SWAS: A shear-wave analysis system for semi-automatic measurement of shear-wave splitting above small earthquakes

The complexity of shear wave-arrivals above small earthquakes makes the polarisations and time-delays of shear-wave splitting above small earthquakes difficult to measure. We report a semi-automatic shear-wave analysis system, SWAS, that appears to combine the benefits of both visual and automatic techniques. Initially, SWAS automatically estimates shear-wave polarisations and picks shear-wave arrivals by an expert system, which provides sufficiently accurate initial measurements for visual adjustment. SWAS then allows easy comparison and adjustment of picks between screen images of original seismograms, seismograms rotated into anisotropic polarisations, and polarisation diagrams (hodograms), with immediate plotting in various standard or non-standard configurations. This speeds up visual measurements by well over an order of magnitude, and typically allows records of almost all small earthquakes (M ≥ −1.0) to be reliably measured for shear-wave splitting polarisations and time-delays. SWAS was developed and tested for data from the SIL seismic network in Iceland.