Shear wave splitting in the Appalachians and the Urals: A case for multilayered anisotropy

Observations of shear wave splitting in the northeastern U.S. Appalachians and in the foredeep of the Urals vary significantly with the back azimuth a nd incidence angle of the incoming phase. These variations suggest two or m ore layers within the upper mantle with different anisotropic properties. S ynthetic seismograms for simple multilayered anisotropic structures show th at shear wave splitting parameters tend to vary substantially with the dire ction of approach. Relying on a subset of back-azimuth and incidence angle may strongly bias the model inferred, especially if the observations are av eraged. On the other hand, the azimuthal splitting pattern provides additio nal constraints on vertical or lateral variation of anisotropic properties in the Earth. Using a new error estimation technique for splitting, we find that individual measurements from broadband data have errors of the order of delta phi = 3 degrees-7 degrees for the fast direction and 0.1 - 0.2 s f or the delay of split shear waves. The azimuthal variation of splitting par ameters is broadly consistent throughout the Appalachian terranes in the no rtheast United States, especially for two long-running stations in the nort heast United States, HRV (Harvard, Massachusetts) and PAL (Palisades, New Y ork). Observations can be separated into two distinct populations, with mea n fast-axis azimuths of N60 degrees E+/-4 degrees and N119 degrees E+/-2 de grees. Delay values within each population range from near zero to similar to 1 s. Azimuthal splitting variation for ARU (Arti, Russia) in the foredee p of Uralian mountains is characterized by sharp transitions between differ ent groups of observations. Using synthetic seismograms in simple structure s, we develop one-dimensional anisotropic models under stations HRV and ARU . The model for HRV includes two layers of anisotropic material under an is otropic crust, with fast-axis azimuths N53 degrees E and N115 degrees E for the bottom and the top layers, respectively. The model for the upper mantl e under ARU includes a layer with a fast-axis at N50 degrees E atop a layer with fast axis azimuth N90 degrees E. Our modeling confirms the need for a layer of strong anisotropy with a slow axis of symmetry in the lower crust under ARU, reported by Levin and Park [1997a]. Our results suggest that bo th Urals and Appalachians possess a relict anisotropy in the tectosphere, a ssociated with past continental collision and accretion, underlain by aniso tropy with orientation similar to the local absolute plate motion, suggesti ng an asthenospheric component to the signal.