Shear wave splitting across Australia

Globally, fast polarization directions determined from shear wave splitting (SWS) analysis commonly align with plate motion, likely the result of mechanical coupling between the overriding lithosphere and the asthenosphere. However, in some regions there is considerable mismatch between plate motion and fast polarization direction, likely the result of regional dynamic processes or lithospheric anisotropy. The Australian plate is one of the fastest moving, yet shear wave splitting results across the continent do not align with absolute plate motion, and exhibit significant lateral variations. Additionally, anisotropic tomography models of Australia require layered lithospheric anisotropy, suggesting that complex shear wave splitting observations are tied to the heterogeneous lithospheric structure, a product of the complex deformational history of the continent.

Reconciling regional differences begins by characterizing depth-based variations in Australian anisotropy. To begin our assessment of depth-dependent anisotropy we have made SWS measurements at 14 stations distributed across Australia. Key to our analysis is repeated measurements of SKS/SKKS phases over a range of frequency bands (6 in total), similar to the methodology employed by Eakin and Long (2013). In total, 171 measurements (not including nulls) were made, for an average of 12 measurements per station. Calculation of SWS was performed in Splitlab following methodology set out in Wüstefeld et al. (2008. Initial calculations of shear wave splitting show some variation from previously published results, but in general results are in good agreement and exhibit complex anisotropic variation across the continent. To increase the number of potential measurements, further analysis will be performed on particularly noisy stations using StackSplit, a Splitlab plugin, following methodology developed by Grund et al. (2017). In addition to SWS measurements, our research group is utilizing a complementary data set derived from aninsotropic receiver functions (see Small et al., AGU 2018). Both sets of data will be used to further model the nature of anisotropy and a joint inversion will be performed to provide an accurate, depth-dependent model of seismic anisotropy in Australia's lithosphere.