Radial anisotropy and prior petrological constraints: A comparative study

Click Here JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 115, B03303, doi:10.1029/2008JB005842, 2010 for Full Article Radial anisotropy and prior petrological constraints: A comparative study C. Beghein 1 Received 31 May 2008; revised 8 September 2009; accepted 6 October 2009; published 6 March 2010. [ 1 ] Radial seismic anisotropy models are traditionally obtained using empirical constraints based on laboratory experiments and petrological considerations. We tested the hypothesis that such petrological constraints affect the uppermost mantle models of S wave anisotropy using a statistical approach. In addition, we were able to determine which model features are constrained by the data and which are dominated by the prior. We focused on large-scale models and found that the most likely models obtained in both cases are highly correlated. This demonstrates that for the best data-fitting solution, the geometry of uppermost mantle radial anisotropy is not strongly affected by prior petrological constraints. The amplitude of the anomalies, however, can change significantly: The best data-fitting model obtained without petrological constraints displays stronger amplitudes than the one obtained with prior. This could become an issue when quantitatively interpreting seismic anisotropy models, and thus emphasizes the importance of accurately accounting for parameter uncertainties and trade-offs, and of understanding whether the seismic data or the prior constraints the model. We showed that model uncertainties are strongly affected by the prior as the relative RMS uncertainties were reduced by a factor of 2. In addition, we showed that while the model distributions are not necessarily Gaussian a priori, imposing petrological constraints can force the distributions to be narrower and more Gaussian-like, as expected from inverse theory. Finally, we demonstrated that the age dependence of seismic wave velocities is robust and independent of prior constraints. A similar age signal exists for anisotropy, but with larger uncertainties without prior constraints. Citation: Beghein, C. (2010), Radial anisotropy and prior petrological constraints: A comparative study, J. Geophys. Res., 115, B03303, doi:10.1029/2008JB005842. 1. Introduction [ 2 ] Accurately modeling mantle seismic anisotropy, that is the dependence of seismic wave velocity with the direction of propagation or polarization, can help us under- stand mantle deformation [Karato and Toriumi, 1989; Kendall, 2000; Becker et al., 2003], the coupling between lithosphere and asthenosphere [Silver and Holt, 2002; Becker et al., 2006b], mantle composition [Montagner and Anderson, 1989], rheology [Becker et al., 2008], and the net rotation of the lithosphere [Becker, 2008]. However, despite numerous efforts to model mantle seismic anisotropy over the past 20 years, uncertainties remain on its exact depth extent and lateral variations in the uppermost mantle, on its presence in the transition zone, and on its global nature in the D 00 layer [Fouch and Fischer, 1996; Montagner and Kennett, 1996; Ekstro¨m and Dziewonski, 1998; Lay et al., 1998; Trampert and van Heijst, 2002; Wookey et al., 2002; Gung et al., 2003; Panning and Romanowicz, 2006; Earth and Space Sciences Department, UCLA, Los Angeles, California, USA. Copyright 2010 by the American Geophysical Union. 0148-0227/10/2008JB005842$09.00 Beghein and Trampert, 2004a, 2004b; Beghein et al., 2006; Panning and Romanowicz, 2006; Zhou et al., 2006; Marone et al., 2007; Nettles and Dziewonski, 2008; Beghein et al., 2008]. [ 3 ] Discrepancies between models can arise for a variety of reasons. To fully describe Earth’s elastic properties one would ideally want to determine the 21 independent ele- ments of the fourth-order elastic stiffness tensor, at a given time and location inside Earth. In practice, this is challeng- ing because seismic data are only sensitive to subsets of those 21 elements [Tanimoto, 1986; Chen and Tromp, 2007; Beghein et al., 2008], and different types of data depend on different subsets of elastic coefficients [see summary tables in Chen and Tromp [2007] and Beghein et al. [2008]]. In addition, while some data, such as shear wave splitting measurements, can provide precise constraints on lateral changes in seismic anisotropy, their depth resolution is very poor. Surface wave and free oscillation data are better suited to constrain depth changes in structure, but their lateral resolution is lower than that of body waves. This can yield apparent discrepancies and make model comparisons diffi- cult. Moreover, three-dimensional models of seismic anisot- ropy are typically obtained by data inversion, which is often an ill-posed and ill-conditioned problem. This means that B03303 1 of 17