Wavefield polarization in seismograms of Darfield aftershocks: orthogonality relation with shear wave velocity anisotropy

The Greendale fault was discovered after the 7.2 magnitude earthquake that occurred on September 4, 2010 at Darfield, New Zealand. It is a right-lateral strike slip fault that released an average slip of 2.5m during the main shock. In previous works, the presence of structure/stress induced anisotropy was investigated by analyzing the aftershocks recorded by 13 stations installed around the fault in the period September 2010 - January 2011. Near-parallel fast directions were observed for split shear waves at several stations close to the fault zone, interpreting the effect as related to aligned fault fabric. In order to assess whether the anisotropy of rock properties could also affect ground motion polarization, we compute the wavefield polarization in the horizontal plane using about 40 seismic events among those recorded during the deployment. We apply an approach based on the computation of horizontal-to-vertical spectral ratios (HVSRs) and the diagonalization of the covariance matrix. The distribution of instantaneous polarization angles computed throughout the seismograms consists of narrow rose diagrams with low standard deviation at stations in the fault zone, where predominant horizontal polarization varies from N78° to N122°. In contrast, stations outside the fault zone show very scattered rose diagrams with no preferential orientation. We compare the horizontal polarization directions with fast directions. We find a near-orthogonal relation for stations in the fault zone. The same relationship between polarization and anisotropy was found in previous studies in the Parkfield area and in the Val d'Agri Basin, where polarization also forms a high-angle with the main fracture fields. The same cause of shear wave splitting is therefore also responsible for a stiffness anisotropy that produces more efficient propagation of ground motions polarized orthogonally to the fault fabric.