Three‐dimensional seismic models of the Earth's mantle

Abstract

Accurate models of the distribution of elastic heterogeneity in the Earth's mantle are important in many areas of geophysics. The purpose of this paper is to characterize and compare quantitatively a set of recent three‐dimensional models of the elastic structure of the Earth, to assess their similarities and differences, and to analyze their fit to one class of data in order to highlight fruitful directions for future research. The aspherical models considered are the following: M84C (Woodhouse and Dziewonski, 1984), LO2.56 (Dziewonski, 1984), MDLSH (Tanimoto, 1990a), SH.10c.17 (Masters et al., 1992), and S12_WM13 (Su et al., 1994). Through much of the discussion, M84C and LO2.56 are combined into a single whole mantle model, M84C + LO2.56. The fit of each model to previously tabulated even degree normal mode structure coefficients taken from Smith and Masters (1989a) and Ritzwoller et al. (1988) for multiplets along the normal mode fundamental and first, second, and fifth overtone branches is also presented. Rather than concentrating on detailed comparisons of specific features of the models, analyses of these models are general and statistical in nature. In particular, we focus on a comparison of the amplitude and the radial and geographical distribution of heterogeneity in each model and how variations in each affect the fit to the normal mode observations. In general, the results of the comparisons between the models are encouraging, especially with respect to the geographical distribution of heterogeneity and in the fit to the normal mode data sensitive to the upper mantle and lowermost lower mantle. There remain, however, significant discrepancies in amplitude and in the radial distribution of heterogeneity, especially near the top of the upper mantle and near the top of the lower mantle. The confident use of these models to constrain compositional and dynamical information about the mantle will await the resolution of these discrepancies. The factors that may be responsible for the differences in the models and/or for the misfit between the observed and predicted normal mode data are divided into two types: intrinsic (or procedural) and extrinsic (or structural). We discuss only three extrinsic factors at length here, including errors in the reference crustal models, unmodeled topography on discontinuities in the interior of the mantle, and errors in the assumed relationships between shear (υs) and compressional (υp) heterogeneity.

Global surface heat flux anomalies from seismic tomography‐based models of mantle flow: Implications for mantle convection

“…Here, the qualifter "important" obviously refers to long-wavelength heterogeneity whose omission would otherwise seriously affect the conclusions reached for theoretical parameters (such as viscosity) or the preferred theoretical framework (such as whole-mantle versus layered-mantle) within the context of the internal loading formalism. Whether or not mantle heterogeneity is adequately resolved is a question that is at the heart of ongoing seismic tomographic modeling efforts [e.g., Ritzwoller and Lavely, 1995;Natal and Ricard, 1996]. As discussed in section 1, however, it is clear that the current generation of long-wavelength tomographic models does not unambiguously constrain either transition zone or midmantle heterogeneity [Forte et al, 1994;Forte and Woodward, 1997a,b].…”

Towards a quantitative interpretation of global seismic tomography

“…Trampert and Woodhouse [2000] tested all recent long-wavelength fundamental mode phase velocity models against independent waveform data and found a reassuring agreement between them. In a test against independent splitting functions, Ritzwoller and Lavely [1995] showed that a robust pattern of the Earth's mantle emerged at the lowest degrees, even if different data and different mapping strategies were employed. Although this quantitative comparison is now almost 10 years old, more recent long-wavelength models correlate highly with the models in the Ritzwoller and Lavely study.…”

“…Among the success stories of global tomography are the delineation of long-wavelength variations in elastic properties in Earth's mantle, which started in the early 1980s, and the detailed delineation, over the last decade or so, of trajectories of mantle convection [see reviews by, e.g., Dziewonski and Woodhouse, 1987;Woodhouse and Dziewonski, 1989;Masters, 1989;Romanowicz, 1991;Montagner, 1994;Masters and Shearer, 1995;Ritzwoller and Lavely, 1995;Dziewonski, 1996;Masters et al, 2000;Kárason and van der Hilst, 2000;Fukao et al, 2001;Romanowicz, 2003]. It is encouraging to see that increasingly consistent information on the spatial patterns of wave speed variations is emerging from tomographic studies that use different data and/or techniques.…”

High‐resolution analysis of the gravest seismic normal modes after the 2004 Mw = 9 Sumatra earthquake using superconducting gravimeter data

“…The precise determination of the frequency splitting of seismic modes below 1 mHz is a way to improve 1D density models without any trade-off with elastic parameters, as they are directly linked to the 1D-density profile [Widmer-Schnidrig, 2003]. The splitting of the gravest modes also possesses high sensitivity to the 3D-density structure in the Earth's mantle and core, so their observation can constrain the Earth's lateral density structure [Ritzwoller and Lavely, 1995]. The recent Sumatra-Andaman earthquake on 2004 December 26 is a rare opportunity to improve Earth's models, as this huge event has strongly excited the low-frequency seismic modes both on SG and seismic records.…”