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Rock avalanches display a distinct and complicated behavior that is not yet completely understood. Analyzing the deposit characteristics is crucial for studying the comminution mechanism during its emplacement. Combining field investigation, image processing, and numerical modeling, we explore the deposit characteristics and comminution process of the Wangjiapo rock avalanche triggered by the Mw6.2 Ludian earthquake. First, we use UAV photogrammetry to obtain a three-dimensional point cloud model and a digital orthophoto, from which we semi-automatically extract the geometry and position of 80,602 blocks by adopting a new block photo-analytical procedure, in addition to a further 4388 coarse blocks that were mapped from manual procedure. We analyze the longitudinal and transversal variation of the block distribution within the deposit. Large boulder content was more abundant in the middle part, while the fine blocks were distributed more evenly. Blocks were prevalently equant-sphere (ES) shaped with no dependence on volume and location, but the roundness increased significantly with runout. Additionally, we find fractal dimensions ranging from 1.42 to 1.83, lower with respect to other rock avalanches, probably due to the high weakness of materials. Furthermore, we propose two comminution models based on the significant differences observed between the eastern deposit II and western deposit III, which were caused by variably-cemented breccias. Finally, numerical modeling indicates that the large amount of fine material damped the grain-grain velocity during runout, leading to a limited spread of the rock avalanche and so explaining the short displacement of the front observed.

The process and dynamics of rock fragmentation during the collapse of rockfalls and rock avalanches is a poorly developed topic. The most severe fragmentation often leads to the formation of a rock dust that rises to form a cloud suspended in the air. The understanding of fragmentation processes is hampered by the environmental disturbances that alter the dust cloud deposit shortly after deposition. Here, we study the fragmentation of the October 2017 Pousset rockfall, detached from a NNE facing steep bedrock wall in the permafrost zone, that involved 8,300m3 of metamorphic rock and fell about 800 m. The collapse generated large boulders which rolled downslope and a thick and large dust cloud. The source and deposit were investigated, and dust cloud material was sampled at different locations to reconstruct an exponential thickness distribution and perform grain size characterization. The fragmentation energy was estimated by integrating the spectrum of the grains assuming that the fragmentation energy is proportional to the generated area. The fragmentation energy was found to be about 0.4% of the initial potential energy. Most probable fragmentation points and block deposition areas were evaluated and positioned by means of the HyStone 3D rockfall simulator. Furthermore, we calculated the flow rate of the suspended powder generated by the fragmentation process and compared the results with observations available for the evolution of the phenomenon and the collected samples. The Pousset event, in its relatively simple dynamics, may be a good testing ground to address the current theories of rockfall and rock avalanche fragmentation and dust cloud behavior.

Rock avalanches are extremely rapid, flow-like movements of dry debris mass with extreme destructive potential. Among the dynamical aspects of rock avalanches still inadequately understood, the flow along an erodible substrate stands out as a particularly significant one. We carried out a series of flume experiments to investigate the mobility and the erosion of granular flows on an erodible substrate. It was found that granular flows with a large total mass could evolve into a stable multi-layer flow when suffering the enhanced basal resistance (e.g., loose and dry erodible substrate). Without the variation of the properties of erodible material, the increase of total mass (from 5 kg to 15 kg) as well as the reduction of grain size (from 4 to 8 mm to 1–2 mm) has a positive influence on the mobility of the granular flows (max. 12% and max. 3.4% growths in runout distance, respectively), emphasizing the important roles of both the event magnitude and the internal friction. Total mass and the grain size have prominent influences also on the erosion effect of granular flow. An increase in flow mass causes an enhancement of the eroded depth and of the expansion in both backward and forward directions, while grain size enhanced only forward expansion in addition to the eroded depth. Altogether, the experiments indicate that an erodible substrate may have dramatic effects on the dynamics of an experimental granular flow. When applied to the field, they point to the importance of the nature of the basal material onto which a natural landslide collapses.

The exploration of the solar system has shown that mass movement is a common process on the surface of the terrestrial planets (Mercury, Venus, Mars) and on many moons, including our own. However, it is on Mars that mass movements represent a major geomorphologic force both in terms of frequency, volume (often greater than 10km3, with a record volume of 106 km3), and runout, normally longer than several tens of kilometers. The study of landslides and more in general of mass movements on Mars has important implications for assessing the rock and solid properties and is a tool for understanding the geologic and climatic history of the planet. In contrast to our planet, where a landslide deposit is erased or covered after few thousands of years, mass movements on Mars are still perfectly preserved after times that may be greater than 1 billion years, making of such impulsive events a key for understanding the conditions of the planetary surfaces deep in time. In particular, landslides and their associated primary and secondary deposits may shed light on the possible presence of water or ice on the planet at the instant of flow, which is of great astrobiological importance. Firstly, we describe the different types of landslides on Mars. Many types are similar to those on Earth, albeit often at a larger scales. We document the characteristics of slumps and rock avalanches, most of which occur in Valles Marineris, a 4000km-long system of 6-8km deep gorges. Landslides in Valles Marineris exhibit a variety of morphologies on their surfaces, such as Toreva blocks, longitudinal grooves, pressure ridges, run-ups on pre-existing mounds indicating high speed of emplacement. Other forms of mass movements are dubious of at least not as common as they are on Earth. Rockfalls have been documented in some cases, but their smaller size can be spotted only with high-resolution cameras. The presence of slow-expanding lateral spreads may indicate deep clay layers. Some landforms may be i

Mars has held large amounts of running and standing water throughout its history, as evidenced by numerous morphologies attributed to rivers, outflow channels, lakes, and possibly an ocean. This work examines the crater Antoniadi located in the Syrtis Major quadrangle. Some parts of the central area of the crater exhibit giant polygonal mud cracks, typical of endured lake bottom, on top of which a dark, tens of kilometers-long network of dendritic (i.e., arborescent) morphologies emerges, at first resembling the remnant of river networks. The network, which is composed of tabular sub-units, is in relief overlying hardened mud, a puzzling feature that, in principle, could be explained as landscape inversion resulting from stronger erosion of the lake bottom compared to the endured crust of the riverine sediments. However, the polygonal mud cracks have pristine boundaries, which indicate limited erosion. Furthermore, the orientation of part of the network is the opposite of what the flow of water would entail. Further analyses indicate the similarity of the dendrites with controlled diffusion processes rather than with the river network, and the presence of morphologies incompatible with river, alluvial, or underground sapping processes, such as overlapping of branches belonging to different dendrites or growth along fault lines. An alternative explanation worth exploring due to its potential astrobiological importance is that the network is the product of ancient reef-building microbialites on the shallow Antoniadi lake, which enjoyed the fortunate presence of a heat source supplied by the Syrtis Major volcano. The comparison with the terrestrial examples and the dating of the bottom of the crater (formed at 3.8 Ga and subjected to a resurfacing event at 3.6 Ga attributed to the lacustrine drape) contribute to reinforcing (but cannot definitely prove) the scenario of microbialitic origin for dendrites. Thus, the present analysis based on the images available from the

The recurrent failures of the 8 to 10-km high basal scarp of the Olympus Mons volcano on Mars has resulted in enormous landslides merging into a composite deposit known as the aureole. The largest of these landslides, the western (W) aureole, appears as a hummocky and extensive tongue reaching the distance of 700 ​km from the present Olympus Mons edge. Totalizing a volume of 106 Km3, the W aureole was created by a single, 100°-wide sector collapse of the Olympus Mons flank between north and west directions. Whereas the western segment of the landslide fell onto the flat area of Amazonis Planitia, the northern section came across the gently sloping ridge of Acheron Fossae after 550 ​km of travel, and subsequently rose against slope for some additional 50–80 ​km. Analysis of imagery on Acheron Fossae ridge beyond the front of the landslide shows a peculiar deposit and associated morphologies. Termed here Frontal Aureole Deposits (FAD), it occurs in two units: a smooth, uniform deposit immediately beyond the aureole terminus, followed in the distal part by distinct lobes directed against slope. These deposits are interpreted as the remnants of debris-rich sandy and muddy waves thrust by the W aureole landslide that inundated the Acheron Fossae ridge. Numerical and analytical calculations set up to study the dynamics of inundation show that the length of the deposits was in the reach of water thrust by a fast-travelling landslide. A possible explanation for the presence of water is that the front was travelling across an outlet of the ancient Martian ocean which, consequently, must have persisted when the landslide event took place. According to this hypothesis, FAD represents the remnant of a giant tsunami deposit similar, albeit at a much larger scale, to those that occur on Earth.

The mobility of landslides on Mars is studied based on a database of 3,118 events. To establish the volume of the landslides for the whole data set based on the deposit area, a new volume-area relationship based on a representative data set of 222 landslides is used. By plotting the H/L ratio between fall height H and runout L versus volume, the landslide mobility is analyzed and compared with existing empirical relationships for Martian and terrestrial landslides. By analyzing the mobility in terms of normalized residuals, that is, the relative deviation of the H/L ratio from the data set best-fit line, mobility is found to depend on both the landslide location on Mars and the landslide typology. This allows us to identify four different types of high-mobility (hypermobile) landslides. Three classes of high-mobility landslides are associated respectively to meteoroid impact, the Olympus Mons aureoles, and landslides with Toreva-block failure style, and their mobility can be explained by the peculiar flow mechanics. The fourth class includes landslides associated with isolated craters, those in the regions wetted by the putative Oceanus Borealis, and the ones at high latitudes. We suggest that the common factor behind all the hypermobile landslides of this fourth kind is the presence of ice. This is confirmed by our data showing that landslides increase in mobility with latitude. The latitudinal trend mirrors the distribution of ice as detected by radar, neutron probes, and the presence of glacial and layered ejecta morphologies. Because the overall landslide distribution supports the presence of ice-lubricated conditions, two ice lubrication models are presented showing how ice melting within or underneath the landslides could enhance mobility. By proper analysis in terms of apparent friction residuals, we find that the mobility of landslides in Valles Marineris with the largest landslide concentration is lower than average. We explain this circumstance partly from

The new exploration programs of the Moon set up by various countries require a good understanding of the mechanical behavior of lunar soil, especially for the construction of facilities and the soft-landing process of lunar rovers. Plate load tests (PLTs) of lunar soil simulants are routinely used to gain accurate understanding of engineering properties of real lunar soil and to predict the load-settlement behavior of foundation systems on the Moon. In this paper, a test apparatus and testbed are carefully designed to account for the grain-size effect and boundary effect. A series of plate load tests of Tongji-1 lunar soil simulant (TJ-1 simulant) is conducted with a foot pad and four circular plates at different loading velocities using the strain-loading method (soft-landing process of a lunar rover). For comparison, tests using the stress-loading method are also carried out (traditional shallow foundation system). In addition, the experimental results are compared with those predicted by the Terzaghi and the modified Terzaghi methods. The results show that with the same loading method and loading velocity, the bearing capacity of TJ-1 simulant under a foot pad with top diameter Rt and bottom diameter Rb, can be estimated using the data under the plates with diameters Rt and Rb, whereas the deformation modulus is close to the data under the plate with diameter Rb; the values of ultimate bearing capacity and bearing capacity factor using the stress-loading method lie in the range of those using the strain-loading method at different velocities, whereas the values of the deformation modulus using the stress-loading method are larger than those using the strain-loading method; and the Terzaghi method overpredicts the bearing behavior of TJ-1 simulant, whereas the modified Terzaghi method underpredicts it. Hence, more experimental and engineering tests are required to find a better modified method.

Sedimentary structures within rock avalanche deposits have gained increasing attention in recent years, since they may provide useful information about the dynamics of such energetic events. This work then is aimed at better defining the physical processes arising during the propagation, paying particular attention to the kinetic sieving mechanism, and strengthening the assumption (widely diffused in the literature) that such a process does not occur for similar events. Specifically, after the examination of two rock avalanche deposits in Central Italy, where cuts through the fragmented deposits are accessible and illustrative of the sediment texture, a series of laboratory flume tests have been performed in order to investigate in detail the flowing process. A simplified physical model for granular agitation has been then introduced to explain how and why kinetic sieving may occur at the laboratory scale and, in the case of natural granular flows of reduced size, also at the field scale.

Radiocarbon isotopic ages and sedimentological data are presented for material recovered from three adjacent translational submarine landslides (YS1, YS2 and YS3) identified on the upper-continental slope offshore Yamba, New South Wales, Australia. The age data indicate that these three co-located upper-slope slides probably occurred independently of each other and not in a single, widespread regional-scale failure event. Numerical estimates of the likely runout distances for slide blocks corresponding to the entire landslide scar volumes range between 10 and 27 km, and represent a ‘runout zone’ in which landslide blocks or debris might reasonably be expected to be located. There is no morphological evidence for large blocks or debris fields derived from two of the Yamba landslide scars within their identified runout zones (YS1 and YS2), suggesting these two failures involved complete disintegration of large slide blocks after failure or the removal of sediment from the landslide sites as grainflows or turbidites. In contrast, the third runout zone (YS3) presents good evidence of at least 12 slide blocks between 100 and 200 m in diameter, suggesting that they were shed as relatively small individual blocks or they were generated due to the dismemberment of a larger slab.

Extremely Energetic Rockfalls (EERs for brevity) are here defined as rockfalls for which the combination of both large volume (at least some thousands of m3) and free fall height in the range of hundreds of metres, results in a total released energy larger than 1/50 of kilotons. Documented examples include several events with different size in the Alps (Civetta, 2013, ca 50,000 m3; Dru, 2005, 2012, 265,000, 59,200 m3; valFiscalina-Cima Una, 40,000 m3; and Thurwieser 2004, ca 3 Mm3; Cengalo, 2011, 1.5 Mm3 in Switzerland), in the Apennines (Gran Sasso, 2006, 30,000 m3), Rocky Mountains (Yosemite, Happy Isles, 38,000 m3), and Himalaya. EERs may become more frequent as a consequence of permafrost thawing at higher altitudes and on steep and sharp mountain peaks. In contrast to low energy rockfalls where block fragmentation is limited, in EERs the impact after free fall causes an immediate and efficient release of energy much like an explosion. The severe disintegration of the rock and air blast are capable of snapping trees many hundreds of metres ahead of the fall point. Pulverized rock at high speed can abrade tree logs, and the resulting suspension flow may travel much further the impact zone, blanketing vast surrounding areas. Using published accounts of some of these events, we introduce some basic physical considerations of the involved phenomena based on analogies with explosions and explosive fragmentation. Of the initial energy, one part is used up in the rock disintegration, and the rest is shared between the shock wave and air blast. Simple and practical model results expressed by analytical formulas may be useful in estimating the entity of devastation in future events.

Fragmentation can occur in rapid solid mass movements due to different controlling factors. In particular, here we consider the initial state of stress, rapid unloading, shearing of highly stressed grain chains and extremely rapid loadings. The last two cases could also be described as dynamic fragmentation, while the first two are more typical of instability phenomena. In this contribution, we present a series of simulations by Discrete Element Method (DEM) for a simple block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. These conditions have been chosen because two sets of experiments with the same settings (Bowman et al. 2010) are available in the literature and resemble simplified natural geometries. In this study, we analyze how dynamic fragmentation occurs in the numerical model and the modes of failure for different rock blocks. The rock fragmentation occurs mainly in the lower part of the rock block after direct impacts with the ground floor, producing several small fragments. The upper part of rock block suffers little influence from the impacts, creating relatively large fragments. Rock masses with many initial weak zones (e.g. fractured or highly weathered rock) exhibit very poor performance of integrity, as represented by their short final runout distances, large breakage ratios, small fragment mass ratios and low final energy.

There are strong propositions in the literature that abiotic factors override biotic drivers of diversity on time scales of the fossil record. In order to study the interaction of biotic and abiotic forces on long term changes, we devise a spatio-temporal discrete-time Markov process model of macroevolution featuring population formation, speciation, migration and extinction, where populations are free to migrate. In our model, the extinction probability of these populations is controlled by latitudinally and temporally varying environment (temperature) and competition. Although our model is general enough to be applicable to disparate taxa, we explicitly address planktic organisms, which are assumed to disperse freely without barriers over the Earth's oceans. While rapid and drastic environmental changes tend to eliminate many species, generalists preferentially survive and hence leave generalist descendants. In other words, environmental fluctuations result in generalist descendants which are resilient to future environmental changes. Periods of stable or slow environmental changes lead to more specialist species and higher population numbers. Simulating Cenozoic diversity dynamics with both competition and the environmental component of our model produces diversity curves that reflect current empirical knowledge, which cannot be obtained with just one component. Our model predicts that the average temperature optimum at which planktic species thrive best has declined over the Neogene, following the trend of global average temperatures.

Landslides have been observed in different terrestrial environments and also on planets, satellites, and asteroids. Long runout landslides are strongly dependent on the initial mass position, material and slope path properties, topographic relief, and presence of volatiles. Therefore, landslides represent a means for the description of rock properties and environment of deposition prevailing at the time of occurrence and may assist in understanding the geological and climatological history of the planetary surfaces. Concerning Mars, previous studies have concentrated on Valles Marineris, where among the largest and longest landslides have been observed. Using different imagery, we present and analyze an original database of 3,118 Martian landslides of deposit area greater than 0.1 km2 throughout the planet between 60°N and 60°S, resulting in a data set far richer than previously done. After a distinction is made between different typologies of landslides, their position and the statistical distribution of their geometrical properties are examined. Large landslides cluster along the Noctis Labyrinthus-Valles Marineris-Margaritifer Terra system. Rock avalanches within craters are widespread, but no significant large landslides have been found at latitudes higher than 40°S and 46°N. The magnitude-frequency distribution follows a power law with scaling exponent ranging between 1.02 and 1.57, for the entire data set and varies according to the geomorphological settings, the landslide typology, and mobility. A volume-area power law relationship (exponent: 1.12–1.24) is proposed, based on the reconstruction of 222 landslide geometries, and compared to those for similar terrestrial landslides (1.39). Similarities with respect to terrestrial landslide, distribution with respect to impact craters and impact energy, and cryosphere extent are also discussed.

Extremely energetic rockfalls (EERs) are defined here as rockfalls for which a combination of both large volume and free fall height of hundreds of meters results in energy larger than about 80 GJ released in a short time. Examples include several events worldwide. In contrast to low energy rockfalls where block disintegration is limited, in EERs the impact after free fall causes immediate release of energy much like an explosion. The resulting air blast can snap trees hundreds of meters ahead of the fall area. Pulverized rock at high speed can abrade vegetation in a process of sandblasting, and particles suspended by the blast and the subsequent debris cloud may travel farther than the impact zone, blanketing vast areas. Using published accounts and new data, we introduce physically based models formulated on analogies with explosions and explosive fragmentation to describe EERs. Results indicate that a portion of the initial potential energy of the block is spent in rock disintegration at impact (typically 0.2%–18%), while other sources of energy loss (air drag, seismic, sound, and ground deformation) are negligible; consequently, more than 80% of the potential energy is converted to kinetic energy of the fragmented block (ballistic projection, shock wave, sand blast, and dust cloud). We also propose simple estimates for the flow of the dust cloud associated with an EER and its long settling time. The areal extent of the affected zone is estimated from the energy balance and an empirical power law relationship.

The tallest volcano in the solar system, Olympus Mons on Mars, is bordered by at least ten enormous sub-circular hummocky deposits forming a welded halo, termed the aureole. The aureole units (or simply aureoles), which are the deposits of landslides from Olympus Mons, have dramatically transformed the pristine size and shape of the volcanic edifice. Topographic data are used to determine the amount of collapsed material, and so reconstruct the original outline of Olympus Mons before the landslides took place, under the assumption that the edifice did not start failing until it had reached this maximum size. Due to post-aureole deposition on the eastern and southern flanks and to the uncertainty of the slippage level on the northern side, the reconstruction is sufficiently precise along the western and north-western Olympus Mons flank, it is more uncertain for the northern flank, while it is not feasible around the rest of the volcano. It appears that the radius along the western and north-western Olympus Mons before the collapse of the aureole landslides was approximately 200 km longer or more. The results show that the volume of the aureoles, even if enormous, is insufficient to fill the ideal conical edifice which would be obtained prolonging the Olympus Mons flanks with present slope angles. Thus, an overhang remains in the pre-aureole reconstructed Olympus Mons, which would also explain the onset of instability that led to the aureole collapse. Further, a drape deposit blanketing the southern Acheron Fossae ridge just at the front of the western (W) aureole landslide deposit and a fan-channel system carved on the same W aureole are investigated, and it is suggested that these morphologies have been emplaced in subaqueous setting. While the drape may indicate a landslide-thrust water splash akin to a tsunami deposit caused by the fast travelling W aureole landslide, the fan-channel system is similar to certain morphologies in the terrestrial oceans. A numerical

An erodible substrate and a sharp slope break affect the dynamics and deposition of long runout landslides. We study the flow evolution of a granular mass (1.5–5.1 l of sand or gravel) released on a bilinear chute, i.e., an incline (between 35 and 66°) followed by a horizontal sector, either sand-free or covered (1–2-cm-thick sand layer). Monitoring the time evolution of the falling mass profiled at 120 Hz, the impact dynamics, erosion of the basal layer, and modes of deposition are studied. The frontal deposition is followed by a backward propagating shock wave at low slope angles (<45°), or by a forward prograding flow at greater angles. Experiments with colored sand layers show a complex sequence of dilation, folding and thrusting within both the collapsing sand flow and the substrate. Experimental results are compared with real rock avalanche data and nearly vertical collapses. The observed increase of the drop height divided by the runout (H/L or Heim’s ratio) with both chute slope angle and thickness of the erodible substrate is explained as an effect of vertical momentum loss at the slope break. Data suggest a complex evolution, different from that of a thin flow basal shear flow. To provide an approximate explanation of the dynamics, three analytical models are proposed. Erosion of a 1-cm-thick substrate is equivalent to 8–12 % increase of the apparent friction coefficient. We simulate the deposition and emplacement over an erodible layer with a FEM arbitrary Lagrangian Eulerian code, and find a remarkable similarity with the time evolution observed in the experiments. 2D models evidence the internal deformation with time; 3D models simulate deposition.

The Acheron Dorsum north of Olympus Mons on Mars is a unique 800 km-long arcuate ridge apparently unrelated to other adjacent morphologies. Acheron Dorsum is crossed by deep and wide fractures (“fossae”, here referred to also as Linear Depressions or “LDs”), commonly interpreted as grabens formed by local lithosphere stress in response to endogenous mantle upwelling. We suggest an alternate view that the linear depressions may result from gravitational self-adjustment of a loose deposit similar to the Deep-Seated Gravitational Slope Deformations (DSGSD or Sackung), a kind of slow mass movement well documented on Earth. Numerical simulations based on a finite difference numerical model devised for terrestrial DSGSDs reproduce the timing and position of LDs reasonably well provided that the viscosity of the viscoplastic layer is between ≈71015 and ≈1017 Pas. While rock masses typically exhibit much higher values, based on comparison with glacial examples, we find that these viscosities could be compatible to a rock–ice mixture. This leads to the hypothesis that the rock underneath Acheron Dorsum may be highly fragmented and that ice might have been involved in its deformation. Our hypothesis could shed light on the puzzling stress pattern on Acheron Dorsum, which follows approximately the ridge, without requiring the presence of hypothetical small-scale, endogenous currents.

Friction rocks are produced by concentrated shear and partial melting at the Earth surface. Although the commonest examples are known from earthquake-generated layers (pseudotachylytes), such rocks may also derive from the slippage of rock avalanches. While numerous studies have been dedicated to earthquake-generated pseudotachylytes from a petrological, geochemical and physical viewpoint, fewer investigations have focused on the corresponding rock avalanche rocks (also termed frictionites), especially concerning the mechanics of formation and its relationship to the mineralogical composition of the original rock. In this work, we introduce a numerical model for the melting of a crystalline micro-breccia (gouge) in the shear layer of a rock avalanche due to heat generated at the sliding surface. The motion of the landslide is calculated and the resulting frictional heat is used to compute the melting of the crystalline gouge. We constrain the model based on calibration of the Köfels frictionite in Austria, the best-known example of landslide-frictionite association. We have collected samples of frictionite and of the original rock consisting of gneiss rich in alkaline feldspar and have analysed it chemically and mineralogically. Not only should the model calculate the temperature increase to fuse the average gneissic rock, we also require that the simulated percentages of mineral species should reasonably reproduce the data. The compositional data of both the original rock and the frictionite, whose mineralogical species have different melting temperatures, latent heats and thermal conductivities, allow us to constrain the numerical simulations. A second aspect considered in this work is the particular pumiceous texture derived from growth and coalescence of bubbles as a consequence of decreasing ambient pressure. The simulations show a satisfactory agreement with data but also discrepancies that are probably due to the limitations of the numerical model and to the

Extremely energetic rockfalls (EER) are phenomena for which the combination of a large volume (at least some thousands of m ) and a free fall height of hundreds of metres, results in a large released energy. We fix a threshold value of around 1/50 of kilotons to define such a type of events. Documented examples include several events with dif-ferent size in the Alps (Dru, 2005, 2011, 265,000, 59,200 m3; val Fiscalina – Cima Una, 2007, 40,000 m3; Thurwieser 2004, ca 2 Mm3; Cengalo, 2011, 1.5*105 m3 in 2016, in Switzerland; Civetta, 2013, ca 50,000 m3;), in the Apennines (Gran Sasso, 2006, 30,000 m3), Rocky Mountains (Yosemite, Happy Isles, 38,000 m3), and Himalaya. EERs may become more frequent on steep and sharp mountain peaks as a consequence of permafrost thawing at higher altitudes. In contrast to low energy rockfalls where block disintegration is limited, in EERs the impact after free fall causes an immediate and efficient release of energy much like an explosion. The severe disintegration of the rock and the corresponding air blast are capable of snapping trees many hundreds of metres ahead of the fall area. Pulverized rock at high speed can abrade tree logs, and the resulting suspension flow may travel much further the impact zone, blanketing vast surrounding areas. Using both published accounts of some of these events and collecting direct data for some of them, we present some basic models to describe the involved processes based on analogies with explosions and explosive fragmentation. Of the initial energy, one part is used up in the rock disintegration, and the rest is shared between the shock wave and air blast. The fragmentation energy is calculated based on the fitting of the dust size spectrum by using different proba-bilistic distribution laws and the definition of a surface energy and by considering the involved strain rate. We find the fragmentation is around one third of the initial boulder energy. Finally, we evaluate the velocity of the correspo

Multiple layered ejecta (MLE) associated to impact craters on Mars are characterized by typical crater diameter 10 to 60 km and extensive superposed sheets of ejecta material (Fig. 1). These peculiar crater typologies often appear, but are not limited to, the lowlands near the dichotomy at the highlands lowlands boundary [1-4]. Figure 1 shows the prominent example of Tooting crater. Close relatives of DLEs are double-layered ejecta in which the inner layer of ejecta has a very sharp outline, and are usually associated to smaller crater, [5] (Fig. 1). Kotka crater on Mars (18°30’ N; 171°16’ W) is an example of crater with multiple layer ejecta. Ejecta surrounding Kotka typify many of the characteristics exhibited by MLE: ejecta appear as frozen, earlier fluidized lobes elongated radially to indicate the flow direction from the crater rims to the outward edges (Fig. 2). In an ongoing project, we are studying the geometry of the Kotka ejecta remnants to infer their flow dynamics following the impact. The rationale for the choice of the Kotka crater is the presence of several mounds protruding from the ground. During their flow, the ejecta have impacted against such mounds, from which the flow speed can be computed based on run-up and other dynamical criteria.

Landslides on Mars exhibit features such as steep collapse, extreme deposit thinning, and long runout. We study the flow dynamics of Martian landslides particularly in Valles Marineris, where landslides are among the largest and longest. Firstly, we observe that landslides in Valles Marineris share a series of features with terrestrial landslides fallen onto glaciers. The presence of suspected glacial and periglacial morphologies from the same areas of Valles Marineris, and the results of remote sensing measurements suggest the presence of ice under the soil and into the rock slopes. Thus, we explore with numerical simulation the possibility that such landslides have been lubricated by ice. To establish a plausible rheological model for these landslides, we introduce two possible scenarios. One scenario assumes ice only at the base of the landslide, the other inside the rock-soil. A numerical model is extended here to include ice in these two settings, and the effect of lateral widening of the landslide. Only if the presence of ice is included in the calculations, do results reproduce reasonably well both the vertical collapse of landslide material in the scarp area, and the extreme thinning and runout in the distal area, which are evident characteristics of large landslides in Valles Marineris. The calculated velocity of landslides (often well in excess of 100 m/s and up to 200 m/s at peak) compares well with velocity estimates based on the run-up of the landslides on mounds. We conclude that ice may have been an important medium of lubrication of landslides on Mars, even in equatorial areas like Valles Marineris.

This paper investigates the characteristics of dynamic rock fragmentation and its influence on the postfailure fragment trajectory. A series of numerical simulations by discrete element method (DEM) were performed for a simple rock block and slope geometry, where a particle agglomerate of prismatic shape is released along a sliding plane and subsequently collides onto a flat horizontal plane at a sharp kink point. The rock block is modeled as an assembly of bonded spherical particles with fragmentation arising from bond breakages. Bond strength and stiffness were calibrated against available experimental data. We analyzed how dynamic fragmentation occurs at impact, together with the generated fragment size distributions and consequently their runout for different slope topographies. It emerges that after impact, the vertical momentum of the granular system decreases sharply to nil, while the horizontal momentum increases suddenly and then decreases. The sudden boost of horizontal momentum can effectively facilitate the transport of fragments along the bottom floor. The rock fragmentation intensity is associated with the input energy and increases quickly with the slope angle. Gentle slopes normally lead to long spreading distance and large fragments, while steep slopes lead to high momentum boosts and impact forces, with efficient rock fragmentation and fine deposits. The fragment size decreases, while the fracture stress and fragment number both increase with the impact loading strain rate, supporting the experimental observations. The fragment size distributions can be well fitted by the Weibull's distribution function.

The flow characteristics and velocities of three landslides in Valles Marineris on Mars are investigated using detailed morphological analyses of high-resolution images and dynamical calculations based on the run-up and curvature of the landslide deposits. The morphologies of the landslides are described, especially concerning those characteristics that can provide information on the dynamics and velocity. The long runout and estimated high velocities, often exceeding 100 m/s, confirm a low basal friction experienced by these landslides. Because subaqueous landslides on Earth exhibit reduced friction, we explore the scenario of sub-lacustrine failures, but find little support to this hypothesis. The environmental conditions that better explain the low friction and the presence of longitudinal furrows suggest an aerial environment with a basal soft and naturally lubricating medium on which friction diminished gradually; in this perspective, ice is the most promising candidate.

Numerous field surveys have provided quantitative information on the characteristics of talus deposits. Much less has been done to quantify the basic dynamics processes of blocks involved in talus evolution. In this work, we perform a set of experiments at the reduced scale of some metres using an inclined board covered with a loose granular medium. The complexity of the processes forming a talus is simplified by studying the interaction of only two kinds of grain sizes at a time. Grains of one class size are dropped from a fixed height onto the board covered by a layer of grains of a different class size, and their final distribution is recorded. We find that when small grains fall on large grains, the granular abundance decreases rapidly as a function of the distance from the fall point, which is explained by the effect of multiple bouncing on the irregular surface. In contrast, large grains falling on a bed of smaller grains lose much more energy at impact. They may stop at once, or roll down slope, often reaching the whole board length; as a consequence, their abundance peaks in the fall zone and at the change of slope. Experiments also show that grains travel longer with increasing fall height and sloping angle. The results clarify in a physical manner one mechanism that might explain why large blocks are typically found in the distal part of a talus slope, while smaller blocks remain near the fall zone. Based on these and previous experiments, a schematic view of talus evolution is discussed.

Debris-flows are widespread natural phenomena characterized by high mobility (high velocity and long runout distance) and impact forces, which frequently cause human casualties and significant damage to infrastructure. To better understand the dynamics of such events, analyzing in particular the effect induced by the valley geometry on flow velocity, runout and mobilized volumes, in this work we reconstruct a real debris-flow event through numerical modeling. Specifically, we used a modified version of the BING model, a fluid-dynamic depth-integrated numerical model for debris flows, which has been properly modified to account for width changes along the valley. The studied event, which occurred in Scaletta Zanclea (Messina, north-eastern Sicily, Italy) on October 1, 2009, is exceptionally well constrained by field and topographic information. In this respect the flow velocity, estimated from two specific locations on the basis of field evidence, the distribution of erosional and depositional areas along the Racinazzo valley, based on the comparison of pre- and post-event digital elevation models (DEMs), and the runout distance were used as constraints to calibrate the model. Furthermore, we report a detailed description of the main event characteristics based on hydrological records and witness reports. The numerical modeling results are consistent with witness reports and the severe damage recorded in the Scaletta Marina village, and highlight the effect of the valley geometry on both the debris flow velocity and the erosion/deposition processes. The effect of changing valley width has been also quantified, resulting in accelerations of the debris in correspondence of the valley narrowing and stagnations at the plateaus. © 2015 John Wiley

In this contribution we tackle the problem of a fast moving granular mass flowing on a steep slope and successively along a horizontal erodible layer. This setting is typical at many sites where rock and debris avalanching can occur or already occurred. In such a common condition, the high energy content and the entrapment of erodible materials along the path can strongly control the evolution of the landslide motion. We briefly present preliminary experimental tests performed on a simplified slope with just one kind of sand and one volume of granular material.We find a significant role of the erodible layer, which can modify the flow dynamics and the shape of the final deposit compared to flow without layer. .

Landslides often collapse in areas covered by alluvial deposits forming an erodible layer. This erodible substrate may deform plastically under the intense shear stress of the landslide mass. In other cases, the collapse occurs onto a water basin or tidal flat, creating impulse water waves whilst the landslide may be lubricated by a water layer underneath. In either cases the presence of a medium underneath the landslide will change its dynamics introducing complex processes. While frictional, dry masses and taluses generally hamper the landslide motion. In this work, we present some experiments mimicking the collapse of a landslide onto shallow erodible or water layers. The landslide is simulated with a granular material (sand or gravel) flowing on an incline (35-66°) followed by a horizontal sector covered with a granular bed 1 to 2 cm thick or with a 0.5-1 cm of water. Monitoring evolution in time allows us to describe in detail the process of fluidization of the material at impact, the generation of impact waves, and the erosion process. Concerning impact on a sand layer, the apparent friction coefficient (H/L) is found to increase with the chute slope angle and with the thickness of the erodible layer, and to decrease with the volume. At low slope angles, the material accumulates backwards in a shock wave mode, while at larger slope angles (>45°) it accumulates by prograding forward. A granular avalanche falling from the slope is partially reflected at the sharp slope break where erosion occurs and then propagates initially as a wave partially eroding the superficial material. Folding and thrusting occur within the dense shear flow and the erodible layer. Experiments with a water layer show that the dynamics depends much on the permeability of the granular avalanche. FEM numerical simulations replicate and allow to describe and understand both the spreading and the erosion, and internal deformation recorded in the erodible layer. Experimental findings are compa

Fragmentation has been proposed as an important dynamical process in the propagation of rock avalanches. The occurrence of an intense block fragmentation inside a rock avalanche traveling on smooth inclined terrain followed by a flat region (a geometry common to many landslides) is first studied numerically by means of a discrete element 2-D code in which blocks are subjected to mutual collisions and impact with the terrain. This numerical model confirms that the locations where fragmentation is more likely to occur are those in correspondence of abrupt slope changes. We thus analyze this geometry of impact in two cases: low- and high-impact energy. In the first case, where recent experimental data are available, a kinematic analysis shows that the energy released at impact causes high velocities of the smallest fragments, a highly probable scenario in rock avalanche dynamics. When the impact is so energetic to disintegrate the landslide, we find that explosive fragmentation at the slope break provides an extra horizontal boost to a rock avalanche via a peculiar mechanism coupling the geometry of the slope path to the dynamics of the rock avalanche. As a consequence, a net momentum gain (boost) results along the horizontal direction due to the terrain asymmetry. However, under normal field conditions, only when the slope angle is greater than 70° and fragmentation produces clasts of fairly uniform size, the momentum and runout distance are significantly enhanced. Allowing for a spectrum of fragment sizes and velocities, we find a relationship between the degree of fragmentation and the magnitude of the extra boost. Key Points Excessive runout of granular flows on steep slopes Impact energy effect on rock fragmentation Topographic and geometric effects on fragmentation and mobility

The BIG’95 landslide was emplaced 11,500 years ago and is one of the largest known submarine landslides in the Mediterranean Sea. The simulated landslide dynamics matches the observed run-out and deposited thickness. Water elevation simulated by using a dispersive tsunami model exceed 10 m close to the landslide area and at the nearest shorelines. Modeling further indicates that the tsunami probably had widespread consequences in the Mediterranean. Compared to previous studies, this new simulation provides larger waves. There is, however, still a need to better constrain the landslide dynamics in order to illuminate the uncertainties related to the tsunamigenic power of this, and other, submarine landslides.

The rock walls of the Valles Marineris valleys (VM) in the equatorial area of Mars exhibit several gravitational failures which resulted in a series of large landslides up to several hundred cubic kilometers in volume. Questions arise as to forces at play and rock strength in the stability of the walls of VM. In this work we address the stability analysis of the walls of VM by considering the strength of the materials of the walls and the causes of landslides. Using finite element calculations and the limit analysis upper bound method, we explore the range of cohesion and friction angle values associated with realistic failure geometries, and compare predictions with the more classical Culmann's translational failure model. Our analysis is based both on synthetic, simplified slope profiles, and on the real shape of the walls of VM taken from the MOLA topographic data. Validation of the calibrated cohesion and friction angle values is performed by comparing the computed unstable cross sectional areas with the observed pre- and post-failure profiles, the estimated failure surface geometry and ridge crest retreat. This offers a link between rock mass properties, slope geometry and volume of the observed failure, represented in dimensionless charts. The role of groundwater flow and seismic action on the decrease of slope stability is also estimated. Pseudo-static seismic analyses provide another set of dimensionless charts and show that low seismicity events induced by meteoroid impacts, consistent with the size of craters, could be a cause for some of the observed landslides, if poor rock properties for VM are assumed. Analyses suggest that rock mass properties are more similar to their earth equivalents with respect to what has been previously supposed. © 2013 Elsevier B.V.

Rock avalanches are the largest granular flows on Earth. In contrast to artificial, small-scale granular avalanches, they exhibit a large degree of fragmentation with reduction of average grain volume by a factor of up to 1015–1018. Even though fragmentation likely affects the whole dynamics of the rock avalanche, as yet the basic mechanics of the process is poorly known. In this work, a simple model is presented for the fragmentation of rock avalanches, assuming that most of the fragmentation occurs along force chains in the granular medium. The landslide motion is simulated along a curved bumpy profile path. The predicted grain spectra are found to agree reasonably with field data