A RECENT MUDFLOW IN THE NILI FOSSAE REGION OF MARS: MORPHOLOGY AND NUMERICAL SIMULATIONS

International audienceIntroduction: There is a wide morphological range of landslides on Mars [1-3]. They can mobilize large volumes of material: estimates range from 10 6 to 10 12 m 3 [4]. Those formed during Amazonian are interpreted as being dry landslides [e.g., 5,6]. This study focus-es on a small landslide, which we estimate to have a volume of 10 6 m 3 , located in the Nili Fossae region. We performed a morphological analysis of this landslide and found similarities with mudslides on Earth. Terrestrial mudslides necessarily involve the presence of liquid water. The morphological similarities between the martian landslide and terrestrial mudslides raises the question of the potential role of water on Mars. To analyze the rheology of the martian landslide we conduct numerical simulations using the numerical model SHALTOP [e.g., 7,8]. The initial results from our simulations lend support to our interpretation that the landslide in question flowed like a terrestrial mudslide and had a viscous, rather than granular, behavior. Dry simulations could not reproduce the mass distribution of the landslide, as well as being poor matches from terrestrial analogy. Approach: To conduct the morphological analysis of the landslide we used High Resolution Imaging Science Experiment (HiRISE) images with a resolution of 25-50 cm/pix and ConTeXt imager (CTX) at 6m/pix. In order to perform the numerical simulation we created a Digital Elevation Model (DEM) at 2 m/pix from HiRISE stereo images using the Ames Stereo Pipeline [9]. We estimated the pre-landslide topography using the methods in Conway & Balme [10] and then calculated the mobilized volume. The DEM and the estimated release volume were used as an inputs to SHALTOP. The aim of the simulations was to vary the types of dry or viscous behavior in order to reproduce the runout and the mass distribution of the deposit of the landslide. For dry flows we used the Pouliquen and Forterre frictional law which parametrizes a dense granular flow in terms of Froude number, friction coefficient , velocity and thickness of the flowing layer [11]. For viscous flows we used the Bingham frictional law, which parametrizes amongst others the yield stress and viscosity [12]. Morphological results: The landslide studied here is located in a 25 km diameter impact crater to the north of the Nili Fossae. The landslide detached from the inner crater wall with a slope of 21°. It is composed of at least of two depositional lobes (Fig. 1a). The smallest is about 1000 m long by 230 m wide. Th

Broad-band ambient noise characterization by joint use of cross-correlation and MUSIC algorithm

International audienceSeveral days of passive seismic broad-band recordings (vertical component) from a dense 3 × 6 km array installed near Chémery (France), with about 100 seismometers, are analysed for wavefield characterization between 0.1 and 3 Hz. Backazimuth is determined by using the Multiple Signal Characterization (MUSIC) algorithm at frequencies below 1 Hz, and non-coherent cross-correlation beamforming above 1 Hz, since the latter is less sensitive to aliasing issues. A novel method of determining the wavefield velocity is introduced, consisting of processing a cross-correlation common-offset gather by the MUSIC algorithm. The fundamental and three higher modes of Rayleigh waves (R0, R1, R2 and R3) are identified under 1 Hz. Above 1.5 Hz, the Lg phase is detected, while R0 and R1 are also present. Roughly between 1 and 1.5 Hz, a quicker phase, probably Pg, is detected. Both Pg and Lg are dominant during night time, suggesting they have a natural origin, which is also consistent with their backazimuth pointing towards the Atlantic. Large scale 2-D spectral-element simulations using deep- and shallow-water ocean sources confirm the possibility of the Lg phase excitation. Thus, even above 1 Hz, natural sources can explain the major part of the ambient noise energy during quiet time periods

Synthetic benchmarking of concentrated pyroclastic current models

International audienceValidation and benchmarking of pyroclastic current (PC) models is required to evaluate their performance and their reliability for hazard assessment. Here we present results of a benchmarking initiative built to evaluate four models commonly used to assess concentrated PC hazard: SHALTOP, TITAN2D, VolcFlow and IMEX_SfloW2D. The benchmark focuses on the simulation of channelized flows with similar source conditions over five different synthetic channel geometries: 1) a flat incline plane, 2) a channel with a sharp 45° bend, 3) a straight channel with a break-in-slope, 4) a straight channel with an obstacle, and 5) a straight channel with a constriction. Several outputs from 60 simulations using three different initial volume fluxes were investigated to evaluate the performance of the four models when simulating valley-confined PC kinematics, including overflows induced by topographic changes. Quantification of the differences obtained between model outputs at t = 100 s allowed us to identify: 1) issues with the Voellmy-Salm implementation of TITAN2D and 2) small discrepancies between the three other codes that are either due to various curvature and velocity formulations and/or numerical frameworks. Benchmark results were also in agreement with field observations of natural PCs: a sudden change in channel geometries combined with a high-volume flux are keys to generate overflows. The synthetic benchmarks proved to be useful for evaluating model performance, needed for PCs hazard assessment. The overarching goal is to provide an interpretation framework for volcanic mass flow hazard assessment studies to the geoscience community

Performance and limits of a shallow-water model for landslide-generated tsunamis: from laboratory experiments to simulations of flank collapses at Montagne Pelée (Martinique)

International audienceWe investigate the dynamics and deposits of granular flows and the amplitude of landslide-generated water waves using the HySEA depth-averaged shallow-water numerical model, both at laboratory and field scales. We evaluate the different sources of error by quantitatively comparing the simulations with (i) new laboratory experiments of granular collapses in different conditions (dry, immersed, dry flow entering water) and slope angles and (ii) numerical simulations made with the SHALTOP code that describes topography effects better than most depth-averaged landslide-tsunami models. For laboratory configurations, representing the limits of the shallow-water approximation in such models, we show that topography and non-hydrostatic effects are crucial. When topography effects are accounted for empirically—by artificially increasing the friction coefficient and performing non-hydrostatic simulations—the model is able to reproduce the granular mass deposit and the waves recorded at gauges located at a distance of more than two to three times the characteristic dimension of the slide with an error ranging from 1 to 25 per cent depending on the scenario, without any further calibration. Taking into account this error estimate, we simulate landslides that occurred on Montagne Pelée volcano, Martinique, Lesser Antilles as well as the generated waves. Multiple collapse simulations support the assumption that large flank collapses on Montagne Pelée likely occurred in several successive subevents. This result has a strong impact on the amplitude of the generated waves and thus on the associated hazards. In the context of the ongoing seismic volcanic unrest at Montagne Pelée volcano, we calculate the debris avalanche and associated tsunamis for two potential flank-collapse scenarios

Dynamics of recent landslides (<20 My) on Mars: Insights from high-resolution topography on Earth and Mars and numerical modelling

Landslides are common features found on steep slopes on Mars and the role of water in their formation is an open question. Our study focuses on three young martian landslides whose mechanism of formation is unknown and knowing their formation mechanism could give us key information on recent martian climate and/or tectonics. They are less than 5 km long, and formed during the Late Amazonian Epoch, with an age <20 Ma when Mars is thought to have had a hyperarid climate. To better understand the dynamics and formation mechanism of these landslides, we combine two approaches: geomorphic comparison between martian and terrestrial landslides using remote sensing data from the High Resolution Imaging Science Experiment (HiRISE) and the Colour and Stereo Surface Imaging System (CaSSIS), and numerical modelling using a dry granular flow dynamical model. Our geomorphic analysis revealed two contrasting morphologies suggesting differing dynamics and formation mechanisms. Two of the three martian landslides resemble terrestrial rockslides, while the third is more akin to terrestrial mudslides. The numerical modelling, although not fully conclusive, broadly supports our in- terpretations from the morphological observations. We suggest that the two landslides resembling terrestrial rockslides could have been triggered by shaking by meteorite impact or marsquakes in the absence of water. On the contrary, we suggest liquid water (originating from ground-ice melted by geothermal heat flux) may have been involved in the initiation of the landslide resembling a terrestrial mudslide. Our results show the value of using morphological comparison between martian and terrestrial landslides combined with numerical modelling to inform the hypotheses of landslide-formation on Mars where in situ analysis is not usually possible