Observatoire des Sciences de l'Univers Nantes Atlantique

L'un des principaux défis dans l'élaboration d'un catalogue d'événements de faible amplitude est la discrimination entre les événements naturels (événements tectoniques) et les événements anthropiques (causés par les activités humaines). Pour parvenir à une discrimination automatique, une méthode basée sur du Deep Learning a été développée : grâce à une base de données suffisamment importante, des algorithmes d'intelligence artificielle peuvent s'entraîner à reconnaitre des objets naturels et prendre des décisions. Les spectrogrammes de formes d'ondes sismiques ont été choisis pour constituer cette base de données.

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Abstract Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at determining the efficiency of surface dissolution as a landshaping process on Titan, in a comparative planetology perspective with the Earth as reference. Our approach is based on the calculation of solutional denudation rates and allow inference of formation timescales for topographic depressions developed by chemical erosion on both planetary bodies. The model depends on the solubility of solids in liquids, the density of solids and liquids, and the average annual net rainfall rates. We compute and compare the denudation rates of pure solid organics in liquid hydrocarbons and of minerals in liquid water over Titan and Earth timescales. We then investigate the denudation rates of a superficial organic layer in liquid methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth depending on its composition, which means that dissolution processes would likely occur but would be 30 times slower on Titan compared to the Earth due to the seasonality of precipitation. Assuming an average depth of 100 m for Titan's lacustrine depressions, these could have developed in a few tens of millions of years at polar latitudes higher than 70°N and S, and a few hundreds of million years at lower polar latitudes. The ages determined are consistent with the youth of the surface (<1 Gyr) and the repartition of dissolution‐related landforms on Titan.

Titan's polar surface is dotted with hundreds of lacustrine depressions. Based on the hypothesis that they are karstic in origin, we aim at determining the efficiency of surface dissolution as a landshaping process on Titan, in a comparative planetology perspective with the Earth as reference. Our approach is based on the calculation of solutional denudation rates and allow inference of formation timescales for topographic depressions developed by chemical erosion on both planetary bodies. The model depends on the solubility of solids in liquids, the density of solids and liquids, and the average annual net rainfall rates. We compute and compare the denudation rates of pure solid organics in liquid hydrocarbons and of minerals in liquid water over Titan and Earth timescales. We then investigate the denudation rates of a superficial organic layer in liquid methane over one Titan year. At this timescale, such a layer on Titan would behave like salts or carbonates on Earth depending on its composition, which means that dissolution processes would likely occur but would be 30 times slower on Titan compared to the Earth due to the seasonality of precipitation. Assuming an average depth of 100 m for Titan's lacustrine depressions, these could have developed in a few tens of millions of years at polar latitudes higher than 70{\deg} N and S, and a few hundreds of million years at lower polar latitudes. The ages determined are consistent with the youth of the surface (<1 Gyr) and the repartition of dissolution-related landforms on Titan.

Abstract We present inversions for the structure of Mars using the first Martian seismic record collected by the InSight lander. We identified and used arrival times of direct, multiples, and depth phases of body waves, for 17 marsquakes to constrain the quake locations and the one‐dimensional average interior structure of Mars. We found the marsquake hypocenters to be shallower than 40 km depth, most of them being located in the Cerberus Fossae graben system, which could be a source of marsquakes. Our results show a significant velocity jump between the upper and the lower part of the crust, interpreted as the transition between intrusive and extrusive rocks. The lower crust makes up a significant fraction of the crust, with seismic velocities compatible with those of mafic to ultramafic rocks. Additional constraints on the crustal thickness from previous seismic analyses, combined with modeling relying on gravity and topography measurements, yield constraints on the present‐day thermochemical state of Mars and on its long‐term history. Our most constrained inversion results indicate a present‐day surface heat flux of 22 ± 1 mW/m 2 , a relatively hot mantle (potential temperature: 1740 ± 90 K) and a thick lithosphere (540 ± 120 km), associated with a lithospheric thermal gradient of 1.9 ± 0.3 K/km. These results are compatible with recent seismic studies using a reduced data set and different inversion approaches, confirming that Mars' potential mantle temperature was initially relatively cold (1780 ± 50 K) compared to that of its present‐day state, and that its crust contains 10–12 times more heat‐producing elements than the primitive mantle.

Abstract. The geochemical composition of deep-sea benthic foraminiferal calcite is widely used to reconstruct sea floor paleoenvironments. The calibration of the applied proxy methods has until now been based on field observations in complex natural ecosystems where multiple factors are interfering. However, laboratory experiments with stable physico-chemical conditions appear to be the ideal way to evaluate the influence of a single parameter. In this paper, we present the oxygen isotopic composition of deep-sea benthic foraminiferal shells entirely calcified under controlled experimental conditions over a large temperature range (4 to 19 °C). The new laboratory protocols developed for this study allowed us to produce large quantities of shells in stable conditions, so that also the shell size effect could be investigated. It appears that when considering a narrow test size range, the curve describing the temperature dependency of δ18O in Bulimina marginata is parallel to the thermodynamically determined curve observed in inorganically precipitated calcite (−0.22‰ °C−1). This observation validates the use of δ18O of this benthic species in paleoceanographical studies. Over the studied size range (50 to 300 μm), the effect of test size was 0.0014‰ μm−1, confirming previous suggestions of a substantial test size effect on δ18O of benthic foraminifera. This study opens new perspectives for future proxy calibrations in laboratory set-ups with deep-sea benthic foraminifera (e.g. quantification of the influence of the carbonate chemistry).

Recent research in northern Iceland has highlighted a significant period of rock slope instability during the early Holocene due to the combined effects of postglacial rebound, relative sea-level fall, and glacially oversteepened mountain slopes. Using the Vatn landslide (Skagafjörður, central northern Iceland) as an example, this paper focuses on this period and describes the sequence of events that led to landsliding. Geomorphic mapping, stratigraphical evidence, and both radiocarbon and tephra dating were applied. Collectively, the data acquired indicate that the landslide occurred between 11,400 and 10,790 cal. yr BP. However, while rock slope failure represents a significant disintegration of mountain slopes, this study suggests that large postglacial landslides might also play a role in arresting sediment transport from other hillslope processes rather than contributing large volumes of sediment.

Since early 2019, the InSight mission has proven that Mars is seismically active, with more than 900 seismic events recorded. Among them, several events have characteristics close to terrestrial tectonic earthquakes. Most of these events are located on the major graben system of Cerberus fossae and, a little further north, on the secondary system of Grjotá Valles. In this study, we invert the seismic moment tensors for nine of these tectonic marsquakes characterized by high quality data. Seven of them are located on Cerberus fossae/Grjotá Valles and two of them are located near the Martian dichotomy. The moment tensors allow us to interpret the nature and depth of the seismic sources at the origin of these events. In our approach, we invert the P and S body waveforms, the PP, SS, PPP and SSS secondary phase maximum amplitudes and we look for solutions with surface waves weaker than the Martian noise. From our results on moment tensors, we determine that all our events have been triggered at moderate depths of 15–36 km. We deduce that the seven events located on Cerberus fossae have geometries similar to the fractures and are generated by tectonics. This activity is supposed to result from the reactivation of previous faults and fractures, which would have been initially induced by the propagation of volcanic dikes at depth. The two dichotomy events are due to deep compressive fracturing of the Martian lowlands. They are therefore triggered by the planetary thermal contraction. Our results are in strong agreement with recent studies on the event depths and on the previous moment tensors calculated for two events.

This paper reviews the climatological influences on major past storm events in the North-east Atlantic. Analyses are based on a millenary record of sedimentological and historical impacts affecting coastal societies. The effects of 20 past storms have been found from sedimentary deposits from the last 1,000 years. Historical archives confirmed these events. This paper highlights five major storms that have markedly impacted coastal populations. They date back to 1351-1352, 1469, 1645, 1711 and 1751 AD. The 1351-1352 AD event is defined as a millennium storm that was "likely apocalyptical", provoking serious damage and long lasting floods on much of the European coast. Major storm impacts have mostly been recorded during positive North Atlantic Oscillation phases. Four decreasing temperature phases are concomitant with 1300-1355, 1420-1470, 1560-1590 and 1690-1715 AD periods, during which much of the northern Atlantic coast of France underwent severe storm damages.

The rate of occurrence of High Frequency (HF) marsquakes, as recorded by InSight at Homestead Hollow, Elysium Planitia, increased after about LS=33∘, and ceased almost completely by LS=187∘, following an apparently seasonal variation with a peak rate near aphelion. We define seismic rate models based on the declination of the Sun, annual solar tides, and the annual CO2 cycle as measured by atmospheric pressure. Evaluation of Akaike weights and evidence ratios shows that the declination of the Sun is the most likely, and the CO2 cycle the least likely driver of this seismic activity, although the discrimination is weak, and the occurrence of a few events in August 2020 is in favor for a triggering by CO2 ice load. We also show that no periodicity related to Phobos' orbit is present in the HF event sequence. Event rate forecasts are presented to allow further discrimination of candidate mechanisms from future observations.

Ontario Lacus is thus far the largest flat‐floored topographic depression of Titan's southern hemisphere interpreted as a permanent or ephemeral lake. From 2005 to 2010, it was imaged several times and at various wavelengths by ISS, VIMS and RADAR instruments onboard Cassini's spacecraft. We analyze the position and uncertainty of Ontario Lacus' margin in all these images using an edge detection method based on image derivation. We find that, given the range of uncertainties in contour locations derived from images, no measurable displacement of Ontario Lacus' margin can be detected between 2005 and 2010 at the actual image spatial resolutions. The discrepancy between this result and previous ones is attributable to differences in (1) the basics behind the methods used, (2) the actual spatial resolutions and contrasts of the available images due to differential atmospheric scattering effects at different wavelengths, and (3) the geomorphological interpretation of contours derived from images acquired at different wavelengths. This lack of measurable displacement in the images suggests that the imaged contour corresponds either (1) to the border of a surface liquid body, provided that potential changes in its extent over five terrestrial years were not sufficiently large to be measured, or (2) to the stationary topographic border between Ontario Lacus' depression and the surrounding alluvial plain. Potential displacements of Ontario Lacus' margin between 2005 and 2010 are thus below the actual resolution of currently available images or have to be sought for within the extent of the topographic depression rather than along its borders.

In Iceland, the Late Weichselian deglaciation led to paraglacial topographic readjustments in the form of extensive deep-seated gravitational slope deformation (DSGSD) and large rockslope failures (landslides and rock-avalanches). Here we provide age estimates for the Stífluhólar landslide (Tröllaskagi Peninsula, Northern Central Iceland), which indicate that this occurred between the end of the Late Weichselian deglaciation (15-13 ka cal. yr. BP) and 8200 ± 1400 cal. yr. BP. Our surface observations also suggest that the Stífluhólar landslide was probably initiated by propagation and connection of DSGSD failure planes. This observation suggests that the DSGSD can act as a precursor for landsliding in paraglacial settings. However, further investigations are still required to fully assess the role of DSGSD as a landslide precursor, both at the local and at a regional scale. Comparisons with other examples of paraglacial rock slope failures in Northern Iceland and in the world suggest that DSGSD is not the main or sufficient precondition for large landslides. The landslide of Stífluhólar occurred due to some combination of favorable preconditioning, predisposing and triggering controls. Predisposing controls include: the favourable dip of the bedding planes in the basaltic bedrock and the presence of intercalated poorly cohesive layers of palagonite between the lava layers, probably acting as basal sliding planes for the landslide.

Abstract Geological mapping in the Ogaden region of SE Ethiopia, integrated with aeromagnetic data, has revealed a large dyke swarm extending SSE more than 600 km from the southern Afar margin across the Somali Plate to the Ethiopia–Somalia border. 40 Ar/ 39 Ar age dating shows that emplacement occurred at 24–27 Ma, contemporaneous with early rifting and dyking in the Red Sea. Slab-pull forces generated at the Zagros subduction zone strained the lithosphere in the Afro-Arabian plate, and dyking began to extend south from the Red Sea at c. 27 Ma, extending across Afar, fed by a plume-related magma source, rather than the rift-related source prevailing along the Red Sea. Immediately south of the Afar margin, the dyke system was emplaced along the Precambrian Marda Fault Zone, and the continuation across the Ogaden may have been controlled by lithospheric weakness associated with a splay of the Marda Fault. We suggest that the Ogaden Dyke Swarm is a zone of crustal dilation continuing the Red Sea trend across the Horn of Africa and constituting the original third ‘arm’ of the Afar triple junction. Geochemical and geochronological analyses indicate that basaltic outpourings from the Ogaden Dyke Swarm flowed at least as far east as the Ethiopia–Somalia border and emanated from the same magma source as the Ethiopian flood basalts, which had erupted earlier at c. 30 Ma. Dykes are emergent only occasionally and are marked at the surface by linear sand-filled troughs varying from 2 to 20 m deep caused by tensional collapse above the dyke tip. Magnetic anomalies associated with the dykes vary in width up to 1 km and likely identify dyke zones. Supplementary material: Geological map of the Marda Fault Zone and surroundings; argon dating analytical protocol and isotopic data corrected from baseline; petrographic description of representative samples of the Ogaden Dyke Swarm and analytical protocol used for geochemistry are available at http://www.geolsoc.org.uk/SUP18829 .

Cartographic products of the Martian trough system, Valles Marineris, are useful to identify the diversity and complexity of geological activity that has occurred there. A huge fraction of the processes that have shaped the surface of Mars are also concentrated there. A geomorphological map of Ius Chasma in western Valles Marineris is presented. The map is published in three sheets at 1:260,000. It was drawn on the basis of 100 Mars Reconnaissance Orbiter’s Context Camera images of 12 m/pixel resolution, mosaiced using the USGS ISIS Planetary Image Processing Software, and subsequently mapped and interpreted for geomorphology in ArcGIS. The map displays 52 main geomorphological units of which some are further subdivided. They include both well-established features (e.g. spur-and-gully morphology on trough walls, landslide scars, and deposits), and newly reported landforms (e.g. alluvial fans with dendritic channels, moraines in western Ius Chasma). The proposed classifications of landslide deposits, glacial landforms, and floor areas are more detailed than on any previous map of Valles Marineris. The Ius Chasma map is the first cartographic product presenting a full inventory of dune fields, impact craters, light-toned outcrops, and mass-wasting features.

This study presents an assessment of the diversity and spatial distribution of benthic macrofauna communities along the Moulay Bousselham lagoon and discusses the environmental factors contributing to observed patterns. In the autumn of 2018, 68 stations were sampled with three replicates per station in subtidal and intertidal areas. Environmental conditions showed that the range of water temperature was from 25.0 °C to 12.3 °C, the salinity varied between 38.7 and 3.7, while the average of pH values fluctuated between 7.3 and 8.0. In vegetated habitats, biomass values of the seagrass Zostera noltei Hornemann ranged between 31.7 gDW/m² and 170.2 gDW/m² while the biomass of the seagrass Ruppia cirrhosa (Petagna) Grande between 54.2 gDW/m² and 84.7 gDW/m². Sediment analyses showed that the lagoon is mainly composed of sandy and silty sediments. We recorded 37,165 individuals of macrofauna distributed in 63 taxa belonging to 50 families, with a mean abundance value of 4582.8 ind/m² and biomass average of 22.2 g/m². Distance-based linear modeling analysis (DISTLM) identified sediment characteristics, water parameters and habitat type (biomass of Z. noltei) as the major environmental drivers influencing macrozoobenthos patterns. Our results clearly revealed that the hydrographic regime (marine and terrestrial freshwater), sediment distribution and characteristics and the type of habitat (vegetated vs. unvegetated substrate) are the key factors determining the species composition and patterns of macrozoobenthos assemblages.

Abstract We measure group velocity dispersion of surface waves generated by two meteoroid impacts on Mars close to the lander of the InSight mission. This allows us to probe the crustal structure in the first few kilometers beneath the InSight lander. In combination with body wave arrival times from five impact events, we obtain direct seismic constraints on the seismic velocity of the crust in the vicinity of the InSight landing site. We confirm the existence of a uppermost low‐velocity layer with a mean thickness of ∼1.2 km, interpreted as layered volcanic materials, possibly interstratified with sedimentary and altered materials. Our joint inversion of surface and body waves shows a four‐layer model for the Martian crust, compatible with high‐ and low‐frequency P‐to‐S receiver functions estimated in previous studies.

We present the first experimental results on the evaporation of liquid CH 4 under simulated Titan surface conditions similar to those observed at the Huygens landing site. An average evaporation rate of (3.1 ± 0.6) × 10 −4 kg s −1 m −2 at 94 K and 1.5 bar was measured. While our results are generally higher than previous models based on energy balance, they show an excellent match with a theoretical mass transfer approach. Indeed, we find that evaporation in the Titan environmental chamber is predominantly diffusion driven and affected by the buoyancy of lighter CH 4 in the heavier N 2 atmosphere. After correcting for the difference in gravity of Earth and Titan, the resulting evaporation rate is (1.6 ± 0.3) × 10 −4 kg s −1 m −2 (or 1.13 ± 0.3 mm hr −1 ). Using our experimental evaporation rates, we determine that the low‐latitude storm recently observed by Cassini ISS would have resulted in a maximum evaporated mass of (5.4 ± 1.2) × 10 10 kg of CH 4 equivalent to a 2.4 ± 0.5 m thick layer over 80 days. Based on our results, a sufficient amount of CH 4 can accumulate in the otherwise arid equatorial regions to produce transient ponds and liquid flows.

Abstract NASA's Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight) mission records several high‐frequency (&gt;0.5 Hz) dispersive seismic signals on Mars. These signals are due to the acoustic‐to‐seismic coupling of infrasound generated by the entry and impact of meteorites. This dispersion property is due to infrasound propagating in a structured atmosphere, and we refer to this dispersive infrasound as guided infrasound. We propose to model the propagation of guided infrasound and the seismic coupling to the ground analytically; we use a 1D layered atmosphere on a three‐layer solid subsurface medium. The synthetic ground movements fit the observed dispersive seismic signals well and the fitting indicates that the regolith beneath InSight is about 40‐m in thickness. We also examine and validate the previously‐published subsurface models derived from InSight ambient seismic vibration data.

The contribution of paraglacial rock-slope failure (RSF) to the excavation of cirques in glacial landscapes is explored in the Tindastóll Mountain, Skagi peninsula, northern Iceland. We analyze 8 cirques that developed during the Quaternary and 13 paraglacial RSF cavities that developed during the Holocene in this paleo-plateau. From a reconstruction of the pre-Quaternary surface of the plateau, we calculate the excavated volumes of the cirques and RSF cavities and quantify the contribution of Holocene RSF to cirque growth. By extrapolating this contribution to the whole Quaternary, we find that paraglacial RSF is a first-order contributor to the development of cirques. This contribution is twofold: first, paraglacial RSF creates cavities along valley slopes, in which cirque glaciers can develop during subsequent glaciations; second, paraglacial RSF along pre-existing cirque walls promotes cirque widening and deepening. Our results also reveal that Quaternary glacial/paraglacial erosion rates range from 0.02 to 0.17 mm yr‑¹ in the studied cirques. We infer that glaciers are (i) efficient preparatory factors for the destabilization of slopes by paraglacial RSF, (ii) efficient conveyors to evacuate deposits produced by paraglacial RSF in cirques, but (iii) not necessarily predominant agents in the excavation of cirque bedrocks.

Abstract Using body wave arrival times from 31 seismic events recorded on Mars by the InSight mission, combined with topography and gravity field modeling, we constrained lateral variations of crustal thickness through a Bayesian inversion approach. The parameterization of the seismic structure relies on quantities that influence the thermochemical evolution of Mars, enabling the seismic velocities and densities in the different planetary envelopes to be consistently linked through common physical assumptions. Compared to a 1D structure, models with lateral variations of crustal thickness show two possible interpretations of the thermal evolution of Mars, with either a hot or cold scenario at the present‐day. We found the hot scenario to be more compatible with InSight's radiotracking data and the tidal Love number. We relocated the marsquakes and derived maps of seismicity recorded by InSight, which is mostly located along or North of the boundary between the Northern lowlands and the Southern highlands.