INVESTIGATING SEISMIC ANISOTROPY BENEATH LONG-RUNNING SEISMIC STATIONS IN NORTH AMERICA

Numerous observational and laboratory studies have indicated that seismic anisotropy is a near ubiquitous characteristic of the Earth’s crust and mantle. However, whether the azimuthal anisotropy structure beneath a given seismic station can be represented by simple anisotropy, which is defined as anisotropy originating from a single layer with a horizontal axis of symmetry, or complex anisotropy remains debated, mostly due to the inadequate azimuthal coverage of the teleseismic phases used in the studies. Here we utilize the shear wave splitting method, which is one of the routinely used techniques in structural seismology, to investigate the pervasiveness of complex anisotropy. This method uses P-to-S converted waves at the core-mantle boundary, including SKS, SKKS, and PKS. Data from several long-running broadband seismic stations across the United States are acquired from the Incorporated Research Institutions for Seismology data management center. Shear wave splitting parameters are measured based on the transverse energy minimization method. The resulting measurements are visually checked and manually ranked to ensure quality. At station CCM located in NE Missouri, the fast orientations show a systematic azimuthal variation indicative of complex anisotropy, probably suggesting anisotropy from the lithospheric fabrics and simple shear at the top of the asthenosphere related to absolute plate motion of the North American plate. Results from additional stations in the tectonically active western U.S. and the stable North American craton will also be presented to explore possible relationship between shear wave splitting measurements, lithospheric thickness, direction and rate of plate motion, and tectonic history.