A Systematic Comparison of the Transverse Energy Minimization and Splitting Intensity Techniques for Measuring Shear-Wave Splitting Parameters

Over the past several decades, shear-wave splitting (SWS) analyses have been increasingly utilized to delineate mantle structure and probe mantle dynamics. However, the reported splitting parameters (fast polarization orientations and splitting times) are frequently inconsistent among different studies, partially due to the different techniques used to estimate the splitting parameters. Here, we report results from a systematic comparison of the transverse minimization (TM) and the splitting intensity (SI) techniques. The study was motivated by the fact that recent comparative studies led to conflicting conclusions, which include the suggestion that TM, which is arguably the most widely used SWS-measuring technique, performs significantly poorly relative to SI under most circumstances in terms of stability and reliability of the resulting splitting parameters. We use both synthetic and real seismograms to evaluate the performance of the techniques for noise resistance, dominant period dependence, and complex anisotropy recognition. For one-layer anisotropy models with a horizontal axis of symmetry, our results show the two techniques can provide measurements with similar reliability. The testing confirms conclusions from previous studies that, although SI cannot distinguish between simple and complex anisotropy models with a horizontal axis of symmetry, TM can serve as a powerful tool in recognizing the existence of complex anisotropy, which is characterized by a systematic dependence of the splitting parameters on the back azimuth of the events. Therefore, when the existence of complex anisotropy beneath a study area is unknown, TM is a better choice. E A FORTRAN program for the calculation of Wiener-filtered wavelet and splitting intensity using SI technique is provided as an electronic supplement to this article. Online Material: FORTRAN code to calculate splitting intensity. Introduction As demonstrated by numerous previous studies, shearwave splitting (SWS) analyses using P-to-S converted phases at the core–mantle boundary (including PKS, SKKS, and SKS; collectively called XKS) have been widely utilized as a powerful tool for detecting and characterizing deformational processes in the Earth’s mantle (Bowman and Ando, 1987; Silver and Chan, 1991; Gao et al., 1994, 2010; Savage, 1999; Long and Silver, 2009; Liu et al., 2014). However, the resulting SWS parameters, the fast polarization orientation (φ), and the splitting time (δt) are frequently dependent on the measuring methodologies (Vecsey et al., 2008). Significant discrepancies exist in published papers conducted by different groups, which led to heated debates concerning the reliability of the resulting splitting parameters and, consequently, their geodynamic implications (e.g., Liu et al., 2008). Arguably, the most popularly used SWS measuring method is the transverse minimization (TM) method proposed by Silver and Chan (1991). It estimates splitting parameters based on a grid-search technique for the optimal fast orientation and splitting time that can best remove energy on the corrected transverse component. TM is an event-specific measuring technique; that is, each event with a sufficient signal-to-noise ratio (SNR) on both the radial and transverse components will lead to an optimal pair of parameters. For simple anisotropy (i.e., anisotropy characterized by a single layer with a horizontal axis of symmetry), the observed splitting parameters are independent of the back azimuth (BAZ) of the events. For events with a BAZ that is orthogonal or parallel to the fast orientation, or for stations below which the Earth’s crust and mantle is isotropic, no energy on the transverse component will be observed, resulting in a null (N) 230 Bulletin of the Seismological Society of America, Vol. 105, No. 1, pp. 230–239, February 2015, doi: 10.1785/0120140108