Autocorrelation of the Ground Vibrations Recorded by the SEIS-InSight Seismometer on Mars

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 “Université Fédérale de Toulouse Midi Pyrénées” and the “Région 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

The seismicity of Mars

The InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) mission landed in Elysium Planitia on Mars on 26 November 2018 and fully deployed its seismometer by the end of February 2019. The mission aims to detect, characterize and locate seismic activity on Mars, and to further constrain the internal structure, composition and dynamics of the planet. Here, we present seismometer data recorded until 30 September 2019, which reveal that Mars is seismically active. We identify 174 marsquakes, comprising two distinct populations: 150 small-magnitude, high-frequency events with waves propagating at crustal depths and 24 low-frequency, subcrustal events of magnitude Mw 3–4 with waves propagating at various depths in the mantle. These marsquakes have spectral characteristics similar to the seismicity observed on the Earth and Moon. We determine that two of the largest detected marsquakes were located near the Cerberus Fossae fracture system. From the recorded seismicity, we constrain attenuation in the crust and mantle, and find indications of a potential low-S-wave-velocity layer in the upper mantle. © 2020, The Author(s), under exclusive licence to Springer Nature Limited.We acknowledge NASA, CNES and its partner agencies and institutions (UKSA, SSO, DLR, JPL, IPGP-CNRS, ETHZ, IC and MPS-MPG) and the flight operations team at JPL, SISMOC, MSDS, IRIS-DMC and PDS for providing SEIS data. The Swiss co-authors were jointly funded by (1) the Swiss National Science Foundation and French Agence Nationale de la Recherche (SNF-ANR project 157133 ‘Seismology on Mars’), (2) the Swiss National Science Foundation (SNF project 172508 ‘Mapping the internal structure of Mars’), (3) the Swiss State Secretariat for Education, Research and Innovation (SEFRI project ‘MarsQuake Service-Preparatory Phase’) and (4) ETH Research grant no. ETH-06 17-02. Additional support came from the Swiss National Supercomputing Centre (CSCS) under project ID s922. The Swiss contribution in the implementation of the SEIS electronics was made possible by funding from the federal Swiss Space Office (SSO) and contractual and technical support from the ESA-PRODEX office. The French Team acknowledge the French Space Agency CNES, which has supported and funded all SEIS-related contracts and CNES employees, as well as CNRS and the French team universities for personal and infrastructure support. Additional support was provided by ANR (ANR-14-CE36-0012-02 and ANR-19-CE31-0008-08) and, for the IPGP team, by the UnivEarthS Labex programme (ANR-10-LABX-0023), IDEX Sorbonne Paris Cité (ANR-11-IDEX-0005-0). SEIS-SP development and delivery were funded by the UK Space Agency. A portion of the work was carried out at the InSight Project at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. The MPS SEIS team acknowledges funding for development of the SEIS leveling system by the DLR German Space Agency. We thank gempa GmbH for software development related to the MQS tools. This paper is InSight contribution number 102.Peer reviewe

Constraints on the shallow elastic and anelastic structure of Mars from InSight seismic data

Mars’s seismic activity and noise have been monitored since January 2019 by the seismometer of the InSight (Interior Exploration using Seismic Investigations, Geodesy and Heat Transport) lander. At night, Mars is extremely quiet; seismic noise is about 500 times lower than Earth’s microseismic noise at periods between 4 s and 30 s. The recorded seismic noise increases during the day due to ground deformations induced by convective atmospheric vortices and ground-transferred wind-generated lander noise. Here we constrain properties of the crust beneath InSight, using signals from atmospheric vortices and from the hammering of InSight’s Heat Flow and Physical Properties (HP3) instrument, as well as the three largest Marsquakes detected as of September 2019. From receiver function analysis, we infer that the uppermost 8–11 km of the crust is highly altered and/ or fractured. We measure the crustal diffusivity and intrinsic attenuation using multiscattering analysis and find that seismic attenuation is about three times larger than on the Moon, which suggests that the crust contains small amounts of volatiles

Analyse des divergences entre les prix à la production et les prix de détail dans la filière porc, de 1985 à 1991

National audienceDes divergences d'évolution entre les prix à la production et les prix de détail sont apparues depuis 1985 sur les produits carnés. Les principales explications seraient la déflation des prix à la production, alors que les charges des opérateurs continuent à suivre l'inflation générale, et la libération des prix de détail. Au niveau de l'abattage-découpe en porc, la faible divergence des prix s'explique pour l'essentiel par l'inflation sur les charges. Au niveau des ventes au détail en grande surface, le rayon boucherie et le rayon charcuterie à la coupe auraient augmenté leur marge brute, sans que l'on puisse observer une augmentation semblable de leurs frais : ces rayons auraient perdu ainsi leur ancien statut de rayon à faible marge. Toutefois, en viande fraîche de porc, la divergence des prix s'explique assez bien par l'inflation sur les coûts des distributeurs

Les divergences de prix dans les filières viandes de 1985 à 1991

National audienceL'évolution des prix à la production reste un facteur essentiel pour comprendre l'évolution des prix de détail en viandes. Sur la période 1962-1976, le Centre d'étude des revenus et des coûts (CERC) avait observé des augmentations de prix similaires. La présente étude décrit et explique les changements apparus depuis 1985 : les prix à la production baissent alors que les prix de détail augmentent excepté en volailles et en agneau. L'étude des divergences de prix a été scindée en deux parties, l'abattage-découpe et la distribution de détail. Au niveau abattage, l'essentiel des divergences s'explique par l'inflation sur les différents éléments de la marge brute et la déflation des prix d'achat à la production. Au détail en grandes surfaces, la suppression de la taxation en 1986 a donné une grande liberté aux distributeurs en matière de stratégies de prix : ainsi les baisses de prix à la production ont été de plus en plus mises à profit pour organiser des ventes en promotion. Cependant, si, pour une raison ou pour une autre, une catégorie de viande ne se prête pas bien à ces promotions, alors la marge brute du distributeur tend à augmenter : ceci expliquerait les hausses spécifiques de marges sur le veau, le boeuf et le jambon cuit, dont l'inflation n'explique qu'une faible partie. En revanche le porc frais, l'agneau et les volailles ont été de plus en plus vendus en promotion et la simple répercussion de l'inflation sur leurs marges justifie l'essentiel des divergences de prix observées

Kelvin waves activity in the Eastern Tropical Atlantic

International audienc

Monitoring Saturation Changes with Ambient Seismic Noise and Gravimetry in a Karst Environment

On a heterogeneous karstic site in the Larzac plateau (France), we performed cross-correlations of ambient seismic noise recorded at two broadband seismometers to obtain daily seismic velocity changes. Rayleigh velocity changes at the 6- to 8-Hz frequency band show variations of ±0.2% over 1 yr. Assuming a simple velocity profile, changes are expected to come from depths of tens of meters. Therefore velocity changes at 6 to 8 Hz were interpreted as induced by water saturation changes. A slow infiltration rate would explain the delay of several months between the rainy season (November) and the minimum velocity (June). Superconducting gravimeter, evapotranspiration, and magnetic resonance sounding (MRS) measurements were then combined with seismic data in one-dimensional physical simulations. Velocity changes clearly constrain hydrological parameters, like saturated hydraulic conductivity, even if the Biot–Gassmann theory does not explain all of the amplitude observed. Nevertheless, this nondestructive method demonstrates great potential in hydrological model calibration. It overcomes the lack of depth resolution of gravimetry and the lack of temporal resolution of MRS. The combination of ambient seismic noise with gravimetry and MRS could fill the instrumental gap currently existing in hydrology for the study of deep and/or complex critical zones

Diminution de la vitesse des ondes de cisaillement précédant la fluidification d'un glissement de terrain argileux reproduit en laboratoire

International audienceClay slopes are susceptible to suddenly liquefy into rapidly accelerating landslides, thereby threatening people and facilities in mountainous areas. Because the shear-wave velocity (V s ) characterizes the medium stiffness, this parameter can potentially be used to investigate the rheological behavior of clay materials before and during the solid-to-fluid transition associated to such landslide failures. Previous rheometrical studies performed on clay samples coming from Trièves landslides (French Alps) have established that this material behaves as a yield stress fluid with a marked viscosity bifurcation. When the applied stress reaches a critical level, the viscosity decreases abruptly, along with V s which tends to zero in the fully fluidized material. Here, we monitor the Rayleigh wave velocity (V R ) variations in a saturated clay layer placed in a flume and progressively brought to failure by tilting the device. Experiments performed on clay samples with different water contents show a significant relative drop in V R values (and hence in V s ) before the onset of the mass movement. Additional rheometrical analyses point out that this precursory drop in V s is presumably due to a complex transient rheological response of the clay. These new results confirm that V s variations constitute a good indicator for monitoring clay slope stability

The solid-to-liquid transition in the Trieves clay: the lessons from rheometric and seismic tests

The Trièves clay (Western French Alps) is a thick Quaternary formation affected by numerous slow earthslides (slide velocity of a few cm/year to a few dm/year). Under extreme meteorological conditions (heavy rain falls, quick melting of the snow cover), these slides can turn into devastating earthflows. In order to study this solid-to-liquid transition, identification, rheometric and seismic laboratory studies were per-formed on clay samples collected at the border between the Avignonet and Harmalière landslides. Identifica-tion tests showed the wide range of liquid limits characterizing the Trièves clay. Rheometric creep tests were performed on two samples with different gravimetric water content. The clay material behaves as a thixotrop-ic yield stress fluid and is characterized by a marked viscosity bifurcation. This rheological behavior could be a key point explaining the solid-to-liquid behavior of this material. The seismic study aims at characterizing the clay in different moisture conditions and to test our capacity to measure Rayleigh wave velocity (VR) var-iations in clay from the liquid limit to a drier state. VR values ranging from 22m/s to 120m/s were successfully measured for a gravimetric water content decreasing from 40% to 26%, respectively. These results show that VR (and indirectly the shear-wave velocity) is very sensitive to a change in the clay physical state and could be used as a monitoring parameter