SUMMARY
Tomographic-geodynamic model comparisons are a key component in studies of the present-day state and evolution of Earth’s mantle. To account for the limited seismic resolution, ‘tomographic ...filtering’ of the geodynamically predicted mantle structures is a standard processing step in this context. The filtered model provides valuable information on how heterogeneities are smeared and modified in amplitude given the available seismic data and underlying inversion strategy. An important aspect that has so far not been taken into account are the effects of data uncertainties. We present a new method for ‘tomographic filtering’ in which it is possible to include the effects of random and systematic errors in the seismic measurements and to analyse the associated uncertainties in the tomographic model space. The ‘imaged’ model is constructed by computing the generalized-inverse projection (GIP) of synthetic data calculated in an earth model of choice. An advantage of this approach is that a reparametrization onto the tomographic grid can be avoided, depending on how the synthetic data are calculated. To demonstrate the viability of the method, we compute traveltimes in an existing mantle circulation model (MCM), add specific realizations of random seismic ‘noise’ to the synthetic data and apply the generalized inverse operator of a recent Backus–Gilbert-type global S-wave tomography. GIP models based on different noise realizations show a significant variability of the shape and amplitude of seismic anomalies. This highlights the importance of interpreting tomographic images in a prudent and cautious manner. Systematic errors, such as event mislocation or imperfect crustal corrections, can be investigated by introducing an additional term to the noise component so that the resulting noise distributions are biased. In contrast to Gaussian zero-mean noise, this leads to a bias in model space; that is, the mean of all GIP realizations also is non-zero. Knowledge of the statistical properties of model uncertainties together with tomographic resolution is crucial for obtaining meaningful estimates of Earth’s present-day thermodynamic state. A practicable treatment of error propagation and uncertainty quantification will therefore be increasingly important, especially in view of geodynamic inversions that aim at ‘retrodicting’ past mantle evolution based on tomographic images.
SUMMARY
The mantle transition zone (TZ) is expected to influence vertical mass flow between upper and lower mantle as it hosts a complex set of mineral phase transitions and an increase in viscosity ...with depth. Still, neither its seismic structure nor its dynamic effects have conclusively been constrained. The seismic discontinuities at around 410 and 660 km depth (‘410’ and ‘660’) are classically associated with phase transitions between olivine polymorphs, the pressure of which is modulated by lateral temperature variations. Resulting discontinuity topography is seismically visible and can thus potentially provide insight on temperature and phase composition at depth. Besides the olivine phase changes, the disassociation of garnet may additionally impact the 660 at higher temperatures. However, the volume of material affected by this garnet transition and its dynamic implications have not yet been quantified. This study presents hypothetical realizations of TZ seismic structure and major discontinuities based on the temperature field of a published 3-D mantle circulation model for a range of relevant mineralogies, including pyrolite and mechanical mixtures (MM). Systematic analysis of these models provides a framework for dynamically informed interpretations of seismic observations and gives insights into the potential dynamic behaviour of the TZ. Using our geodynamic-mineralogical approach we can identify which phase transitions induce specific topographic features of 410 and 660 and quantify their relative impact. Areal proportions of the garnet transition at the 660 are ∼3 and ∼1 per cent for pyrolite and MM, respectively. This proportion could be significantly higher (up to ∼39 per cent) in a hotter mantle for pyrolite, but remains low (<2 per cent) for MM. In pyrolite, both slabs and plumes are found to depress the 660—with average deflections of 14 and 6 km, respectively—due to the influence of garnet at high temperatures indicating its complex dynamic effects on mantle upwellings. Pronounced differences in model characteristics for pyrolite and MM, particularly their relative garnet proportions and associated topography features, could serve to discriminate between the two scenarios in Earth.
Modeling past states of Earth's mantle and relating them to geologic observations such as continental‐scale uplift and subsidence is an effective method for testing mantle convection models. However, ...mantle convection is chaotic and two identical mantle models initialized with slightly different temperature fields diverge exponentially in time until they become uncorrelated, thus limiting retrodictions (i.e., reconstructions of past states of Earth's mantle obtained using present information) to the recent past. We show with 3‐D spherical mantle convection models that retrodictions of mantle flow can be extended significantly if knowledge of the surface velocity field is available. Assimilating surface velocities produces in some cases negative Lyapunov times (i.e., e‐folding times), implying that even a severely perturbed initial condition may evolve toward the reference state. A history of the surface velocity field for Earth can be obtained from past plate motion reconstructions for time periods of a mantle overturn, suggesting that mantle flow can be reconstructed over comparable times.
Key Points
Long‐term retrodictions of mantle convection are hindered by chaotic drift
Reconstructions of past plate motion provide fundamental information about mantle flow in the past
Assimilation of past plate motion data greatly improves long‐term retrodictions of mantle convection
SUMMARY
Current interpretations of seismic observations typically argue for significant chemical heterogeneity being present in the two large low shear velocity provinces under Africa and the ...Pacific. Recently, however, it has been suggested that large lateral temperature variations in the lowermost mantle resulting from a strong thermal gradient across D″ may provide an alternative explanation. In case of a high heat flux from the core into the mantle, the magnitude of shear wave velocity variations in tomographic models can be reconciled with isochemical whole mantle flow and a pyrolite composition. So far, the hypothesis of strong core heating has been tested in a consistent manner only against tomographic S‐wave velocity models, but not against P‐wave velocity models. Here, we explore a new approach to assess geodynamic models and test the assumption of isochemical whole mantle flow with strong core heating directly against the statistics of observed traveltime variations of both P and S waves. Using a spectral element method, we simulate 3‐D global wave propagation for periods down to 10 s in synthetic 3‐D elastic structures derived from a geodynamic model. Seismic heterogeneity is predicted by converting the temperature field of a high‐resolution mantle circulation model (MCM) into seismic velocities using thermodynamic models of mantle mineralogy. Being based on forward modelling only, this approach avoids the problems of limited resolution and non‐uniqueness inherent in tomographic inversions while taking all possible finite‐frequency effects into account. Capturing the correct physics of wave propagation allows for a consistent test of the assumption of high core heat flow against seismic data.
The statistics of long‐period body wave traveltime observations show a markedly different behaviour for P and S waves: the standard deviation of P‐wave delay times stays almost constant with turning depth, whereas that of the S‐wave delay times increases strongly throughout the mantle. Surprisingly, synthetic traveltime variations computed for the isochemical MCM reproduce these different trends. This is not expected from a ray‐theoretical point of view and highlights the importance of finite‐frequency effects. Most importantly, the large lateral temperature variations in the lower mantle related to strong core heating are able to explain most of the standard deviation of observed P‐ and S‐wave delay times. This is a strong indication that seismic heterogeneity in the lower mantle is likely dominated by thermal variations on the length scales relevant for long‐period body waves.
SUMMARY
Resolution and covariance of global seismic tomography models are most often unknown quantities. However, there are many potential applications of these matrices in the broad solid Earth ...research community as well as more focused scientific groups including the nuclear explosion monitoring research community. In this study, we construct both the resolution and covariance matrices for the recent LLNL-G3D-JPS global joint model of P- and S-wave velocity. The global model consists of >1 million free parameters, creating matrices with >1 trillion elements. Given the scale of the problem and computational limitations, we used a custom method to calculated impulse responses at every node in the earth model and produced sparse, yet representative, resolution and covariance matrices that can be practically used for several real applications. We apply the matrices to real problems as example use cases. Utilizing the covariance matrix, we computed traveltime uncertainties for thousands of P waves emanating from (or coming to) specified points around the globe and constructed maps of the traveltime error to illustrate the variability of path-specific traveltime uncertainty. Utilizing the resolution matrix as a tomographic filter, we converted geodynamically derived renditions of Earth structure to images that may be visible through the often-distorted lens of seismic tomography.
We study wavefield effects of direct P- and S-waves in elastic and isotropic 3-D seismic structures derived from the temperature field of a high-resolution mantle circulation model. More ...specifically, we quantify the dispersion of traveltime residuals caused by diffraction in structures with dynamically constrained length scales and magnitudes of the lateral variations in seismic velocities and density. 3-D global wave propagation is simulated using a spectral element method. Intrinsic attenuation (i.e. dissipation of seismic energy) is deliberately neglected, so that any variation of traveltimes with frequency can be attributed to structural effects. Traveltime residuals are measured at 15, 22.5, 34 and 51 s dominant periods by cross-correlation of 3-D and 1-D synthetic waveforms. Additional simulations are performed for a model in which 3-D structure is removed in the upper 800 km to isolate the dispersion signal of the lower mantle. We find that the structural length scales inherent to a vigorously convecting mantle give rise to significant diffraction-induced body-wave traveltime dispersion. For both P- and S-waves, the difference between long-period and short-period residuals for a given source–receiver pair can reach up to several seconds for the period bands considered here. In general, these ‘differential-frequency’ residuals tend to increase in magnitude with increasing short-period delay. Furthermore, the long-period signal typically is smaller in magnitude than the short-period one; that is, wave-front healing is efficient independent of the sign of the residuals. Unlike the single-frequency residuals, the differential-frequency residuals are surprisingly similar between the ‘lower-mantle’ and the ‘whole-mantle’ model for corresponding source–receiver pairs. The similarity is more pronounced in case of S-waves and varies between different combinations of period bands. The traveltime delay acquired in the upper mantle seems to cancel in these differential signals depending on the associated wavelengths and the length scales of structure at shallow depth. Differential-frequency residuals may thus prove useful to precondition tomographic inversions for the lower-mantle structure such as to reduce the influence of the upper mantle for certain length scales. Overall, standard deviations of the diffraction-induced traveltime dispersion between the longest (51 s) and the shortest (15 s) period considered here are 0.6 and 1.0 s for P- and S-waves, respectively. For comparison, the corresponding standard deviations of the 15 s residuals are 1.0 s and 2.8 s. In the lower-mantle model, standard deviations are 0.3 and 0.6 s, respectively, which gives an average lower-mantle contribution to the total dispersion of 50 per cent for P-waves and 60 per cent for S-waves.
The plate tectonic history of the hypothesized “proto‐South China Sea” (PSCS) ocean basin and surrounding SE Asia since Cenozoic times is controversial. We implement four diverse proto‐South China ...Sea plate reconstructions into global geodynamic models to constrain PSCS plate tectonics and possible slab locations. Our plate reconstructions consider the following: southward versus double‐sided PSCS subduction models; earlier (Eocene) or later (late Oligocene) initiation of Borneo counterclockwise rotations; and larger or smaller reconstructed Philippine Sea plate sizes. We compare our modeling results against tomographic images by accounting for mineralogical effects and the finite resolution of seismic tomography. All geodynamic models reproduce the tomographically imaged Sunda slabs beneath Peninsular Malaysia, Sumatra, and Java. Southward PSCS subduction produces slabs beneath present Palawan, northern Borneo, and offshore Palawan. Double‐sided PSCS subduction combined with earlier Borneo rotations uniquely reproduces subhorizontal slabs under the southern South China Sea (SCS) at ~400 to 700 km depths; these models best fit seismic tomography. A smaller Philippine Sea (PS) plate with a ~1,000‐km‐long restored Ryukyu slab was superior to a very large PS plate. Considered together, our four end‐member plate reconstructions predict that the PSCS slabs are now at <900 km depths under present‐day Borneo, the SCS, the Sulu and Celebes seas, and the southern Philippines. Regardless of plate reconstruction, we predict (1) mid‐Cenozoic passive return‐flow upwellings under Indochina; and (2) late Cenozoic downwellings under the SCS that do not support a deep‐origin “Hainan plume.” Modeled Sundaland dynamic topography strongly depends on the imposed plate reconstructions, varying by almost 1 km.
Plain Language Summary
The past motion of tectonic plates (i.e., plate tectonic reconstructions) is the source of fundamental boundary conditions for many studies of Earth history. The South China Sea lies at a key junction between NE and SE Asia, which is one of the most tectonically complex regions in the world. A great diversity of plate reconstructions has been proposed for the South China Sea area. In this work we assimilate multiple SE Asia plate tectonic reconstructions into numerical models of mantle convection, in the form of time‐dependent velocity boundary conditions at the Earth's surface. This technique leads to a prediction for the time evolution of mantle structure and its flow field that is consistent with the reconstructed plate motions. Each reconstruction produces a different prediction of where subducted, colder oceanic lithosphere (i.e., slabs) should exist within the Earth's mantle. These predictions are transformed into a “synthetic” seismic tomography using published tomographic resolution filters and compared against seismic tomographic images of the Earth's mantle with special emphasis on the present South China Sea area, and its past plate tectonic history. For each plate reconstruction we also computed the mantle flow history and warping of the Earth's surface induced by mantle flow.
Key Points
Four fully kinematic, end‐member SE Asian plate tectonic models were input into global geodynamic models
Geodynamic models compared to tomography by converting temperature fields to seismic velocities and using P and S wave tomographic filters
Double‐sided proto‐South China Sea subduction and early Borneo rotation during Eocene produces mantle structures that best fit tomography
A striking feature of the Indian Ocean is a distinct geoid low south of India, pointing to a regionally anomalous mantle density structure. Equally prominent are rapid plate convergence rate ...variations between India and SE Asia, particularly in Late Cretaceous/Paleocene times. Both observations are linked to the central Neo‐Tethys Ocean subduction history, for which competing scenarios have been proposed. Here we evaluate three alternative reconstructions by assimilating their associated time‐dependent velocity fields in global high‐resolution geodynamic Earth models, allowing us to predict the resulting seismic mantle heterogeneity and geoid signal. Our analysis reveals that a geoid low similar to the one observed develops naturally when a long‐lived back‐arc basin south of Eurasia's paleomargin is assumed. A quantitative comparison to seismic tomography further supports this model. In contrast, reconstructions assuming a single northward dipping subduction zone along Eurasia's margin or models incorporating a temporary southward dipping intraoceanic subduction zone cannot sufficiently reproduce geoid and seismic observations.
Key Points
Geodynamic modeling results provide evidence for the former existence of large back‐arc basins in the Neo‐Tethys
Comparisons to seismic tomography and the geoid show that standard reconstructions with purely northward dipping subduction are outdated
The speed up of the Indian Plate at ~65 Ma may be due to a combination of a coupled double subduction zone system and plume push
Spatial derivatives of the seismic wave field are known to be sensitive to various site effects (e.g., cavity effects, topography, and geological inhomogeneities). In this study, the focus is on ...strain rotation coupling that can cause significant differences between point measurements compared to array-derived rotational motions. The strain rotation coupling constants are estimated based on finite element simulations for inhomogeneous media as well as for the 3D topography around Wettzell, Germany (the location of the G ring laser). Using collocated array and ring laser data, the coupling constants of the ring laser itself are shown to be small. Several examples are shown to illustrate the order of magnitude that strain-induced rotation might have on the seismograms in the near field of volcanoes as well as in the far field and in the low-frequency spectrum (free oscillations).
PDF
