GyPSuM: A joint tomographic model of mantle density and seismic wave speeds

Abstract

[1] GyPSuM is a 3-D model of mantle shear wave (S) speeds, compressional wave (P) speeds, and density. The model is developed through simultaneous inversion of seismic body wave travel times (P and S) and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density. Geodynamic observations include the global free air gravity field, divergence of the tectonic plates, dynamic topography of the free surface, and the flow-induced excess ellipticity of the core-mantle boundary. GyPSuM is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution. Models of the density field from Earth's free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions. Alternatively, scaling higher-resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations. The current study provides a 3-D density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic-geodynamic inversion process. Notable density field observations include high-density piles at the base of superplume structures, supporting the general results of past normal mode studies. However, we find that these features are more localized and have lower amplitude than past studies would suggest. When we consider both fast and slow seismic anomalies in GyPSuM, we find that P and S wave speeds are strongly correlated throughout the mantle. However, we find a low correlation of fast S wave zones in the deep mantle (>1500 km depth) with the corresponding P wave anomalies, suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies. The cratonic lithosphere and D″ regions are shown to have strong compositional signatures. However, we argue that temperature variations are the primary cause of P wave speed, S wave speed, and density anomalies throughout most of the mantle.

Morphology of seismically slow lower-mantle structures

“…We have carried out the same analysis on HSML-S , GyPSuM (Simmons et al 2010), TX2011 (Grand 2002) and SP12RTS (Koelemeijer et al 2016), and obtained fairly similar results. However, in order to keep the vote map more easily understandable, we present the results on the five most recent models which are parametrized at least up to degree 18.…”

“…When applied to profiles of the entire lower mantle, k-means clustering of five recent tomographic models (Mégnin & Romanowicz 2000;Houser et al 2008;Kustowski et al 2008;Simmons et al 2010;Ritsema et al 2011) retrieves the geographic extents of the LLSVPs, as well as the LLSVP-like, but geographically isolated meso-scale structure called the Perm anomaly (Lekic et al 2012).…”

“…(ii) GyPSuM (Simmons et al 2010): Joint inversion of S-and P-wave velocities and density based on body waves, geodynamical observations and mineral physical constraints, parametrized with 275 km grid spacing and 22 layers.…”

Global Imaging of the Earth's Deep Interior: Seismic Constraints on (An)isotropy, Density and Attenuation

“…The only concrete structural inference relates to the presence of high-density material in the lowermost mantle beneath the African and Pacific superplumes that has been found consistently by several independent studies (e.g. Ishii & Tromp, 2001, 2004, Trampert et al, 2004;Simmons et al, 2010;Mosca et al, 2012). All of these results must be interpreted with caution, given the relatively large standard deviations that range around 50% of the maximum relative density perturbations, even when large data sets of surface waves, normal modes and body waves are combined.…”

“…Along similar lines, Simmons et al, 2010 assimilated seismic travel times, the global free air gravity field, the divergence of tectonic plates, dynamic surface topography and topography of the core-mantle boundary into a joint velocity and density model. Their method relies on the radial viscosity profile of the Earth (Mitrovica & Forte, 2004), and on the assumption that lateral heterogeneities are most likely of thermal origin.…”