Including anisotropy in 3-D velocity inversion and application to Marlborough, New Zealand

Abstract

SUMMARY Two types of seismic velocity heterogeneity are often observed in a given region: pronounced patterns of anisotropy shown by local earthquake shear wave splitting and seismic velocity structure imaged by local earthquake traveltimes. We seek to combine these types of observations by including anisotropy in 3-D velocity inversion. The fast polarization directions from local earthquake shear wave splitting are used to define an initial anisotropy model. The ray paths are estimated for each of the shear wave splitting observations and the 3-D initial model is determined via an averaging process and a coherence analysis. The initial magnitude of anisotropy at a node is dependent on the number of adjacent ray paths and the consistency of the shear wave splitting anisotropy measurements. The traveltime inversion is parametrized with an isotropic component and two azimuthal anisotropy parameters for each node. The ray paths are defined with approximate 3-D ray tracing, and for each point along a ray path, the velocity is defined as a function of the ray path azimuth at that point. A blended approach is used to damp the perturbations to the magnitude of anisotropy and/or perturbations to the fast direction. Tests with synthetic traveltime data show that the anisotropic velocity inversion method is reliable. Application to the Marlborough region shows variations in anisotropy in the brittle crust, ductile lower crust, mantle and subducted slab. The patterns are consistent with the shear wave splitting observations, but show some additional features. The maximum anisotropy is approximately 12 per cent oriented northeast, between the Awatere and Wairau faults. This high anisotropy may reflect deformation and extensive fracturing in a region of high total shear strain. In the region of interaction of the ductile lower crust with the shallow subducting slab, moderate anisotropy with east–west orientation is imaged. This is consistent with ductile deformation roughly parallel to the plate velocity.