Strain modelling, seismic anisotropy and coupling at strike-slip boundaries: applications in New Zealand and the San Andreas fault

Abstract

AbstractDespite similar surface transform faulting behaviour, observed shear-wave splitting patterns in the California and New Zealand plate boundary regions are markedly different. To better understand the origin of the seismic anisotropy in these regions we model mantle flow and strain for a variety of strike-slip plate boundary scenarios. Simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths. Strain-controlled models fit the observations in New Zealand and California better than simplified flow-controlled models. Fast shear polarizations are progressively rotated toward the shear plane over time, and even a constant-viscosity model provides a good fit to the fast directions in New Zealand and southern California. The constant viscosity implies strong coupling between the surface and the deeper mantle. To fit the lack of decrease in delay times with distance from the fault, the relationship between delay time and strain must saturate at small strains. If this is the case, then strain in southern New Zealand and southern California may be small, equivalent to that achieved along an infinite fault by about 3–10 Ma of their present motion. Stratified viscosity allows more rapid rotation of fast directions toward fault-parallel than occurs in isoviscous models, and can explain the nearly fault-parallel fast directions in the central South Island. Different aspects of the northern California results are fitted with different models, but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy suggesting vertical decoupling.