Spatio-temporal analysis of seismic anisotropy associated with the Cook Strait and Kaikōura earthquake sequences in New Zealand

© 2020 The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. Large earthquakes can diminish and redistribute stress, which can change the stress field in the Earth's crust. Seismic anisotropy, measured through shear wave splitting (SWS), is often considered to be an indicator of stress in the crust because the closure of cracks due to differential stress leads to waves polarized parallel to the cracks travelling faster than in the orthogonal direction. We examine spatial and temporal variations in SWS measurements and the Vp/Vs ratio associated with the 2013 Cook Strait (Seddon, Grassmere) and 2016 Kaikōura earthquakes in New Zealand. These earthquake sequences provide a unique data set, where clusters of closely spaced earthquakes occurred. We use an automatic, objective splitting analysis algorithm and automatic local S-phase pickers to expedite the processing and to minimize observer bias. We present SWS and Vp/Vs measurements for over 40 000 crustal earthquakes across 36 stations spanning close to $5\frac{1}{2}$ yr between 2013 and 2018. We obtain a total of 102 260 (out of 398 169) high-quality measurements. We observe significant spatial variations in the fast polarization orientation, φ. The orientation of gravitational stresses are consistent with most of the observed anisotropy. However, multiple mechanisms (such as structural, tectonic stresses and gravitational stresses) may control some of the observed crustal anisotropy in the study area. Systematic analysis of SWS parameters and Vp/Vs ratios revealed that apparent temporal variations are caused by variation in earthquake path through spatially varying media.