Silver [1996] summarizes arguments, developed over the previous decade, that anisotropy is primarily a feature of the uppermost few hundred kilometers of the Earth. This view has been recently challenged by observations of seismic anisotropy near the core-mantle boundary [ Kendall and Silver, 1996; Garnero and Lay, 1997] and within the transition zone [Vinnik and Montagner; 1996]. Nevertheless, the presence of seismic anisotropy within the lithosphere is well-documented.
The variety of mechanisms that produce seismic anisotropy in the lithosphere centers on a handful of scenarios. In the upper crust the strongest influence is believed to be that of aligned cracks and/or pore spaces [Babuska and Pros 1984], for which slower velocities are found for waves that propagate normal to the average crack plane. The aspect ratio of pore/cracks and type of fluid determine the extent and proportion of anisotropy [Hudson, 1981; Crampin, 1991]. Alternating thin isotropic layers of higher and lower velocity can also produce an overall anisotropic effect [Backus, 1962; Helbig, 1994], with the velocities slower normal to the bedding than along it. In the lower crust and the uppermost mantle, cracks are assumed to close in response to increasing overburden pressure [ Babuska and Pros 1984; Kern et al, 1993], though field exposures of (formerly) deep-crustal fluid-filled cracks can be found [ Ague, 1995]. In the absence of cracks and inclusions, the lattice-preferred orientation (LPO) of mineral crystals is taken as the main cause of seismic anisotropy. Most minerals composing the bulk of the crust are anisotropic to some degree [Babuska and Cara, 1991], as are the olivine and orthopyroxene that predominate in the upper mantle anisotropy. Different deformation mechanisms can lead to the alignment of either the slow or the fast crystallographic direction in olivine grains [Nicolas et al., 1973; Ribe, 1992], but LPO caused by dislocation creep in the shallow mantle is commonly believed to lead to preferred alignment of the fast axis [Zhang and Karato, 1995].
It is natural to expect that strain-induced seismic anisotropy would be particularly prominent in plate boundary regions, where deformations are concentrated. World-wide observations of shear-wave splitting support this notion [Silver, 1996]. Present-day regions of active compression commonly have fast axis of seismic anisotropy aligned sub-parallel to the strike of the orogen. One explanation for such orientation is the preferred alignment of "slow" axes of olivine along the direction of compression [Nicolas et al., 1973]. Vauchez and Nicolas [1991] propose an alternative mechanism: preferred alignment of the olivine "fast" axes along the orogen as a result of concurrent strike-slip deformation commonly observed during mountain building.
Some stable continental interiors have anisotropic intensity equal, if not superior, to actively deforming regions, perhaps because many now-stable continental regions have experienced plate-boundary deformation in the past, and have retained a fossil deformation. Patterns of seismic anisotropy within stable continental masses may therefore record the tectonic history of these regions. Seismic stations examined in this work lie within two Paleozoic mountain belts, the Appalachians and the Uralides. Both have been loci of continent-building accretionary episodes in early Paleozoic time. Both regions are presently embedded within stable continental region, the North American and Eurasian plates, respectively.
A number of shear-wave splitting studies have examined the Northeastern
US, using both permanent and temporary stations [e.g., Silver and
Chan, 1991; Fouch and Fischer, 1995; Levin et al., 1996;
Barruol et al., 1997; Fouch et al., 1999].
In most cases a single "average" set of parameters was reported for
selected sites (Figure 5).
The average of all values shown in the map may be treated as a
"regional average", and comes out as delay 
s and
a fast direction 
CW from N.
Helffrich et al. [1994] reported average splitting parameters of
and 
s for station ARU, the station we
examine in the foredeep of the Urals.
It is interesting to note that, in both the northeastern Appalachians and
the central Uralides, the fast axes of seismic anisotropy are often not
parallel to the strike of the orogenic belt, and thus do not follow the
pattern reported in active-tectonic regions like the Pyrenees [
Vauchez and Nicolas, 1991] .