[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.

S U M M A R YThe joint interpretation of seismic and geodynamic data requires mineral physical parameters linking seismic velocity to density perturbations in the Earth's mantle. The most common approach is to link velocity and density through relative scaling or conversion factors: R ρ/s = dlnρ/dlnV S . However, the range of possible R ρ/s values remains large even when only considering thermal effects. We directly test the validity of several proposed depthdependent conversion profiles developed from mineral physics studies for thermally-varying properties of mantle materials. The tests are conducted by simultaneously inverting shear wave traveltime data and a diverse suite of convection-related constraints interpreted with viscous-flow response functions of the mantle. These geodynamic constraints are represented by surface spherical harmonic basis functions (up to harmonic degree 16) and they consist of the global free-air gravity field, tectonic plate divergences, dynamic surface topography and the excess ellipticity of the core-mantle boundary (CMB). The tests yield an optimum 1-D thermal R ρ/s profile that is generally compatible with all considered data, with the exception of the dynamic surface topography that is most sensitive to the shallow upper mantle. This result is not surprising given that cratonic roots are known to be compositionally-distinct from the surrounding ambient mantle. Moreover, it is expected that the temperature-dependence of the thermal R ρ/s in the upper mantle is significant due to the temperature-dependence of seismic attenuation or Q. Therefore, a simple 1-D density-velocity relationship is insufficient. To address this problem, we obtained independent estimates of the first-order correction factors to the selected R ρ/s profile within the cratonic roots and in the ambient (thermal) upper mantle. These correction factors, defined as ∂ R ρ/s /∂lnV S , are highly negative within the cratons signifying considerable compositional buoyancy. This result confirms that the negative buoyancy associated with the low temperatures in the cratons is significantly counteracted by the composition-induced positive buoyancy resulting in near-neutral buoyancy of the cratonic roots. Within the ambient upper mantle, the correction factors are found to be positive which is consistent with the expected behaviour of the R ρ/s relationship in thermally-varying upper-mantle material. We obtain a significantly greater reconciliation of the global gravity anomalies and dynamic surface topography when applying these laterally-varying corrections compared to a simple 1-D R ρ/s relationship. Inversion for a fully 3-D R ρ/s relationship reveals secondary effects including additional compositional variation within the cratonic roots and the deep-mantle superplume structures. We estimate the relative magnitude of the thermal and compositional (non-thermal) contributions to mantle density anomalies and conclude that thermal effects dominate shear wave and density heterogeneity throughout the non-cratonic mantle....

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[1] We develop a global-scale P wave velocity model (LLNL-G3Dv3) designed to accurately predict seismic travel times at regional and teleseismic distances simultaneously. The model provides a new image of Earth's interior, but the underlying practical purpose of the model is to provide enhanced seismic event location capabilities. The LLNL-G3Dv3 model is based on $2.8 million P and Pn arrivals that are re-processed using our global multiple-event locator called Bayesloc. We construct LLNL-G3Dv3 within a spherical tessellation based framework, allowing for explicit representation of undulating and discontinuous layers including the crust and transition zone layers. Using a multiscale inversion technique, regional trends as well as fine details are captured where the data allow. LLNL-G3Dv3 exhibits large-scale structures including cratons and superplumes as well numerous complex details in the upper mantle including within the transition zone. Particularly, the model reveals new details of a vast network of subducted slabs trapped within the transition beneath much of Eurasia, including beneath the Tibetan Plateau. We demonstrate the impact of Bayesloc multiple-event location on the resulting tomographic images through comparison with images produced without the benefit of multiple-event constraints (single-event locations). We find that the multiple-event locations allow for better reconciliation of the large set of direct P phases recorded at 0-97 distance and yield a smoother and more continuous image relative to the single-event locations. Travel times predicted from a 3-D model are also found to be strongly influenced by the initial locations of the input data, even when an iterative inversion/relocation technique is employed.Citation: Simmons, N. A., S. C. Myers, G. Johannesson, and E. Matzel (2012), LLNL-G3Dv3: Global P wave tomography model for improved regional and teleseismic travel time prediction,