Null Detection in Shear-Wave Splitting Measurements

Abstract

Shear-wave splitting measurements are widely used to analyze orientations of anisotropy. We compare two different shear-wave splitting techniques, which are generally assumed to give similar results. Using a synthetic test, which covers the whole backazimuthal range, we find characteristic differences, however, in fast-axis and delay-time estimates near Null directions between the rotation correlation and the minimum energy method. We show how this difference can be used to identify Null measurements and to determine the quality of the result. This technique is then applied to teleseismic events recorded at station LVZ in northern Scandinavia, for which our method constrains the fast-axis azimuth to be 15Њ and the delay time 1.1 sec.

Seismic anisotropy across the Kunlun fault and their implications for northward transforming lithospheric deformation in northeastern Tibet

“…This standard based on the comparison of different signal processing techniques (Long and van der Hilst, 2005;Wüstefeld and Bokelmann, 2007) demonstrates that the methodology of Silver and Chan (1991) is reliable for cases with |ΔBAZ| down to 10°. We defined the null cases of splitting measurements with the RC and SC methods based on the differences between the results obtained by the two methods (Huang et al, 2011;Wüstefeld and Bokelmann, 2007): the angular difference ψ = |φ RC − φ SC | for φ and the ratio ρ = δt RC /δt SC for δt. The criterion taken is 30°b ψ b 60°and ρ b 0.3.…”

Finite-Frequency SKS Splitting: Measurement and Sensitivity Kernels

“…Uncertainties in the measurements are estimated by an F-test (Silver and Chan, 1991). These different techniques give similar results for simple anisotropy, but significant discrepancies have been observed in complex structures like subduction zones (Long and van der Hilst, 2005) or when one is close to a null measurement (Wüstefeld and Bokelmann, 2007). A null measurement occurs when there is no apparent splitting.…”

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

“…Null splitting may be observed for three reasons: (i) if the incoming polarization direction below an anisotropic layer is parallel to the fast or slow axis; (ii) if the region is isotropic in nature due to complex anisotropy (e.g. Saltzer et al, 2000;Wüstefeld and Bokelmann, 2007); (iii) if the region itself is isotropic in nature. Following Eken and Tilmann (2014) and Singh et al (2016), we use the F test with the null-split rejection criteria to be able to avoid the contamination of the null measurement with the good splitting measurements.…”