direct contact throughout their tectonic history. To have a detailed description of the lithospheric layering in both cratons, we employ receiver function analysis with harmonic decomposition to characterize the anisotropic seismic structure beneath 4 long‐operating sites in each craton. We can identify multiple unique anisotropic boundaries above 170 km at all sites in both cratons. Properties of the anisotropic boundaries are distinct both within and across the cratons. Our observation agrees with a commonly accepted view of the cratonic lithosphere consisting of at least two layers. Moreover, it adds new details to the previous view and reveals lateral variations of the anisotropic properties over distances of a few hundreds of kilometers. Such variations in anisotropic properties likely reflect the tectonic history predating the final assembly of cratons, and suggest horizontal movements are necessary for the formation of cratonic lithosphere.