The static and transient deformations produced by earthquakes cause density perturbations which, in turn, generate immediate, long-range perturbations of the Earth's gravity field. Here, an ...analytical solution is derived for gravity perturbations produced by a point double-couple source in homogeneous, infinite, non-self-gravitating elastic media. The solution features transient gravity perturbations that occur at any distance from the source between the rupture onset time and the arrival time of seismic P waves, which are of potential interest for real-time earthquake source studies and early warning. An analytical solution for such prompt gravity perturbations is presented in compact form. We show that it approximates adequately the prompt gravity perturbations generated by strike-slip and dip-slip finite fault ruptures in a half-space obtained by numerical simulations based on the spectral element method. Based on the analytical solution, we estimate that the observability of prompt gravity perturbations within 10 s after rupture onset by current instruments is severely challenged by the background microseism noise but may be achieved by high-precision gravity strainmeters currently under development. Our analytical results facilitate parametric studies of the expected prompt gravity signals that could be recorded by gravity strainmeters.
Réunion Island in the western Indian Ocean is well known as one of the most active volcanic hotspots on Earth. Its birth, ∼65Ma ago, created the Deccan volcanic traps in India (almost 2 million km2), ...associated with the Cretaceous‐Tertiary boundary and with the extinction of about 90% of life on the Earth, including dinosaurs. However, the deep structure of the underlying mantle, the potential presence of a rising plume and its exact geometry in the lower and in the upper mantle are still subjects of debates. The use of seismic data acquired by the French‐German RHUM‐RUM experiment in the Indian Ocean around the Réunion volcanic hotspot (2012–2013) and the collection of broadband seismic data from temporary experiments and from the FDSN (Federation of Digital Seismograph Networks) data center make it possible to investigate the deep structure of the Réunion mantle plume along its complete track, from its birth to its present stage, with a lateral resolution of ∼600 km. So far, global seismic tomography models cannot provide such high resolution images of the transition zone or lower mantle in this region. In this study, we used the spectral element method (SEM) to perform waveform forward modeling for several thousand paths beneath the Indian Ocean, and normal mode perturbation theory to compute the gradient and the Hessian for the inverse part of the tomography. Using this hybrid method, we derived a regional tomographic model (including teleseismic and regional events) beneath the Indian Ocean, down to ∼1200 km depth, from simultaneous inversion of fundamental and higher mode three components waveforms down to 40 s period. Our model retrieves a low‐velocity channel extending from West to East in the western side of the Central Indian Ridge, in the depth range of 150–250 km. It also reveals a plume conduit with a broad head in the upper mantle and narrow tail anchored in the lower mantle at 1,200 km depth or deeper. The connection between the Réunion hotspot and the South‐Africa Large Low‐Shear Velocity Province (LLSVP) is also brought to light. Our findings suggest a long‐lived Réunion hotspot, since the lower part of the conduit appears to be anchored in the lower mantle, likely fed by the African LLSVP. Our results will guide further geochemical and geodynamic studies on the interaction between the lower transition zone (660–1,000 km) and the deep lower mantle beneath the Réunion hotspot.
Plain Language Summary
We present a model of the upper‐mantle structure beneath the Indian ocean obtained by full waveform inversion of three‐component seismograms collected from global and regional seismic networks, including ocean bottom stations of the RHUM‐RUM project, for several thousand paths that provide good coverage of the region, with focus on the deep structure beneath the Reunion hotspot volcano. Our model features a low‐velocity channel extending from West to East in the western side of the Central Indian Ridge, in the depth range of 150–250 km. It also reveals a plume conduit with a broad head in the upper mantle and narrow tail anchored in the lower mantle at 1,200 km depth or deeper, beneath the Reunion hotspot. The connection between the Réunion hotspot and the South‐Africa Large Low‐Shear Velocity Province (LLSVP) is also brought to light. Our findings suggest a long‐lived Réunion hotspot, since the lower part of the conduit appears to be anchored in the lower mantle.
Key Points
We performed a regional tomography of the Indian Ocean upper and mid‐mantle by waveform inversion
Two main low‐velocity anomalies are found in the upper mantle beneath the western (Mascarene Basin) and the Eastern (Central Indian Basin) parts of the Ocean
The source that feeds the hotspot volcano of La Réunion Island is anchored in the lower mantle
Recent studies reported the observation of prompt elastogravity signals during the 2011 M9.1 Tohoku earthquake, recorded with broadband seismometers and gravimeter between the rupture onset and the ...arrival of the seismic waves. Here we show that to extend the range of magnitudes over which the gravity perturbations can be observed and reduce the time needed for their detection, high‐precision gravity strainmeters under development could be used, such as torsion bars, superconducting gradiometers, or strainmeters based on atom interferometers. These instruments measure the differential gravitational acceleration between two seismically isolated test masses and are initially designed to observe gravitational waves around 0.1 Hz. Our analysis involves simulations of the expected gravity strain signals generated by fault rupture, based on an analytical model of gravity perturbations in a homogeneous half‐space. We show that future gravity strainmeters should be able to detect prompt gravity perturbations induced by earthquakes larger than M7, up to 1,000 km from the earthquake centroid within P waves travel time and up to 120 km within the first 10 s of rupture onset, provided a sensitivity in gravity strain of 10−15 Hz−1/2 at 0.1 Hz can be achieved. Our results further suggest that, in comparison to conventional P wave‐based earthquake‐early warning systems, gravity‐based earthquake‐early warning systems could perform faster detections of large offshore subduction earthquakes (at least larger than M7.3). Gravity strainmeters could also perform earlier magnitude estimates, within the duration of the fault rupture, and therefore complement current tsunami warning systems.
Key Points
Future high‐precision gravity strainmeters could record prompt gravity signals before the seismic waves arrival during an earthquake rupture
Planned sensitivity is sufficient to observe gravity perturbations from earthquakes of magnitude larger than 7 at distances up to 1,000 km
Gravity‐based warning system could perform faster detection and magnitude estimation of large earthquakes compared to conventional systems
According to different types of observations, the nature of lithosphere‐asthenosphere boundary (LAB) is controversial. Using a massive data set of surface wave dispersions in a broad period range ...(15–300 s), we have developed a three‐dimensional upper mantle tomographic model (first‐order perturbation theory) at the global scale. This is used to derive maps of the LAB from the resolved elastic parameters. The key effects of shallow layers and anisotropy are taken into account in the inversion process. We investigate LAB distribution primarily below the oceans, according to different kinds of proxies that correspond to the base of the lithosphere from the shear velocity variation at depth, the amplitude radial anisotropy, and the changes in azimuthal anisotropy G orientation. The estimations of the LAB depth based on the shear velocity increase from a thin lithosphere (∼20 km) in the ridges, to a thick old‐ocean lithosphere (∼120–130 km). The radial anisotropy proxy shows a very fast increase in the LAB depth from the ridges, from ∼50 km to the older ocean where it reaches a remarkable monotonic subhorizontal profile (∼70–80 km). The LAB depths inferred from the azimuthal anisotropy proxy show deeper values for the increasing oceanic lithosphere (∼130–135 km). The difference between the evolution of the LAB depth with the age of the oceanic lithosphere computed from the shear velocity and azimuthal anisotropy proxies and from the radial anisotropy proxy raises questions about the nature of the LAB in the oceanic regions and of the formation of the oceanic plates.
Key Points
The anisotropic parameters inverted from surface waves are sensitive to LAB
The shear velocity and azimuthal anisotropy proxies show age‐dependent patterns
The radial anisotropy proxy presents a subhorizontal pattern as age increase
We present a high‐resolution 3‐D lithospheric model of the Indian plate region down to 300 km depth, obtained by inverting a new massive database of surface wave observations, using classical ...tomographic methods. Data are collected from more than 550 seismic broadband stations spanning the Indian subcontinent and surrounding regions. The Rayleigh wave dispersion measurements along ~14,000 paths are made in a broad frequency range (16–250 s). Our regionalized surface wave (group and phase) dispersion data are inverted at depth in two steps: first an isotropic inversion and next an anisotropic inversion of the phase velocity including the SV wave velocity and azimuthal anisotropy, based on the perturbation theory. We are able to recover most of the known geological structures in the region, such as the slow velocities associated with the thick crust in the Himalaya and Tibetan plateau and the fast velocities associated with the Indian Precambrian shield. Our estimates of the depth to the Lithosphere‐Asthenosphere boundary (LAB) derived from seismic velocity Vsv reductions at depth reveal large variations (120–250 km) beneath the different cratonic blocks. The lithospheric thickness is ~120 km in the eastern Dharwar, ~160 km in the western Dharwar, ~140–200 km in Bastar, and ~160–200 km in the Singhbhum Craton. The thickest (200–250 km) cratonic roots are present beneath central India. A low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep.
Key Points
New massive surface wave data set assembled for this work
First high‐resolution 3‐D shear wave velocity model for the Indian continent
Our 3‐D model explains most of the geological features and different cratonic blocks
As soon as an earthquake starts, the rupture and the propagation of seismic waves redistribute masses within the Earth. This mass redistribution generates in turn a long-range perturbation of the ...Earth gravitational field, which can be recorded before the arrival of the direct seismic waves. The recent first observations of such early signals motivate the use of the normal mode theory to model the elastogravity perturbations recorded by a ground-coupled seismometer or gravimeter. Complete modelling by normal mode summation is challenging due to the very large difference in amplitude between the prompt elastogravity signals and the direct P-wave signal. We overcome this problem by introducing a two-step simulation approach. The normal mode approach enables a fast computation of elastogravity signals in layered self-gravitating Earth models. The fast and accurate computation of gravity perturbations indicates instrument locations where signal detection may be achieved, and may prove useful in the implementation of a gravity-based earthquake early warning system.
The mantle transition zone (MTZ) of the Earth lies between 410 and ∼1000 km in depth and has a key role in mantle convection processes. In particular, the discontinuity at 660 km and its associated ...endothermic mineralogical transformation can slow or inhibit the passage of matter between the upper and lower mantle. The MTZ thus acts as a boundary layer within the mantle. The depth variations of radial and azimuthal seismic anisotropies enable the detection of boundary layers within the mantle. However, the 3D imaging is difficult due to the lack of sensitivity of surface waves of fundamental modes, and the poor global coverage of this depth range by body-wave data. We present a new 3D general anisotropy model (both radial and azimuthal anisotropies) of the mantle down to 1200 km in depth using surface-wave overtone datasets. We find that there is little seismic anisotropy in most of the MTZ, except below subduction zones around the Pacific Ocean and, more surprisingly, in a large area beneath eastern Eurasia where the Pacific subducting plate is stagnant. Seismic anisotropy is usually associated with intense deformation processes but also possibly to water transportation or to fine layering. This significant anisotropy in this part of MTZ might reveal a large water ‘reservoir’ associated with hydrous minerals or a strong stratification. It reflects a complex history beneath central Asia, where the Tethys, Izanagi and Pacific plates appear to have strongly interacted during the last 100 My, having subducted in orthogonal directions under the Asian continent, with the Tethys plate descending into the lower mantle, and the Izanagi plate remaining stagnant in the MTZ. The Asian continent is the only region in the world where subducting slabs originating from different plates can interact. This unique slab distribution might explain why some plates descend while others remain in the lower transition zone.
A new 3D anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves. Though seismic heterogeneities are usually interpreted in terms of heterogeneous ...temperature field, a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals. New insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure. The model is best resolved in the Pacific Plate, the largest and the most active tectonic plate. Superimposed on the large-scale radial (
ξ parameter) and azimuthal anisotropy (of
V
SV velocity) within and below the lithosphere, correlated with present or past Pacific Plate motions, are smaller-scale (<1000 km) lateral variations of anisotropy not predicted by plate tectonics. Channels of low anisotropy down to a depth of 200 km (hereafter referred to as LAC) are observed and are the best resolved anomalies: one east–west channel between Easter Island and the Tonga–Kermadec subduction zones (observed on both radial and azimuthal anisotropies) and a second one (only observed on azimuthal anisotropy) extending from the south-west Pacific up to south-east Hawaii, and passing through the Polynesia hotspot group for plate older than about 40 Ma. These features provide strong constraints on the decoupling between the plate and asthenosphere. They are presumably related to cracking within the Pacific Plate and/or to secondary convection below the rigid lithosphere, predicted by numerical and analog experiments. The existence and location of these LACs might be related to the current active volcanoes and hotspots (possibly plumes) in the Central Pacific. LACs, which are dividing the Pacific Plate into smaller units, might indicate a future reorganization of plates with ridge migrations in the Pacific Ocean.
The analysis of rock anisotropy revealed by seismic waves provides fundamental constraints on stress‐strain field in the lithosphere and asthenosphere. Nevertheless, the anisotropic models resolved ...for the crust and the upper mantle using seismic waves sometimes show substantial discrepancies depending on the type of data analyzed. In particular, at several permanent stations located in Africa, previous studies revealed that the observations of SKS splitting are accounted for by models with a single and homogeneous anisotropic layer whereas 3‐D tomographic models derived from surface waves exhibit clear anisotropic stratification. Here we tackle the issue of depth‐dependent anisotropy by performing joint inversion of receiver functions (RF) and SKS waveforms at four permanent broadband stations along the East African Rift System (EARS) and also on the Congo Craton. For three out of the four stations studied, stratified models allow for the best fit of the data. The vertical variations in the anisotropic pattern show interesting correlations with changes in the thermomechanical state of the mantle associated with the lithosphere‐asthenosphere transition and with the presence of hot mantle beneath the Afar region and beneath the EARS branches that surround the Tanzanian Craton. Our interpretation is consistent with the conclusion of earlier studies that suggest that beneath individual stations, multiple sources of anisotropy, chiefly olivine lattice preferred orientation and melt pocket shape preferred orientation in our case, exist at different depths. Our study further emphasizes that multiple layers of anisotropy must often be considered to obtain realistic models of the crust and upper mantle.
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