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

Abstract

S U M M A R YTwo 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.

Seismic anisotropy tomography: New insight into subduction dynamics

“…Fouch and Rondenay (2006) made a detailed review of the methods for studying seismic anisotropy, as well as their advantages and limitations. In the past three decades, many researchers have attempted to use P-wave travel-time data to study anisotropy tomography (e.g., Babuska et al, 1984;Hearn, 1984;Hirahara and Ishikawa, 1984;Hirahara, 1988;Babuska and Cara, 1991;Mochizuki, 1995;Gresillaud and Cara, 1996;Hearn, 1996;Plomerova et al, 1996;Mochizuki, 1997;Lees and Wu, 1999;Wu and Lees, 1999;Bokelmann, 2002;Eberhart-Phillips and Henderson, 2004;Oda, 2005, 2008;Wang and Zhao, 2008;Koulakov et al, 2009;Eken et al, 2010;Plomerova et al, 2011;Tian and Zhao, 2012a;Wei et al, 2013;Koulakov et al, 2015;Menke, 2015;Wei et al, 2015). However, reliable and geologically reasonable results have been obtained only in recent years, thanks to the availability of abundant high-quality arrival-time data recorded by dense seismic arrays of permanent and portable stations at local and regional scales.…”

Tomography and Dynamics of Western-Pacific Subduction Zones

“…Several researchers have tried to determine P-wave anisotropic tomography in the Japan and New Zealand subduction zones, and the results show that complex anisotropic anomalies exist in wide areas of the crust, mantle wedge and the subducting Pacific slab (e.g., Hirahara and Ishikawa, 1984;Eberhart-Phillips and Henderson, 2004;Ishise and Oda, 2005;Wang and Zhao, 2008, 2009, 2010Cheng et al, 2011). Huang et al (2011a) used a large number of arrivaltime data from local earthquakes in the crust and the subducting Pacific slab to determine a detailed P-wave anisotropic tomography of the NE Japan arc from the Japan Trench to the Japan Sea (Figs.…”

Methodology of Seismic Tomography

“…Fortunately, anisotropy with hexagonal symmetry is a good approximation to the rocks in the Earth and reduces the number of free parameters describing the anisotropy (e.g., Christensen 1984;Park and Yu 1993;Maupin and Park 2007). For a further simplification, the hexagonal symmetry is generally assumed to be horizontal when the azimuthal anisotropy is concerned in shear-wave splitting measurements (e.g., Crampin, 1984;Silver, 1996;Savage 1999;Huang et al 2011b, c;Long 2013) and P-wave velocity studies (e.g., Hess 1964;Backus 1965;Raitt et al 1969;Hearn 1996;Eberhart-Phillips and Henderson 2004;Zhao 2008, 2013); whereas the hexagonal symmetry is generally assumed to be vertical when the radial anisotropy is concerned in the form of a Vsh/Vsv variation (Vsh and Vsv are the velocities of shear waves polarized horizontally and vertically, respectively) in surface-wave studies (e.g., Nettles and Dziewonski 2008;Fichtner et al 2010;Yuan et al 2011) and in the form of a Vph/Vpv variation (Vph and Vpv are the velocities of P-waves propagating horizontally and vertically, respectively) in P-wave velocity studies (e.g., Ishise et al 2012;Wang and Zhao 2013;Wang et al 2014). Here, we introduce the recent tomographic methods for P-wave azimuthal and radial anisotropy, following Zhao (2008, 2013).…”

“…For the azimuthal anisotropy (a), the azimuthal angle ψ ′ of the hexagonal symmetry axis is normal to the fastvelocity direction with a horizontal hexagonal symmetry axis (Fig. 2.6a), the P-wave slowness can be approximately expressed as (e.g., Backus 1965;Raitt et al 1969;Hearn 1996;Eberhart-Phillips and Henderson 2004;Zhao 2008, 2013):…”