PN ANISOTROPY STUDIES IN NORTHERN BRITAIN AND THE EASTERN AND WESTERN UNITED-STATES

Summary. Available seismic refraction data from three different continental areas, northern Britain and the eastern and western United States, has been studied for possible Pn, velocity anisotropy using the methods described by Bamford. There are various deficiencies in the time—distance data used in each case but, while the uppermost mantle beneath northern Britain and the eastern United States seems to be isotropic within the limits of measurement error, there is a small but significant anisotropy beneath the western United States. Both the amount (up to 3 per cent) and the direction (70–80° east of north) of this anisotropy are very similar to the results obtained in the Pacific Ocean off California. We tentatively conclude that this anisotropy is present as a consequence of the subduction of oceanic lithosphere beneath the western United States. Copyright © 1979, Wiley Blackwell. All rights reserved