Estimation of the Depth of Anisotropy Using Spatial Coherency of Shear-Wave Splitting Parameters

Shear-wave splitting (SWS) analyses are essential in understanding the structure and dynamics of the Earth’s deep interior. While splitting measurements have excellent horizontal resolution relative to other anisotropy-measuring techniques, their vertical resolution is low due to the steep incidence angle of the seismic phases used by the analyses. Here, using synthetic and real data, we present and test a simple approach to estimate the optimal depth of anisotropy by measuring the spatial coherency of the splitting parameters. The approach searches for the optimal depth by computing a spatial variation factor. Tests using synthetic SWS data produced with varying number of events, number of stations, and levels of noise suggest that the approach can satisfactorily find the depth of the source of anisotropy. Successful application of the depth-estimation procedure requires well-defined splitting parameters obtained from a multistation network and multiple events from a decent back-azimuthal range. It also requires significant and smooth spatial variations of anisotropy with horizontal axis of symmetry within a single layer of anisotropy. We applied the approach to 448 pairs of splitting measurements obtained at about 50 stations on the Ethiopian Plateau and found an optimal depth of anisotropy of about 300 km, suggesting an asthenospheric origin of the observed anisotropy.