Abstract: We present the first application of full-waveform ambient noise inversion to observed correlation functions that jointly constrains 3-D Earth structure and heterogeneous noise sources. For this, we model and interpret ambient noise correlations as recordings of correlation wavefields, which completely eliminates the limiting assumptions of Green's function retrieval, such as equipartitioning and homogeneous random noise sources. Our method accounts for seismic wave propagation physics in 3-D heterogeneous and attenuating media and also for the heterogeneous and nonstationary nature of the ambient noise field. Designed as a proof of concept, the study considers long periods from 100 to 300 s, thus focusing on the Earth's hum. Treating correlations as self-consistent observables allows us to make separate measurements on the causal and acausal branches of correlation functions, without any need to choose one of them or form the average. We validate our approach by assessing the quality of the obtained models and by comparing them to previous studies. This work is a step toward the establishment of full-waveform ambient noise inversion as a tomographic technique with the goal to go beyond ambient noise tomography based on Green's function retrieval.

Abstract: We construct a new seismic model for central and West Antarctica by jointly inverting Rayleigh wave phase and group velocities along with P wave receiver functions. Ambient noise tomography exploiting data from more than 200 seismic stations deployed over the past 18 years is used to construct Rayleigh wave phase and group velocity dispersion maps. Comparison between the ambient noise phase velocity maps with those constructed using teleseismic earthquakes confirms the accuracy of both results. These maps, together with P receiver function waveforms, are used to construct a new 3-D shear velocity (Vs) model for the crust and uppermost mantle using a Bayesian Monte Carlo algorithm. The new 3-D seismic model shows the dichotomy of the tectonically active West Antarctica (WANT) and the stable and ancient East Antarctica (EANT). In WANT, the model exhibits a slow uppermost mantle along the Transantarctic Mountains (TAMs) front, interpreted as the thermal effect from Cenozoic rifting. Beneath the southern TAMs, the slow uppermost mantle extends horizontally beneath the traditionally recognized EANT, hypothesized to be associated with lithospheric delamination. Thin crust and lithosphere observed along the Amundsen Sea coast and extending into the interior suggest involvement of these areas in Cenozoic rifting. EANT, with its relatively thick and cold crust and lithosphere marked by high Vs, displays a slower Vs anomaly beneath the Gamburtsev Subglacial Mountains in the uppermost mantle, which we hypothesize may be the signature of a compositionally anomalous body, perhaps remnant from a continental collision.

Abstract: In this study we map the entire East African Rift System (EARS) within a global surface-wave velocity model to better constrain the structure of the underlying mantle, as well as patterns that might be associated with its continuation in the Mozambique Channel. We use all publicly available seismograms from the African continent, amounting to 1296 stations and more than 30 years of recordings. From these data, we obtain dispersion curves using both ambient noise and teleseismic earthquakes. To our measurements, we add global counterparts and jointly invert them for phase-velocity maps. We exploit a linearized inversion based on the ray theory, with an adaptive parameterization that allows for maximizing the resolution of the final maps based on the density of data coverage. We thus image the main African cratons and also some of the Archean blocks within them. We highlight the discontinuous nature of magmatic activity along the EARS and also display low-velocity anomalies beneath the Comores Archipelago, Madagascar and Bassas da India volcanic islands and seamounts. This last low-velocity anomaly is in the direct continuation of the EARS and could unveil an important magmatic system in the Mozambique Channel.

Abstract: Abstract Observations of seismic anisotropy near the core?mantle boundary may yield constraints on patterns of lowermost mantle flow. We examine seismic anisotropy in the lowermost mantle beneath Australia, bounded by the African and Pacific Large Low Shear Velocity Provinces. We combined measurements of differential splitting of SKS?SKKS and S?ScS phases sampling our study region over a range of azimuths, using data from 10 long?running seismic stations. Observations reveal complex and laterally heterogeneous anisotropy in the lowermost mantle. We identified two subregions for which we have robust measurements of D??associated splitting for a range of ray propagation directions and applied a forward modeling strategy to understand which anisotropic scenarios are consistent with the observations. We tested a variety of elastic tensors and orientations, including single?crystal elasticity of lowermost mantle minerals (bridgmanite, postperovskite, and ferropericlase), tensors based on texture modeling in postperovskite aggregates, elasticity predicted from deformation experiments on polycrystalline MgO aggregates, and tensors that approximate the shape preferred orientation of partial melt. We find that postperovskite scenarios are more consistently able to reproduce the observations. Beneath New Zealand, the observations suggest a nearly horizontal [100] axis orientation with an azimuth that agrees well with the horizontal flow direction predicted by previous mantle flow models. Our modeling results further suggest that dominant slip on the (010) plane in postperovskite aggregates provides a good fit to the data but the solution is nonunique. Our results have implications for the mechanisms of deformation and anisotropy in the lowermost mantle and for the patterns of mantle flow.