The effect of seismic radiation patterns on the interpretation of SKS splitting: evidence from Phase 2 of the WA Array

Miriam Gauntlett, Caroline Eakin, Meghan S Miller

Coherent anisotropic fabrics trending E--W to NE--SW across Western Australia have been revealed by shear-wave splitting of the SKS and PKS (XKS) phases across the WA Array. This large temporary array of 130 stations with 40 km spacing shows that this seismic anisotropy is pervasive; yet individual stations, including permanent ones, produce relatively few results. We investigate the effect of focal mechanism orientation on the XKS energy radiated from an event, and find that XKS energy is at a minimum when the raypath is aligned with an event nodal plane. This coincidence occurs often for events occurring in South America or the Aleutians, two of the main source regions for XKS events that are recorded in Western Australia. We determine that the reduction in XKS energy when the raypath is close to the nodal plane is significant enough to affect our ability to make XKS measurements. We propose that this is why Western Australia has a historic lack of XKS measurements even in the presence of coherent seismic anisotropy. We also suggest that a lack of XKS splitting is not unequivocal evidence for apparent isotropy or weak anisotropy. Focal mechanism orientation is an important consideration when interpreting the productivity of shear-wave splitting studies.