Multimode Rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the Australian upper mantle

Abstract

Summary We present an azimuthally anisotropic 3‐D shear‐wave speed model of the Australian upper mantle obtained from the dispersion of fundamental and higher modes of Rayleigh waves. We compare two tomographic techniques to map path‐average earth models into a 3‐D model for heterogeneity and azimuthal anisotropy. Method I uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure. It performs an iterative inversion with norm damping and gradient regularization. Method II uses a direct inversion of individual depth layers constrained by Bayesian assumptions about the model covariance. We recall that Bayesian inversions and discrete regularization approaches are theoretically equivalent, and with a synthetic example we show that they can give similar results. The model we present here uses the discrete regularized inversion of independent path constraints of Method I, on an equal‐area grid. With the exception of westernmost Australia, we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction, to within 0.8 per cent for the velocity, 20 per cent for the anisotropic magnitude and 20° for its direction. On length scales of 1000 km and longer, down to about 200 km, there is a good correlation between velocity heterogeneity and geologic age. At shorter length scales and at depths below 200 km, however, this relationship breaks down. The observed magnitude and direction of maximum anisotropy do not, in general, appear to be correlated to surface geology. The pattern of anisotropy appears to be rather complex in the upper 150 km, whereas a smoother pattern of fast axes is obtained at larger depth. If some of the deeper directions of anisotropy are aligned with the approximately N–S direction of absolute plate motion, this correspondence is not everywhere obvious, despite the fast (7 cm yr−1) northward motion of the Australian plate. More research is needed to interpret our observations in terms of continental deformation. Predictions of SKS splitting times and directions, an integrated measure of anisotropy, are poorly matched by observations of shear‐wave birefringence.

Seismic and mechanical anisotropy and the past and present deformation of the Australian lithosphere

“…Where data quality allowed we ¢tted the fundamental mode between 5 and 25 mHz (40^200 s period) or a narrower frequency band within this range, while the higher modes were ¢tted between 8 and 50 mHz (20^125 s). With these frequency bounds the fundamental modes can provide sensitivity to structure down to at least 400 km, and, within the limits of our theoretical assumptions, the higher modes yield additional detail [37,47].…”

“…We selected V2250 paths, avoiding regions with complex source and mantle structure by considering events along or within the boundaries of the Indo-Australian plate (see [37], their ¢gure 1). We have performed a tomographic inversion for 3-D Earth structure using the method of Nolet [48].…”

“…This distinguishes our approach from the one taken by [35]. For a more detailed discussion we refer to [37].…”

Shear wave splitting from local events beneath the Ryukyu arc: Trench-parallel anisotropy in the mantle wedge

“…Some contribution to the observed signal from the crust is likely in this splitting dataset, but anisotropy in the crust cannot explain the observed δt values up to nearly 2 s. The crustal thickness in the Ryukyu arc, ∼35-40 km (Taira, 2001), is insufficient to explain such large splitting, and observed crustal splitting times in Japan average about 0.2 s (Kaneshima, 1990). It has been demonstrated that both lithospheric and asthenospheric contributions to anisotropy are important in many regions (e.g., Fouch et al, 2000;Simons et al, 2002;Simons and van der Hilst, 2003;Fischer et al, 2005;Waite et al, 2005), although splitting measurements in subduction zone settings are nearly always interpreted in terms of flow in the asthenosphere (e.g., Fischer et al, 1998Fischer et al, , 2000Smith et al, 2001;Anderson et al, 2004). The lithosphere beneath the Ryukyu arc stations is likely to be thin, due to thermal erosion associated with mantle wedge flow (e.g., Conder et al, 2002), and it is unlikely that the lithosphere is thick enough to explain the large split times.…”

Stepwise joint inversion of surface wave dispersion, Rayleigh wave ZH ratio, and receiver function data for 1D crustal shear wave velocity structure

“…Teleseismic surface wave tomography offers good resolution of the upper mantle (e.g., Trampert and Woodhouse 1995;Shapiro and Ritzwoller 2002;Simons et al 2002), but has difficulty in resolving the shallow to middle crust structure due to its lower frequency content (mostly above 20-s period). In particular, ambient noise tomography provides powerful complementary constraints on regional-or local-scale crustal structures with short to intermediate period surface waves (e.g., Shapiro and Campillo 2004;Shapiro et al 2005;Yao et al 2006;Yang et al 2007;Lin et al 2008;Yao et al 2008;Fang et al 2015).…”