Anisotropic Pn tomography in the western United States

Abstract

Pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy. To investigate uppermost mantle anisotropy, a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy, and it was applied to Pn travel time data from the western United States. Results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations. A detailed resolution study examined the trade-off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage. Pn anisotropy beneath the western United States has maximum amplitudes of +0.3 km/s (7.5%) when resolved on a length scales of around 3 ø. The orientations of Pn anisotropy often correlate well with those inferred from shear-wave splitting results. This correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions. The orientation of the Pn anisotropy can be correlated with some of the tectonic processes which occur within the western United States. For example, the fast direction of Pn anisotropy parallels the northeast direction of subduction of the Juan de Fuca Plate beneath the northwest Pacific coast, suggesting that there the anisotropy is related to subduction-driven deformation. The fast direction of Pn anisotropy also parallels the strike of the San Andreas Fault system in central California, indicating that shear strains along the plate boundary may be responsible for the anisotropy there. Within the Great Basin, the Pn anisotropy varies substantially, and both partial melting and small-scale convection within the uppermost mantle could be responsible for these anisotropy variations. IntroductionMantle seismic anisotropy is a direct indicator of mantle strain history, and it constitutes an important tool in the study of the tectonics. The utility of anisotropy in tectonic studies was first made apparent by the studies of Hess [1964] and Raitt et al. [ 1969, 1971]. These early studies showed that the anisotropic fast axis within the oceanic lithosphere lies parallel to the spreading direction. Since then, many studies have used seismic anisotropy to study regional tectonics beneath both the oceans and continents [e.g., Bamford et al., 1979; Silver and Chan, 1991; Vinnik et al., 1992; Savage and Silver, 1993]. The link between mantle anisotropy and mantle tectonics is a consequence of the aggregate alignment of olivine during mantle deformation. Theoretical studies infer that the anisotropy can be attributed solely to strain and that there are consistent relationships between the mean orientation of the olivine crystal axes and axes of the the strain ellipsoid. During deformation, the olivine slow velocity axes (b axes) tend to align with the shortest strain axis, the olivine fast velocity axes (a axes) tend to align with the longest strain axis, and the olivine inte...

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