Seismic anisotropy from shear wave splitting analysis of EarthScope data

Abstract:

Seismic anisotropy is a powerful tool to study the deformation and dynamics of crust and mantle. We present measurements of anisotropy beneath North American continent, using shear wave splitting (SWS) analysis of SKS and SKKS phases. The SWS measurements are calculated by minimizing the energy on the transverse component of waveform. The data used to calculate the splitting parameters (fast polarization direction and delay time) are from 1949 Earthscope digital broadband seismic stations. We select events with epicentral distances from 98 ^o to 135^o and magnitude greater than 5.8. Each splitting result is visually checked to ensure the quality of the analysis. About 20000 splitting measurements of SKS or SKKS phases are finally obtained. We separate the measurements based on their variations of back-azimuth. A small set of the stations (45) has consistent splitting parameters for events with different back-azimuths; those observations can be explained by a one-layer simple anisotropy. The other stations exhibit splitting patterns varying with back-azimuths; these observations are explained using multiple layers of anisotropy. We present our analysis of anisotropic structure in the mantle based on those measurements, including possible anisotropy in the lower mantle or D".