Shear wave splitting across the rocky mountain front

We search for evidence of seismic anisotropy by determining shear-wave splitting parameters using teleseismic phases from stations placed across the Rocky Mountain Front. Three features are striking: 1) fast polarization orientations are consistent within small geographic regions, but vary rapidly across the network, 2) a large number of high quality ''null'' measurements indicate that little transverse anisotropy with horizontal symmetry axis is present, and 3) stations with well-constrained but inconsistent parameters for rays from different sources imply that a single layer of anisotropic material with a horizontal symmetry axis is an inadequate parameterization. The general pattern suggests fast axes pointing toward (or away from) a central region of little anisotropy. A model of asthenospheric flow converging on or diverging from the central uplifted region is postulated. Within the southern Rocky Mountains, parameters are similar to those in the nearby northern Rio Grande Rift, suggesting that a similar mechanism causes the anisotropy in both regions.