Caroline Beghein

Changes in Seismic Anisotropy Shed Light on the Nature of the Gutenberg Discontinuity

G Below Sea Rheological differences between Earths lithosphere and asthenosphere help drive plate tectonics. Geophysical analyses repeatedly reveal a seismic Gutenberg (G) discontinuity at 40- to 100-kilometer depth in oceanic plates, although the origin of this boundary remains enigmatic. Beghein et al. (p. 1237, published online 27 February) found that vertical stratification of anisotropy aligned with the depths of the G discontinuity, but not with the lithosphere-asthenosphere boundary. It appears that the G discontinuity forms when there are geophysical changes in the mantle, such as dehydration beneath mid-ocean ridges. The Gutenberg discontinuity is distinct from the lithosphere-asthenosphere boundary. The boundary between the lithosphere and asthenosphere is associated with a platewide high–seismic velocity “lid” overlying lowered velocities, consistent with thermal models. Seismic body waves also intermittently detect a sharp velocity reduction at similar depths, the Gutenberg (G) discontinuity, which cannot be explained by temperature alone. We compared an anisotropic tomography model with detections of the G to evaluate their context and relation to the lithosphere-asthenosphere boundary (LAB). We find that the G is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled LAB, implying that the two are not equivalent interfaces. The origin of the G is a result of frozen-in lithospheric structures, regional compositional variations of the mantle, or dynamically perturbed LAB.

Probability density functions for radial anisotropy: implications for the upper 1200 km of the mantle

The presence of radial anisotropy in the upper mantle, transition zone and top of the lower mantle is investigated by applying a model space search technique to Rayleigh and Love wave phase velocity models. Probability density functions are obtained independently for S-wave anisotropy, P-wave anisotropy, intermediate parameter R, Vp, Vs and density anomalies. The likelihoods for P-wave and S-wave anisotropy beneath continents cannot be explained by a dry olivine-rich upper mantle at depths larger than 220 km. Indeed, while shear-wave anisotropy tends to disappear below 220 km depth in continental areas, P-wave anisotropy is still present but its sign changes compared to the uppermost mantle. This could be due to an increase with depth of the amount of pyroxene relative to olivine in these regions, although the presence of water, partial melt or a change in the deformation mechanism cannot be ruled out as yet. A similar observation is made for old oceans, but not for young ones where VSH s VSV appears likely down to 670 km depth and VPH s VPV down to 400 km depth. The change of sign in P-wave anisotropy seems to be qualitatively correlated with the presence of the Lehmann discontinuity, generally observed beneath continents and some oceans but not beneath ridges. Parameter R shows a similar age-related depth pattern as shear-wave anisotropy in the uppermost mantle and it undergoes the same change of sign as P-wave anisotropy at 220 km depth. The ratio between dlnVs and dlnVp suggests that a chemical component is needed to explain the anomalies in most places at depths greater than 220 km. More tests are needed to infer the robustness of the results for density, but they do not affect the results for anisotropy.

Structure and anisotropy of the Mexico subduction zone based on Rayleigh-wave analysis and implications for the geometry of the Trans-Mexican Volcanic Belt

[1] We develop a three-dimensional model of shear wave velocity and anisotropy for the Mexico subduction zone using Rayleigh wave phase velocity dispersion measurements. This region is characterized by both steep and flat subduction and a volcanic arc that appears to be oblique to the trench. We give a new interpretation of the volcanic arc obliqueness and the location of the Tzitzio gap in volcanism based on the subduction morphology. We employ the two-station method to measure Rayleigh phase velocity dispersion curves between periods of 16 s to 171 s. The results are then inverted to obtain azimuthally anisotropic phase velocity maps and to model 3-D variations in upper mantle velocity and anisotropy. Our maps reveal lateral variations in phase velocity at all periods, consistent with the presence of flat and steep subduction. We also find that the data are consistent with two layers of anisotropy beneath Mexico: a crustal layer, with the fast directions parallel to the North American absolute plate motion, and a deeper layer that includes the mantle lithosphere and the asthenosphere, with the fast direction interpreted in terms of toroidal mantle flow around the slab edges. Our combined azimuthal anisotropy and velocity model enables us to analyze the transition from flat to steep subduction and to determine whether the transition involves a tear resulting in a gap between segments or is a continuous deformation caused by a lithospheric flexure. Our anisotropy results favor a tear, which is also consistent with the geometry of the volcanic belt.

How do we understand and visualize uncertainty

Geophysicists are often concerned with reconstructing subsurface properties using observations collected at or near the surface. For example, in seismic migration, we attempt to reconstruct subsurface geometry from surface seismic recordings, and in potential field inversion, observations are used to map electrical conductivity or density variations in geologic layers. The procedure of inferring information from indirect observations is called an inverse problem by mathematicians, and such problems are common in many areas of the physical sciences. The inverse problem of inferring the subsurface using surface observations has a corresponding forward problem, which consists of determining the data that would be recorded for a given subsurface configuration. In the seismic case, forward modeling involves a method for calculating a synthetic seismogram, for gravity data it consists of a computer code to compute gravity fields from an assumed subsurface density model. Note that forward modeling often involves assumptions about the appropriate physical relationship between unknowns (at depth) and observations on the surface, and all attempts to solve the problem at hand are limited by the accuracy of those assumptions. In the broadest sense then, exploration geophysicists have been engaged in inversion since the dawn of the profession and indeed algorithms often applied in processing centers can all be viewed as procedures to invert geophysical data.

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

Three‐dimensional variations in Love and Rayleigh wave azimuthal anisotropy for the upper 800 km of the mantle

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

Surface wave array tomography in SE Tibet from ambient seismic noise and two‐station analysis – II. Crustal and upper‐mantle structure

We present a new mantle model (YB14SHani) of azimuthal anisotropy for horizontally polarized shear waves (SH) in parallel with our previously published vertically polarized shear wave (SV) anisotropy model (YB13SVani). YB14SHani was obtained from higher mode Love wave phase velocity maps with sensitivity to anisotropy down to ∼1200 km depth. SH anisotropy is present down to the mantle transition zone (MTZ) with an average amplitude of ∼2% in the upper 250 km and ∼1% in the MTZ, consistent with YB13SVani. Changes in SV and SH anisotropy were found at the top of the MTZ where olivine transforms into wadsleyite, which might indicate that MTZ anisotropy is due to the lattice-preferred orientation of anisotropic material. Beneath oceanic plates, SV fast axes become subparallel to the absolute plate motion (APM) at a depth that marks the location of a thermally controlled lithosphere-asthenosphere boundary (LAB). In contrast, SH anisotropy does not systematically depend on ocean age. Moreover, while upper mantle SV anisotropy is anomalously high in the middle of the Pacific, as seen in radial anisotropy models, SH anisotropy amplitude remains close to the average for other oceans. Based on the depth at which SV fast axes and the APM direction begin to align, we also found that the average thickness of cratonic roots is ∼ 250 km, consistent with Yuan and Romanowicz (2010) for North America. Here we add new constraints on the nature of the cratonic LAB and show that it is characterized by changes in both SV and SH anisotropy. ©2014. American Geophysical Union. All Rights Reserved.