Seismic anisotropy in the north-eastern US as a source of significant teleseismic P traveltime anomalies

Observations of shear-wave splitting in the north-eastern US and southern Canada provide evidence for seismic anisotropy in the lithosphere throughout most of the region. S-wave splitting times of the order of is are found within the Proterozoic Grenville Province and at a number of sites within the Appalachian Orogen. As a notable exception, seismic anisotropy is weak or absent in Vermont and western New Hampshire-a transitional zone between Proterozoic and Palaeozoic terranes. The fast direction is westerly (260 degrees-280 degrees) within the Grenville Province, and north-westerly (300 degrees-320 degrees) in the Appalachians. The effects of seismic anisotropy on the traveltimes of body waves are modelled in a horizontal layer characterized by an anisotropic elastic tensor of olivine. Simulations are made to study the influence of parameters such as the fraction of anisotropic material, the angle between the tensor symmetry axis and the wave propagation direction, and the type of crystallographic axis aligned in olivine grains. Results indicate that earth models with S-wave splitting times of about 1 s should also have P traveltime anomalies (positive or negative) of the order of 0.2-0.3 s. Also, alignment along either axis (a or b) can produce the combination of P delays between - 0.25 and - 0.75 s and S-wave splitting times between 0.7 and 1.3 s observed in the Adirondack Mountains. We conclude that velocity anomalies found in this region by earlier studies may in part be due to seismic anisotropy.