First-order characteristics of shear-wave splitting parameters beneath the contiguous United States

We report results of preliminary analyses of a growing uniform XKS database that we produced using data recorded by about 2400 USArray and other broadband seismic stations in the contiguous United States. At the present time (August, 2013), the database consists of over 20,000 well-defined XKS measurements, including 14,000 from the SKS, 2900 from the PKS, and 3000 from the SKKS phase. In order to compare with the splitting measurements, we also measured about 480,000 teleseismic P-wave travel-time measurements using the same stations. Virtually all the stations with high-quality data and adequate azimuthal coverage have well-defined observations, suggesting that null measurements are extremely rare and consequently, azimuthal anisotropy is a ubiquitous feature of the contiguous United States. In addition to previously recognized large-scale patterns such as the semi-circular pattern of fast directions observed in the Great Basins, some less well-known or new features can be observed from the database. One of such features is a positive correlation between the splitting times and the P-wave travel-time residuals. Specifically, small (about 0.7 s) splitting times and negative travel-time residuals dominate the continental interior, while both the western and eastern US have positive travel-time residuals and splitting times that are larger than 1 s. The largest splitting times (about 2 s) are found beneath the northern Gulf coast where the fast directions are parallel to the coastline, and in southern Oregon where the fast directions are mostly E-W. For most of the study area, the fast directions are within 30 degrees away from the absolute plate motion direction of the North American plate. Areas with exceptions include the southwestern edge of the North American craton where the fast directions are mostly NS, and the NE US and adjacent SE Canada where the fast directions are mostly E-W or NW-SE. Most of the first order features observed in central and eastern US can be explained using a model involving simple shear-strain originated from partial coupling between the lithosphere and the underlain asthenosphere, and modulation of the flow field by the topography of the base of the lithosphere associated with cratonic boundaries and zones or areas with thinned lithosphere. The database, after the addition of USArray data from the east-most part of the country, will play an important role for the understanding of continental dynamics and evolution.