Systematic bias in shear-wave splitting measurement

SUMMARY Shear-wave splitting measurement returns two parameters related to the fabric of the upper mantle: the orientation of the fast polarization (fast direction), and a measure of the intensity and thickness of the fabric known as the split time. Spatial statistics of compiled splitting measurements indicate that the fast direction is spatially coherent, while the split time is not. We show, through modelling large numbers of noisy measurements, that single-earthquake splitting measurements exhibit a prominent upward bias in split time, the degree of which depends on specifics of the measurement process. Averaging single-event splitting parameters over many measurements does not mitigate this bias; however, stacking of error surfaces from individual measurements does, given sufficient backazimuthal coverage, while also greatly reducing scatter. Published splitting results use a mix of these two averaging techniques, and this inconsistent bias between studies is likely responsible for the lack of spatial coherence in compiled split-time measurements. We demonstrate this in real data by examining a data set from Alberta, Canada and surrounding areas, for which a recent study published parameter-averaged results. By examining a comparable data set using error-surface stacking, we are able to greatly increase the coherence of the split times while obtaining highly similar fast directions. Our coherent split times are mapped to reveal a zone of strong splitting beneath the active Cordillera, and three zones of moderate to low split time within the cratonic lithosphere.