Anisotropic shear zones revealed by backazimuthal harmonics of teleseismic receiver functions

Abstract

Backazimuth-dependent Ps conversion, observed in seismic receiver functions (RFs) is generated by ŚV and ŚH waves that help match the shear-polarized displacement of hybrid Ṕ at the interfaces of an anisotropic layer. The effect of elliptical anisotropy with a tilted or horizontal symmetry axis ŵ on body-wave propagation in horizontal layers can be expressed in terms of the first-order hybridization of upgoing and downgoing P, SV and SH plane waves with a common horizontal slowness p. The first-order perturbations to the upgoing P wave involve hybridization of its polarization eigenvector with the addition of the shear wave polarizations ŚV, S'V, ŚH, SH'. Although the hybridization of plane waves in an anisotropic layer is only one factor in the complicated reflection and transmission problem, these perturbations suggest that the influence of P anisotropy on RFs is much larger than the influence of S anisotropy. Perturbation terms for hybrid Ṕ can be grouped into functional dependencies on the tilt angle Ψ of the symmetry axis ŵ from the vertical. Terms proportional to sin2 Ψ include four-lobed variation (cos 2θ, sin2θ) with backazimuth θ, encompassing the effect of a horizontal axis of symmetry. Terms proportional to sin 2Ψ have two-lobed variation (cos θ, sinθ) with backazimuth θ, encompassing the effects of a tilted symmetry axis. In the perturbation formula (33) for the hybrid Ṕ polarization, the four-lobed terms have pre-factor αp, and the two-lobed terms have pre-factor ανP, leading potentially to larger amplitude in the two-lobed pattern in Ps for the same amount of P anisotropy. For a dipping interface between two isotropic media, the out-of-plane deflections of the P-SV converted wave lead to a two-lobed pattern of P-SV and P-SH amplitudes that resembles the effects of anisotropy with a tilted axis of symmetry. The birefringence of the Moho-converted Ps phase influences the backazimuth harmonics of Ps amplitude, but occurs only in the case where S anisotropy is nonzero. A frequency-byfrequency regression of migrated multiple-taper-correlation RFs HR(f), HT(f) for a collection of seismic records can extractwell the interleaving of their harmonic backazimuth dependence. Application of harmonic stacking to migrated RFs from 471 events recorded at station RAYN (Ar Rayn, Saudi Arabia) of the Global Seismographic Network confirms the general features reported by Levin & Park: a strong two-lobed backazimuth variation in Ps from a Moho near 40-km depth and from an interface 25-30-km deeper, both consistent with a north-striking tilted axis of symmetry. The identical polarity of these two signals indicates that they are not Ps conversions from the top and bottom of a single subcrustal anisotropic layer.