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The physical properties of regional seismic discontinuities in the upper mantle yield insights into lateral and radial thermochemical variations, with implications for our understanding of magmatism and convection in the mantle.The global distribution of the 300-km discontinuity (termed the “X” discontinuity) is relatively poorly resolved, as it is detected infrequently, likely due to its small impedance contrast. Reflectors observed near this depth are usually local and primarily detected beneath continent and subduction zones. Several mechanisms suggest that the X is associated with mineral transformations that occur in basalt-enriched material. Thus, imaging the X-discontinuity holds the key to mapping subducted oceanic crust remnants.Another discontinuity, at around 520 km depth, is detected more frequently and sometimes observed to be split into two signals. Its existence is predicted by the wadsleyite to ringwoodite mineral phase transition. However, the variations in ambient thermochemistry, which influence its visibility, depth variation, reflectivity, and/or splitting, are not fully understood, necessitating further investigations. Improved constraints on the nature of the 520 will inform regarding thermal and compositional gradients within the mantle transition zone.In this study, we use large global datasets of SS and PP precursors to obtain new maps of these discontinuities. Our observations indicate regionally weak yet clear signals at both depths, linked to variations in basalt fraction and potential temperature. We perform mineral physics modeling and investigate the characteristic temperature and composition associated with the signatures of these signals. These results provide insight into our understanding of the chemical segregation and plume stagnation in the upper mantle.

The formation and preservation of compositional heterogeneities inside the Earth affect mantle convection patterns globally and control the long-term evolution of geochemical reservoirs. However, the distribution, nature, and size of reservoirs in the Earth’s mantle are poorly constrained. Here, we invert measurements of travel times and amplitudes of seismic waves interacting with mineralogical phase transitions at 400–700-km depth to obtain global probabilistic maps of temperature and bulk composition. We find large basalt-rich pools (up to 60% basalt fraction) surrounding the Pacific Ocean, which we relate to the segregation of oceanic crust from slabs that have been subducted since the Mesozoic. Segregation of oceanic crust from initially cold and stiff slabs may be facilitated by the presence of a weak hydrated layer in the slab or by weakening upon mineralogical transition due to grain-size reduction.

Seismic topography models reveal that both upwelling plumes and downgoing slabs are deflected or stagnate at various depths in Earth’s mantle transition zone (MTZ) and mid-mantle (MM). Deflection within the MTZ is associated with the mineral physics phase changes at 410 and 660-km depth, however the cause of deflection in the MM remains debated. There are no candidate mineral transformations to explain the varied MM reflectors that have been detected [Waszek et al., 2018], instead indicating widespread compositional heterogeneities. Furthermore, our recent thermal model [Waszek et al., 2021] reveals a link between high temperatures in the MTZ and surface activity, indicating that some plumes are able to traverse this region unimpeded. Illuminating the detailed seismic structures of the upper and mid-mantle is key to determine the link between reflectors, temperature, composition, and dynamics.Here, we present a new large global dataset of ScS reverberations, compiled using an automatic waveform identification code based on Convolutional Neural Networks [Garcia et al., 2021]. Mantle discontinuities and reflectors generate precursors to ScSn phases, and postcursors to sScSn. Here, we present a new method to correct for 3D mantle structure in which we remove the symmetry problem suffered by most of these phases. The data are stacked to reveal the small amplitude reverberation signals, and our correction method allows us to stack for five ScSn and sScSn phases simultaneously to obtain the highest possible data coverage. For the global MTZ discontinuities, we use “adaptive stacking”. Based on Voronoi tessellation, the method automatically adjusts for topography, noise, and data coverage. Regional-scale fixed bin parameterisations of varying sizes are used to search for the intermittent MM reflectors.We incorporate our seismic observations with mineral physics modelling, inverting for a realistic range of potential temperatures and basalt-harzburgite mixtures to obtain the best-matching thermochemical model for the MTZ. We first compare our new ScS MTZ model with its counterpart generated from SS and PP precursors [Waszek et al., 2021], to benchmark observational differences between data types. We next investigate the link or lack thereof between our MTZ model and detections of MM signals, to place improved constraints on variations in properties with depth. The final step is interpretation of our observations and modelling in the context of geodynamical simulations of mantle convection. Our outputs will contribute to greater understanding of the complex relationship between MTZ discontinuities and MM reflectors, with implications for global mantle circulation, compositional layering beneath the MTZ, and even surface activity. References:Waszek, Schmerr, Ballmer. 2018.Garcia, Waszek, Tauzin, Schmerr. 2021.

Large-scale topography is thought to be mainly controlled by active tectonic processes. Fennoscandia is located far from any active tectonic setting and yet includes a mountain range along its passive North Atlantic margin. Models proposed to explain the origin of these enigmatic mountains are based on glacial isostatic adjustments, delamination, long-term isostatic equilibration, and dynamic support from the mantle, yet no consensus has been reached.Here we demonstrate that Precambrian lithospheric structure of Fennoscandia controlled both Cenozoic oceanic breakup and recent mountain rise in the North Atlantic region. Fennoscandia formed by amalgamation of Proterozoic and Archean continental blocks; using both S- and P-receiver functions, we discovered that the Fennoscandian lithosphere still retains the original structural heterogeneity and its western margin is composed of three distinct blocks. The southern and northern blocks have relatively thin crust (~40-45 km), while the central block has thick crust (~60 km) that most likely was formed by crustal stacking during the Proterozoic amalgamation. The boundaries of the blocks continue into the oceanic crust as two major structural zones of the North-East Atlantic, suggesting that the Fennoscandian amalgamation structures determined the geometry of the ocean opening. We found no evidence for mountain root support or delamination in the areas of high topography that could be related to the mountain formation. Instead, our results suggest that the geometry of the observed features creates conditions favorable for edge-driven convection at the adjacent narrow margins that provides dynamic support for the mountains in Scandinavia.

The abrupt changes in mineralogical properties across the Earth’s mantle transition zone substantially impact convection and thermochemical fluxes between the upper and lower mantle. While the 410-km discontinuity at the top of the mantle transition zone is detected with all types of seismic waves, the 660-km boundary is mostly invisible to underside P-wave reflections (P660P). The cause for this observation is debated. The dissociation of ringwoodite and garnet into lower-mantle minerals both contribute to the ‘660’ visibility; only the garnet reaction favours material exchanges across the discontinuity. Here, we combine large datasets of SS and PP precursors, mineralogical modelling and data-mining techniques to obtain a global thermal map of the mantle transition zone, and explain the lack of P660P visibility. We find that its prevalent absence requires a chemically unequilibrated mantle, and its visibility in few locations is associated with potential temperatures greater than 1,800 K. Such temperatures occur in approximately 0.6% of Earth, indicating that the 660 is dominated by ringwoodite decomposition, which tends to impede mantle flow. We find broad regions with elevated temperatures beneath the Pacific surrounded by major volcanic hotspots, indicating plume retention and ponding of hot materials in the mantle transition zone. The mantle transition zone is poorly, mechanically mixed, and acts to impede mantle flow, according to seismic observations integrated with detailed mineral-physics models.

The Seismic Experiment for Interior Structure (SEIS) of the InSight mission to Mars has been providing direct information on Martian interior structure and dynamics of that planet since it landed. Compared with seismic recordings on the Earth, ground‐motion measurements acquired by SEIS on Mars are not only made under dramatically different ambient noise conditions, but also include idiosyncratic signals that arise from coupling between different InSight sensors and spacecraft components. This work is to synthesize what is known about these signal types, illustrate how they can manifest in waveforms and noise correlations, and present pitfalls in structural interpretations based on standard seismic analysis methods. We show that glitches (a type of prominent transient signal) can produce artifacts in ambient noise correlations. Sustained signals that vary in frequency, such as lander modes that are affected by variations in temperature and wind conditions over the course of the Martian sol, can also contaminate ambient noise results. Therefore, both types of signals have the potential to bias interpretation in terms of subsurface layering. We illustrate that signal processing in the presence of identified nonseismic signals must be informed by an understanding of the underlying physical processes in order for high‐fidelity waveforms of ground motion to be extracted. Whereas the origins of the most idiosyncratic signals are well understood, the 2.4 Hz resonance remains debated, and the literature does not contain an explanation of its fine spectral structure. Even though the selection of idiosyncratic signal types discussed in this article may not be exhaustive, we provide guidance on the best practices for enhancing the robustness of structural interpretations., The MPS-MPG SEIS team acknowledges funding for development of the SEIS leveling system by the .DLR German Space Agency Doyeon Kim, Ross Maguire, and Nicholas Schmerr acknowledge NASA Grant 80NSSC18K1628 for support. Ved Lekić acknowledges support from the Packard Foundation. Jessica C. E. Irving was partly funded by UKSA Grant ST/W002515/1. Research by Mark P. Panning and William B. Banerdt was carried out at the JPL, California Institute of Technology, under a contract with the NASA (80NM0018D0004). The authors acknowledge both Université Fédérale de Toulouse Midi Pyrénées and the Région Occitanie for funding the Ph.D. Grant of Nicolas Compaire and Martin Schimmel thanks Seismic Ambient Noise Imaging and Monitoring of Shallow Structures (SANIMS) (RTI2018-095594-B-I00) and Generalitat de Catalunya (2017SGR1022). French authors are supported by Agence nationale de la recherche (ANR) MArs Geophysical InSight (MAGIS) (ANR- 19-CE31-0008-08) and by CNES for SEIS science support. Benoit Tauzin acknowledges support from the European Union’s Horizon 2020 Research and Innovation programme under the Marie Sklodowska-Curie grant agreement 793824. This article is InSight Contribution Number 217.

International audience

Typical seismic waveform data sets comprise hundreds of thousands to millions of records. Compilation is performed by time-consuming handpicking of phase arrival times, or signal processing algorithms such as cross-correlation. The latter generally underperform compared to handpicking. However, differences in picking methods creates variations in models and interpretation of Earth's structure. Here, we exploit the pattern recognition capabilities of Convolutional Neural Networks (CNN). Using a large handpicked data set, we train a CNN model to identify the seismic shear phase SS. This accelerates, automates, and makes consistent data compilation, a task usually completed by visual inspection and influenced by scientists' choices. The CNN model is employed to identify precursors to SS generated by mantle discontinuities. It identifies precursors in stacked and individual seismograms, producing new measurements of the mantle transition zone with quality comparable to handpicked data. This rapid acquisition of high-quality observations has implications for automation of future seismic tomography studies.

Seismological constraints obtained from receiver function (RF) analysis provide important information about the crust and mantle structure. Here, we explore the utility of the free-surface multiple of the P-wave (PP) and the corresponding conversions in RF analysis. Using earthquake records, we demonstrate the efficacy of PPs-RFs before illustrating how they become especially useful when limited data is available in typical planetary missions. Using a transdimensional hierarchical Bayesian deconvolution approach, we compute robust P-to-S (Ps)- and PPs-RFs with InSight recordings of five marsquakes. Our Ps-RF results verify the direct Ps converted phases reported by previous RF analyses with increased coherence and reveal other phases including the primary multiple reverberating within the uppermost layer of the Martian crust. Unlike the Ps-RFs, our PPs-RFs lack an arrival at 7.2 s lag time. Whereas Ps-RFs on Mars could be equally well fit by a two- or three-layer crust, synthetic modeling shows that the disappearance of the 7.2 s phase requires a three-layer crust, and is highly sensitive to velocity and thickness of intra-crustal layers. We show that a three-layer crust is also preferred by S-to-P (Sp)-RFs. While the deepest interface of the three-layer crust represents the crust-mantle interface beneath the InSight landing site, the other two interfaces at shallower depths could represent a sharp transition between either fractured and unfractured materials or thick basaltic flows and pre-existing crustal materials. PPs-RFs can provide complementary constraints and maximize the extraction of information about crustal structure in data-constrained circumstances such as planetary missions., Journal of Geophysical Research: Planets, 126 (11), ISSN:0148-0227, ISSN:2169-9097

Mars is the first extraterrestrial planet with seismometers (Seismic Experiment for Interior Structure, SEIS) deployed directly on its surface in the framework of the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission. The lack of strong Marsquakes, however, strengthens the need of seismic noise studies to additionally constrain the Martian structure. Seismic noise autocorrelations of single-station recordings permit the determination of the zero-offset reflection response underneath SEIS. We present a new autocorrelation study which employs state-of-the-art approaches to determine a robust reflection response by avoiding bias from aseismic signals which are recorded together with seismic waves due to unfavorable deployment and environmental conditions. Data selection and segmentation is performed in a data-adaptive manner which takes the data root-mean-square amplitude variability into account. We further use the amplitude-unbiased phase cross-correlation and work in the 1.2-8.9 Hz frequency band. The main target are crustal scale reflections, their robustness and convergence. The strongest signal appears at 10.6 s, and, if interpreted as a P-wave reflection, would correspond to a discontinuity at about 21 km depth. This signal is a likely candidate for a reflection from the base of the Martian crust due to its strength, polarity, and stability. Additionally we identify, among the stable signals, a signal at about 6.15 s that can be interpreted as the P-wave reflection from the mid-crust at about 9.5 km depth., This is the InSight contribution number 144 and IPGP contribution number 4215. French authors are supported by ANR MAGIS (ANR-19-CE31-0008-08) and by CNES SEIS/InSight Phase E2. MS thanks SANIMS (RTI2018-095594-B-I00), Generalitat de Catalunya (2017SGR1022), and invitations for research stays at IPGP.

Since early February 2019, the SEIS (Seismic Experiment for Interior Structure) seismometer deployed at the surface of Mars in the framework of the InSight mission has been continuously recording the ground motion at Elysium Planitia. In this study, we take advantage of this exceptional data set to put constraints on the crustal properties of Mars using seismic interferometry (SI). To carry out this task, we first examine the continuous records from the very broadband seismometer. Several deterministic sources of environmental noise are identified and specific preprocessing strategies are presented to mitigate their influence. Applying the principles of SI to the single-station configuration of InSight, we compute, for each Sol and each hour of the martian day, the diagonal elements of the time-domain correlation tensor of random ambient vibrations recorded by SEIS. A similar computation is performed on the diffuse waveforms generated by more than a hundred Marsquakes. A careful signal-to-noise ratio analysis and an inter-comparison between the two datasets suggest that the results from SI are most reliable in a narrow frequency band around 2.4 Hz, where an amplification of both ambient vibrations and seismic events is observed. The average autocorrelation functions (ACFs) contain well identifiable seismic arrivals, that are very consistent between the two datasets. Interpreting the vertical and horizontal ACFs as, respectively, the P- and S- seismic reflectivity below InSight, we propose a simple stratified velocity model of the crust, which is mostly compatible with previous results from receiver function analysis. Our results are discussed and compared to recent works from the literature., This study is InSight contribution number 164. The authors acknowledge both "Universite Federale de Toulouse Midi Pyrenees" and the "Region Occitanie" for funding the PhD grant of Nicolas Compaire. The French authors acknowledge the French Space Agency CNES and ANR (ANR-14-CE36-0012-02 and ANR-19-CE31-0008-08) for funding the InSight Science analysis. We acknowledge NASA, CNES, their partner agencies and Institutions (UKSA, SSO, DLR, JPL, IPGP-CNRS, ETHZ, IC, MPS-MPG) and the flight operations team at JPL, SISMOC, MSDS, IRIS-DMC and PDS for providing SEED SEIS data.

Large-scale topography is thought to be mainly controlled by active tectonic processes. Fennoscandia is located far from any active tectonic setting and yet includes a mountain range along its passive North Atlantic margin. Models proposed to explain the origin of these enigmatic mountains are based on glacial isostatic adjustments, delamination, long-term isostatic equilibration, and dynamic support from the mantle, yet no consensus has been reached. We show that topography along the continental margin of Fennoscandia may be influenced by its deep structure. Fennoscandia formed by amalgamation of Proterozoic and Archean continental blocks; using both S- and P-receiver functions, we discovered that the Fennoscandian lithosphere still retains the original structural heterogeneity and its western margin is composed of three distinct blocks. The southern and northern blocks have relatively thin crust (~40-45 km), while the central block has thick crust (~60 km) that most likely was formed by crustal stacking during the Proterozoic amalgamation. The boundaries of the blocks continue into the oceanic crust as two major structural zones of the North-East Atlantic, suggesting that the Fennoscandian amalgamation structures determined the geometry of the ocean opening. We found no evidence for mountain root support or delamination in the areas of high topography that could be related with mountain formation. Instead, our results suggest that both crustal and lithospheric heterogeneity of Fennoscandia along the continental margin might have a control on geodynamic forces that support the rise of Scandinavian mountains.

Since InSight (the Interior Exploration using Geodesy and Heat Transport) landed 26 months ago and deployed an ultra sensitive broadband seismometer(SEIS) on the surface of Mars, around 500 seismic events of diverse variety have been detected, making it possible to directly analyze the subsurface properties of Mars for the very first time. One of the primary goals of the mission is to retrieve the crustal structure below the landing site. Current estimates differ by more than 100% for the average crustal thickness. Since data from orbital gravity measurementsprovide information on relative variations of crustal thickness but not absolute values, this landing site measurement could serve as a tie point to retrieve global crustal structure models. To do so, we propose using a joint inversion of receiver functions and apparent incidence angles, which contain information on absolute S-wave velocities of the subsurface. Since receiver function inversions suffer from a velocity depth trade-off, we in addition exploit a simple relation which defines apparent S-wave velocity as a function of observed apparent P-wave incidence angles to constrain the parameter space. Finally we use the Neighbourhood Algorithm for the inversion of a suitable joint objective function. The resulting ensemble of models is then used to derive the full uncertainty estimates for each model parameter. Before its application on data from InSight mission, we successfully tested the method on Mars synthetics and terrestrial data from various geological settings using both single and multiple events. Using the same method, we have previously been able to constrain the S-wave velocity and depth for the first inter-crustal layer of Mars between 1.7 to 2.1 km/s and 8 to 11 km, respectively. Here we present the results of applying this technique on our selected data set from the InSight mission. Results show that the data can be explained equally well by models with 2 or 3 crustal layers with constant velocities. Due to the limited data set it is difficult to resolve the ambiguity of this bi-modal solution. We therefore investigate information theoretic statistical tests as a model selection criteria and discuss their relevance and implications in seismological framework.

Convective stirring of chemical heterogeneities introduced through oceanic plate subduction results in the marble cake model of mantle composition. A convenient description invokes a chemically unequilibrated mixture of oceanic basaltic crust and harzburgitic lithosphere. Such a composition is required to explain joint observations of shear and compressional waves reflected underneath transition zone (TZ) discontinuities1. The formation of basaltic reservoirs at TZ depth results from complex interaction between phase-change induced chemical segregation, subducted slab downward entrainment, and plume upward advection. However, the dominant mechanism to create and maintain the reservoirs is debated, because both present-day reservoir location and the amount of basalt in these reservoirs are unconstrained. Here, Bayesian inversion of SS- and PP-precursors reflection data indicates that the TZ comprises a global average basalt fraction f = 0.32 ± 0.11. We find the most enriched basaltic reservoirs (f = 0.5-0.6) are associated with recent subduction in the circum-Pacific region. We investigate the efficiency of plate subduction to maintain such reservoirs using global-scale thermochemical convection models2.[1] Waszek, L., Tauzin, B., Schmerr, N.C., Ballmer, M., & Afonso, J.C. (in review). A poorly mixed mantle and its thermal state inferred from seismic waves.[2] Yan, J., Ballmer, M. D., & Tackley, P. J. (2020). The evolution and distribution of recycled oceanic crust in the Earth's mantle: Insight from geodynamic models. Earth and Planetary Science Letters, 537, 116171.

vEGU21: Gather Online | 19–30 April 2021, Since early February 2019, the SEIS seismometer deployed at the surface of Mars in the framework of the NASA-InSight mission has been continuously recording the ground motion at Elysium Planitia. In this work, we take advantage of this exceptional dataset to put constraints on the crustal properties of Mars using seismic interferometry (SI). This method use the seismic waves, either from background vibrations of the planet or from quakes, that are scattered in the medium in order to recover the ground response between two seismic sensors. Applying the principles of SI to the single-station configuration of SEIS, we compute, for each Sol (martian day) and each local hour, all the components of the time-domain autocorrelation tensor of random ambient vibrations in various frequency bands. A similar computation is performed on the diffuse waveforms generated by more than a hundred Marsquakes. For imaging application a careful signal-to-noise ratio analysis and an inter-comparison between the two datasets are applied. These analyses suggest that the reconstructed ground responses are most reliable in a relatively narrow frequency band around 2.4Hz, where an amplification of both ambient vibrations and seismic events is observed. The average Auto-Correlation Functions (ACFs) from both ambient vibrations and seismic events contain well identifiable seismic arrivals, that are very consistent between the two datasets. We interpret the vertical and horizontal ACFs as the ground reflection response below InSight for the compressional waves and the shear waves respectively. We propose a simple stratified velocity model of the crust, which is most compatible with the arrival times of the detected phases, as well as with previous seismological studies of the SEIS record. The hourly computation of the ACFs over one martian year also allows us to study the diurnal and seasonal variations of the reconstructed ground response with a technique call Passive Image Interferometry (PII). In this study we present measurements of the relative stretching coefficient between consecutive ACF waveforms and discuss the potential origins of the observed temporal variations.

International audience; Due to a sharp contrast in elastic properties across the basement rocks of sedimentary basins (SBs), strong reverberations are generated during the passage of seismic waves. Traditional receiver function methods become inadequate for imaging crustal structure due to the existence of these strong reverberations. We investigate the feasibility of an autocorrelation technique to extract vertical component receiver functions from teleseismic earthquake data and the efficiency of the method to image the crustal architecture in presence of a SB. The method involves spectral whitening followed by autocorrelation and stacking in the depth domain. We show promising results when using temporary seismic networks in the eastern United States. Using synthetic and field‐data examples, we demonstrate that vertical autocorrelations are more efficient than classical radial receiver functions for interpretation purposes in an SB context. We also perform a joint analysis of the amplitudes on radial and vertical receiver functions for characterizing the thickness of the Mohorovičić discontinuity (Moho). We find that the Moho in the eastern United States is a transitional layer (up to 5‐km thick) instead of a sharp boundary. Further, we point out that it is challenging to unambiguously pick and interpret reflected phases on autocorrelations because of the effects of reverberations, cross‐mode contaminations, and a narrow frequency band limiting the resolution of velocity gradients. We therefore send a message of caution for future interpretations based on this technique.

International audience; We image the thermo‐chemical structure of crust and mantle underneath the North American continent by inverting recordings of P‐to‐s (Ps) and S‐to‐p (Sp) converted seismic body waves (receiver functions; RF). Through careful data selection and processing, we construct a multi‐frequency Ps (5, 8, 10 s) and Sp (10, 15 s) RF data set from USArray recordings. The inversion is interfaced with petrological phase equilibria computations to build self‐consistent radial seismic velocity and density models for RF waveform simulations. Inverted models are combined through back‐projection along converted raypaths and interpolation to tomographic images of crust and mantle structure. Through clustering analysis we identify three major tectonic regions based on mantle thermo‐chemical and seismic structure: the tectonically active West (TAW), the central transition regions (CTR), and the cratonic‐orogenic East (COE). TAW is chemically more fertile with a Mg#~0.90 (molar Mg#=Mg/Mg+Fe) and an elevated mantle potential temperature of 1490±27°C relative to COE, which is chemically more depleted (Mg#~0.91) and colder (1419±27°C). CTR is intermediate to TAW and COE. We find significant thermo‐chemically‐induced topography associated with the base of the lithosphere (±90 km), while the mantle transition zone is mostly influenced by thermally‐induced topography on the 410‐km discontinuity (±15 km). In contrast, the 660‐km discontinuity, where variations are only ±5 km, reflects a more complex thermo‐chemical interplay. To place the results in a tectonic context, thermobarometric estimates from basaltic rocks across the western USA are integrated with the seismic inversions to produce a thermal model of the underlying mantle.

Typical seismic waveform datasets comprise from hundreds of thousands to several millions records. Compilation is performed by time-consuming handpicking of phase arrival times, or signal processin...

NASA’s InSight mission arrived on Mars in November 2018 and deployed the first very broad-band seismometer, SEIS, on the planet’s surface. SEIS has been collecting data continuously since early February 2019, by now recording more than 400 events of different types. InSight aims at enhancing our understanding of the internal structure and dynamics of Mars, including better constraints on its crustal thickness. Various models based on topography and gravity observed from the orbit currently vary in average crustal thickness from 30 km to more than 100 km, with important implications for Mars’ thermal evolution, and the partitioning of silicates and heat-producing elements between different layers of Mars.We present P-to-S and S-to-P receiver functions, which are available for 4 and 3 marsquakes, respectively, up to now. Out of all of the marsquakes recorded to date, these are the only ones with clear enough P- or S-arrivals not dominated by scattering to make them suitable for the analysis. All of the quakes are located at comparatively small epicentral distances, between 25° and 40°. We observe three consistent phases within the first 10 seconds of the P-to-S receiver functions. The S-to-P receiver functions also show a consistent first phase. Later arrivals are harder to pinpoint, which could be due to the comparatively shallow incidence of the S-waves at the considered distances, which prevents the generation of converted waves. Identification of later multiple phases in the P-to-S receiver functions likewise remains inconclusive. To obtain better constraints on velocity, we also calculated apparent velocity curves from the P-to-S receiver functions, but these provide meaningful results for only one event so far, implying a large uncertainty. Due to difficulties in clearly identifying multiples, the receiver functions can currently be explained by either two crustal layers and a thin (25-30 km) crust or three crustal layers and a thicker (40-45 km) crust at the landing site. This model range already improves the present constraints by providing a new maximum value of less than 70 km for the average crustal thickness. Information from noise autocorrelations as a complementary method, identification of P-reverberations and S-precursors in the event recordings, and more extensive modeling, ultimately including 3D-effects, are considered to further our understanding of the waveforms and tighten the constraints on the crust.

SEIS, the seismometer of the InSight mission, which landed on Mars on 26 November 2018, is monitoring the seismic activity of the planet. The goal of the Mars Structure Service (MSS) is to provide, as a mission product, the first average 1-D velocity model of Mars from the recorded InSight data. Prior to the mission, methodologies have been developed and tested to allow the location of the seismic events and estimation of the radial structure, using surface waves and body waves arrival times, and receiver functions. The paper describes these validation tests and compares the performance of the different algorithms to constrain the velocity model below the InSight station and estimate the 1-D average model over the great circle path between source and receiver. These tests were performed in the frame of a blind test, during which synthetic data were inverted. In order to propagate the data uncertainties on the output model distribution, Bayesian inversion techniques are mainly used. The limitations and strengths of the methods are assessed. The results show the potential of the MSS approach to retrieve the structure of the crust and underlying mantle. However, at this time, large quakes with clear surface waves have not yet been recorded by SEIS, which makes the estimation of the 1-D average seismic velocity model challenging. Additional locatable events, especially at large epicentral distances, and development of new techniques to fully investigate the data, will ultimately provide more constraints on the crust and mantle of Mars. ©2020 The Authors., Xu and Beghein were funded by NASA InSight PSP Grant 80NSSC18K1679. M. P. P., W. B. B., S. E. S., and S. T. were supported the NASA InSight mission and funds from the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The French authors acknowledge the French Space Agency CNES as well as CNRS and the French team universities for personal and infrastructure supports. The development of the MSS and associated software was provided by ANR (ANR‐14‐CE36‐0012‐02 and ANR‐19‐CE31‐0008‐08) and, for IPGP team, by UnivEarthS Labex program (ANR‐10‐LABX‐0023), IDEX Sorbonne Paris Cit (ANR‐11‐IDEX‐0005‐0). M1a and M2b models computation used HPC resources of CINES under the allocation A0050407341 and A0070407341 attributed by GENCI (Grand Equipement National de Calcul Intensif). M1b model is computed at the Centre de calcul des pays de la Loire (ccipl). This is InSight Contribution Number 141 and IPGP Contribution 4104. M. v. D. and S. C. S. were supported by a grant from the Swiss National Science Foundation (SNF‐ANR Project 157133 Seismology on Mars) and the Swiss National Supercomputing Center (CSCS) under Project ID sm682. A. K. and D. G. would like to acknowledge support from the Swiss National Science Foundation (SNF‐ANR project 172508 Mapping the internal structure of Mars).

IRIS Data Services are funded through the Seismological Facilities for the Advancement of Geoscience and EarthScope (SAGE) Proposal of the National Science Foundation under Cooperative Agreement EAR-1261681. We thank the universities of Oregon, California at Berkeley, Rice and the Earthscope program of the National Science Foundation for acquiring and providing the FAME waveform data. B.T. was funded by a Delegation CNRS and Conge pour Recherches et Conversion Thematique from the Universite de Lyon to visit the Research School of Earth Sciences (RSES), The Australian National University (ANU). We thank the CNRS, the Universite de Lyon, and ANU, for providing the funding and research environment.

International audience; We reassess the mantle transition zone structure below the northeast Asia margin in the context of subduction of the Pacific plate below the Eurasian continent. We use two independent approaches of teleseismic imaging, namely, compressional-to-shear converted waves (receiver functions) and shear wave underside reflections (SS precursors), and compare them within their statistical uncertainties. We find localized complexity in the interfaces marking solid phase changes in mantle minerals, in terms of both apparent topography and reflectivity. The 660-km discontinuity is doubled, with approximate to 80-km maximum vertical distance between the interfaces, over an 890 x 350 km(2) region between 36-44 degrees N and 130-133 degrees E at the tip of the subducted Pacific plate. A similar complexity exists on the 410-km discontinuity, coinciding with the presence of a deep cluster of seismicity below the Japan Sea. Both methods suggest the presence of low-velocity zones atop the 410, within the mantle transition zone, and below the 660. This complex seismic signature is related to the Pacific plate and interpreted in light of the subduction thermal regime and phase equilibria for a pyrolitic mantle composition. Phase changes manifest themselves as broad zones of velocity gradients with localized doubled or multiple first-order discontinuities, associated with transitions in the olivine, pyroxene, and garnet systems. An average pyrolitic composition and local temperatures of 1000-1300K can explain the observed velocity gradients and multiple discontinuities. We show that the dissolution of stishovite, a high-pressure polymorph of SiO2, into the higher-pressure perovskite mineral, is a possible explanation for the low-velocity zones at the top of the lower mantle. Plain Language Summary In the early 1990s, the idea emerged that the study of seismic discontinuities near 410- and 660-km depths could inform us about the vertical circulation of material in the mantle. These two discontinuities were assigned to solid phase changes of a silicate mineral rich in magnesium (90%) and iron (10%), olivine. Advances in the field of mineralogy and high-pressure physics suggested, however, that other minerals, garnet and pyroxenes, should contribute to the general seismic signature. Due to the limited imaging capability of seismological methods and station deployments, however, a one-to-one correspondence between observed seismic structure and mineralogy has remained elusive, at best controversial. We provide here from two independent approaches compelling evidence for localized complexity in the seismic structure near 410- and 660-km depths, which we relate, through mineralogical modeling, to phase changes in olivine, garnet, pyroxene, and silica systems. This work provides the observational basis for assessing the viability of geodynamic and thermochemical models of the mantle beneath the northeast Asia margin.

We present a new methodology for inverting P--to--S receiver function (RF) waveforms directly for mantle temperature and composition. This is achieved by interfacing the geophysical inversion with self-consistent mineral phase equilibria calculations from which rock mineralogy and its elastic properties are predicted as a function of pressure, temperature, and bulk composition. This approach anchors temperatures, composition, seismic properties, and discontinuities that are in mineral physics data, while permitting the simultaneous use of geophysical inverse methods to optimize models of seismic properties to match RF waveforms. Resultant estimates of transition zone (TZ) topography and volumetric seismic velocities are independent of tomographic models usually required for correcting for upper mantle structure. We considered two end-member compositional models: the equilibrated Equilibrium Assemblage (EA) and the disequilibrated Mechanical Mixture (MM) models. Thermal variations were found to influence arrival times of computed RF waveforms, whereas compositional variations affected amplitudes of waves converted at the TZ discontinuities. The robustness of the inversion strategy was tested by performing a set of synthetic inversions in which crustal structure was assumed both fixed and variable. These tests indicate that unaccounted-for crustal structure strongly affects the retrieval of mantle properties, calling for a two-step strategy presented herein to simultaneously recover both crustal and mantle parameters. As a proof of concept, the methodology is applied to data from two stations located in the Siberian and East European continental platforms. ISSN:2169-9313 ISSN:0148-0227 ISSN:2169-9356

On 15 February 2013 around 03:20:00 UTC, the largest meteor reported since the 1908 Tunguska event was observed as a fireball traveling through the Earth's atmosphere, exploding in an air burst near the city of Chelyabinsk, Russia. The rarity of such an event provides a unique window on the physics of meteoroid collision. We report the fine seismic detection of Rayleigh waves produced by the coupling of ground motion with the incident shock wave at distances up to 4000 km from the event. Combining information from seismic beam-forming analysis, recon- structed trajectory from casual video records, and remote sensing, we identify the Rayleigh waves as being initiated by the shock wave produced by the main blast that occasioned damages and injuries in Chelyabinsk. From the Rayleigh wave observations, we report a magnitude Ms ~ 3.7 seismic source.

Seismic activity and non-volcanic tremors are often associated with fluid circulation resulting from the dehydration of subducting plates. Tremors in the overriding continental crust of several subduction zones suggest fluid circulation at shallower depths, but potential fluid pathways are still poorly documented. Using receiver function analysis in the Cascadia subduction zone, we provide evidence for a seismic discontinuity near 15 km depth in the crust of the overriding North American plate. This interface is segmented, and its interruptions are spatially correlated with conductive regions of the forearc and shallow swarms of seismicity and non-volcanic tremors. These observations suggest that fluid circulation in the overriding plate is controlled by fault zones separating blocks of accreted terranes. These zones constitute fluid escape routes that may influence the seismic cycle by releasing fluid pressure from the megathrust.

International audience; We exploit conversions between P and S waves for large-scale, high-resolution imaging of the mantle transition zone beneath Northwest Pacific and the margin of Eastern Asia. We find pervasive reflectivity concentrated in two bands with apparent wave-speed reduction of -2% to -4% about 50 km thick at the top of the transition zone and 100 km thick at the bottom. This negative reflectivity associated with the scattered-waves at depth is interpreted jointly with larger-scale mantle tomographic images, and is shown to delineate the stagnant portions of the subducted Pacific plate in the transition zone, with largely positive shear-wave velocity contrasts. The upper reflectivity zone connects to broad low-velocity regions below major intra-plate volcanoes, whereas the lower zone coincides locally with the occurrence of deep-focus earthquakes along the East Asia margin. Similar reflectivity is found in Pacific Northwest of the USA. We demonstrate that the thermal signature of plates alone is not sufficient to explain such features. Alternative explanations for these reflective zones include kinetic effects on olivine phase transitions (meta-stability), compositional heterogeneities within and above stagnant plates, complex wave-propagation effects in the heterogeneous slab structure, or a combination of such factors. We speculate that part of the negative reflectivity is the signature of compositional heterogeneities, as revealed by numerous other studies of seismic scattering throughout the mantle, and that such features could be widespread across the globe.

We report from converted seismic waves, a pervasive seismically anomalous layer above the transition zone beneath the western US. The layer, characterized by an average shear wave speed reduction of 1.6%, spans over an area of similar to 1.8 x10(6) km(2) with thicknesses varying between 25 and 70 km. The location of the layer correlates with the present location of a segment of the Farallon plate. This spatial correlation and the sharp seismic signal atop of the layer indicate that the layer is caused by compositional heterogeneity. Analysis of the seismic signature reveals that the compositional heterogeneity can be ascribed to a small volume of partial melt (0.5 +/- 0.2 vol% on average). This article presents the first high resolution map of the melt present within the layer. Despite spatial variations in temperature, the calculated melt volume fraction correlates strongly with the amplitude of P-S conversion throughout the region. Comparing the values of temperature calculated from the seismic signal with available petrological constraints, we infer that melting in the layer is caused by release of volatiles from the subducted Farallon slab. This partially molten zone beneath the western US can sequester at least 1.2 x 10(17) kg of volatiles, and can act as a large regional reservoir of volatile species such as H or C. (C) 2017 Elsevier B.V. All rights reserved.

International audience; We present a 3-D model of upper mantle seismic discontinuity structure below Cascadia using a receiver function Kirchhoff migration method. A careful analysis of the primary and multiple reverberated phases allows imaging of the Juan de Fuca plate dipping below the North American continent. The subducting slab is observed as an eastward dipping signal at all latitudes. We associate this signal with a thermal gradient between the slab and surrounding mantle, rather than a sharp chemical discontinuity. Our model also shows along-strike variations in the dipping angle and strength of this signal. To the southern and northern ends of the subduction system, the signal is clearly observed down to similar to 300km. However, beneath central Oregon, this structure is missing below similar to 150km depth. We propose that this gap is due to weakening of the slab beneath central Oregon possibly caused by deformation and hydration combined with plume-slab interaction processes after subduction.

International audience; Within the upper mantle, the seismic discontinuity at 410-km depth marks the top of the transition zone and is attributed to pressure-induced transformation of olivine into wadsleyite mineral assemblage. Just above the 410-km discontinuity, a layer characterized by low seismic wave velocities has been identified regionally(1,2). This low velocity layer shows poor lateral continuity and is thought to represent partial melting induced by local effects, such as the dehydration of subducted crust(1) or the dehydration of water-bearing silicates beneath continental platforms in association with mantle plumes(2). However, some models predict that the low-velocity layer should extend globally, because the weaker water storage capacity of upper mantle minerals should induce partial melting of water-bearing silicates throughout this region(3,4). Here we report seismic observations from 89 stations worldwide that indicate a thick, intermittent low-velocity layer is located near 350 km depth in the mantle. The low-velocity layer is not limited to regions associated with subduction or mantle plumes, and shows no affinity to a particular tectonic environment. We suggest that our data image the thickest parts of a more continuous global structure that shows steep lateral variations in thickness. The presence of a global layer of partial melt above the 410-km discontinuity would modify material circulation in the Earth mantle and may help to reconcile geophysical and geochemical observations(3).

Seismic discontinuities at 410 and 660 km depth are usually attributed to solid phase changes within the olivine component of the mantle. The Clapeyron slopes γ 410 and γ 660 , i.e. the thermal dependence of the depths of reactions, have been shown experimentally to be of opposite signs. Yet, their values are not well constrained by laboratory measurements. Seismological observations have not been more precise due to the difficulty to separate the competing effects of background wave-velocities and of temperature on the topography of discontinuities. In this study we use conversion imaging of interfaces under western US. We propose a new approach to derive a seismological estimate of the Clapeyron slopes with respect to γ 410 for the major and minor phase changes of the transition zone. We obtain γ 660 ≈ − 3 MPa K − 1 for γ 410 ≈ + 3 MPa K − 1 . We construct “seismic phase diagrams” of the transition zone that can be directly compared with experimental phase diagrams. We also apply a “Z–Γ” transform to better constrain the Clapeyron slopes γ of the minor phase changes. Although tenuous, signals in seismic phase diagrams suggest that minor phase transitions, both in the olivine and the non-olivine component of the mantle, have visible seismic expressions. They can tentatively be described as follows. The ‘410’ is overlaid at low temperature by an interface corresponding to a decrease of velocity with depth and γ ≈ + 3 MPa K − 1 . The ‘660’ widens at high temperature and is preceded at low temperature by an interface, the ‘620’, with γ ≈ + 7 MPa K − 1 . A ‘520’ is suggested with γ ≈ 2 – 3 MPa K − 1 . These last two interfaces correspond to velocity increases with depth. At last, near 590 km depth, an interface may be associated with a velocity reduction showing a weak dependence on temperature ( γ ∼ 0 MPa K − 1 ).

We employ a new thermodynamic method for self-consistent computation of compositional and thermal effects on phase transition depths, density, and seismic velocities. Using these profi les, we compare theoretical and observed differential traveltimes between P410s and P (T 410 ) and between P600s and P410s (T 660‐410 ) that are affected only by seismic structure in the upper mantle. The anticorrelation between T 410 and T 660‐410 suggests that variations in T 410 and T 660‐410 of ~8 s are due to lateral temperature variations in the upper mantle transition zone of ~400 K. If the mantle is a mechanical mixture of basaltic and harzburgitic components, our traveltime data suggest that the mantle has an average temperature of 1600 ± 50 K, in agreement with temperature estimates from magma compositions of mid-ocean ridge basalts. We infer a 100 K hotter mantle if we assume the mantle to have a homogeneous pyrolitic composition. The transition-zone temperature beneath hotspots and within subduction zones is relatively high and low, respectively. However, the largest variability in T 410 and T 660‐410 is recorded by global stations far from subduction zones and hotspots. This indicates that the 400 K variation in upper mantle temperature is complicated by tilted upwellings, slab fl attening and accumulation, ancient subduction, and processes unrelated to present-day subduction and plume ascent.

We present a new global study of the transition zone from Pds converted waves at the 410- and 660-km discontinuities. Our observations extend previous global Pds studies with a larger data set, especially in oceanic regions where we have been able to measure Pds travel times, sampling the mantle transition zone (MTZ) beneath 26 hotspot locations. We find significant lateral variations of the MTZ thickness. Both the maximum variations (+/- 35 - 40 km) and the long-wavelength pattern are in overall agreement with previous SS precursors studies. The MTZ is generally thick beneath subduction zones, where the observed MTZ variations are consistent with thermal anomalies ranging between -100 degrees K and -300 degrees K. In Central and North America, we observe an NW - SE pattern of thick MTZ, which can be associated with the fossil Farallon subduction. We do not find clear evidence for a thin MTZ beneath hotspots. However, the 410- km discontinuity remains generally deepened after correcting our Pds travel times for the 3D heterogeneities located above the MTZ, and its topography variations can be explained by thermal anomalies between + 100 degrees K and +300 degrees K. The depth of the 660-km discontinuity may be less temperature sensitive in hot regions of the mantle, which is consistent with the effect of a phase transition from majorite garnet to perovskite at a depth of 660 km.

International audience

InSight landed on Mars on November 26, 2018, in western Elysium Planitia. The Mars crust beneath the lander is subject to complex geologic history next to the great topographic and crustal dichotomy of Mars. Understanding this part of the Martian crust in the subsurface would aid future investigations of the internal structure of the planet based on seismic datasets collected by the Seismic Experiment for Interior Structure (SEIS) instrument. Here, we investigate the subsurface structure and composition from the analysis of mineralogy and morphology of exposures in impact craters as well as on scarps and knobs in the general region of Elysium Planitia. Using a combination of orbital datasets, we identify exposures of subsurface materials with distinct composition and physical properties. We find olivine and pyroxene detections associated with small impact craters (1.5–7 km) rim in the vicinity of InSight lander, as well as in the transition unit and Elysium volcanic unit. Fe/Mg phyllosilicates have been identified in the central peak of the 51-km diameter Kalpin crater and on knobs in the transition unit between the dichotomy and the plains. In addition, eroded meter-scale layered unit subject to erosion has been identified in six impact craters to the north-east of the landing site, including Kalpin crater. Massive bedrock and layered, weak materials co-occur in the transition unit, indicating a complex origin. These results together suggest both materials with altered composition and layered deposit occur as distinct geologic units beneath the basaltic lava flow units. Through analogy with terrestrial sedimentary rocks or clay-bearing sediments, we suggest physically weak materials exist beneath the lava flow units in the general region of Elysium Planitia. The spatial distribution and continuity of these materials are unclear due to the lack of exposures within the lava flow unit where the InSight lander is located. These subsurface materials of distinct physical properties may result in increased attenuation or reverberations of seismic waves, to be collected by SEIS. The findings suggest a close investigation of the potential effects of subsurface stratigraphy on seismic data would help inform future data interpretation and understanding of the internal structure of Mars.