How Pervasive of Complex Seismic Azimuthal Anisotropy Beneath Continents

The orientation and strength of seismic azimuthal anisotropy revealed by the splitting of teleseismic P-to-S converted phases at the core-mantle boundary on the receiver side provide us with important information on the structure and deformation of the Earth's deep interior. Most previous shear wave splitting studies assume the existence of simple anisotropy, i.e., a single layer of anisotropy with a horizontal axis of symmetry. For simple anisotropy, the splitting parameters are invariant with the back-azimuth of the events and thus station-averaged splitting parameters are sufficient to represent anisotropy beneath the stations. Recent dramatic increase in the amount of high quality seismic data provides a unique opportunity to investigate the pervasiveness of complex anisotropy, which is characterized by periodic azimuthal variations of the individual splitting parameters. In this study, we systematically examine the spatial distribution of complex anisotropy and its possible relationship with tectonic environments, plate motion rates, and lithospheric thickness using data from long-running seismic stations from different continents. The resulting spatial distribution of stations with confirmed complex anisotropy and the resulting splitting parameters that quantify the anisotropy are compared with the thickness of the lithosphere and plate motion rate determined by previous studies to understand the formation mechanism of seismic anisotropy. Preliminary results show that for areas on a fast-moving (e.g., 20 mm/yr or greater) plate with a thick (e.g., 150 km or greater) lithosphere such as the central United States and most part of South America, seismic anisotropy is caused pervasively by the combination of two layers. For areas on a fast-moving plate with a thin lithosphere such as the western North America, the observed seismic anisotropy is mostly associated with mantle flow in the asthenosphere and the dominant fast polarization orientation is consistent with the absolute plate motion direction of the plate.