Large Rock Avalanches Research Articles

Unravelling driving conditions of rock and ice avalanches and resulting cascading processes in High Mountain Asia

AbstractAmplified climate warming and the tectonically active landscape in High Mountain Asia (HMA) have led to the occurrence of 60 well-documented large rock and/or ice avalanches (RIAs), resulting in at least 1366 fatalities and extensive negative impacts on ecosystems, water resources, infrastructure, and social stability. In this study, we analyzed historical RIAs in HMA using detailed topographic, climatic, glacier, permafrost, and geotechnical data, along with medium- and high-resolution satellite imagery, to identify 23 potential driving factors. Seismic activity, permafrost thaw, lithology, and changes in precipitation were identified as important driving factors for RIAs, while stream flow, stream order, glacier area, glacier slope, glacier length, height differences, and real length were identified as important driving factors for their cascading processes. Our analysis revealed that RIAs have mostly (86%) occurred on steep slopes (> 30°) at altitudes above 3000 m asl, with a prevalence of slopes facing northeast. Almost half of the RIAs produce cascading processes, which on average travel further and lead to a tenfold increase in fatalities compared to single events. Cascading processes are more likely to occur from smaller (glacier area < 3 km2), steeper-sloped (glacier slope > 30°) hanging glaciers. Debris volume, water volume, and topography are three main drivers of cascading processes. Medium-sized source volumes (106 m3 ~ 108 m3) are most susceptible to transform into cascading processes. Statistical analysis indicates that the probability of a RIA transforming into a cascading process significantly increases when the event occurs near high-flow river systems, amplifying potential disasters. These findings offer baseline information on RIA hazards and their cascading processes in HMA, facilitating improved hazard modelling and risk assessment.

Deciphering the dynamics of a Younger Dryas rock avalanche in the Bernese Alps

Large rock avalanches play a key role in shaping alpine landscapes. However, the complex interplay between mass movement and other surface processes poses challenges in identifying these deposits and understanding the underlying process controls. Here, we focus on the rock avalanche deposit of the Lurnigalp valley in the Bernese Alps (Switzerland), originally mapped as till. The Lurnigalp valley is a U-shaped tributary valley located in the southwest of Adelboden, Canton Bern. To explore the timing and dynamics of the rock avalanche event, we employed detailed remote and field mapping, sedimentary petrology, surface exposure dating with cosmogenic 36Cl, and runout modelling with DAN3D®. For the reconstruction of the chronology, we analyzed cosmogenic 36Cl in surface samples from 15 boulders of the rock avalanche deposit. We developed three distinct scenarios to investigate the dynamics and contextual conditions of the rock avalanche event. In the first scenario, we consider a rock avalanche depositing 1 Mm3 of sediment in a valley devoid of ice. The second scenario uses the same deposit volume but introduces a hypothetical glacier occupying the uppermost part of the valley. Finally, the third scenario, similar to the first scenario with a glacier-free valley, assumes a substantially larger volume of collapsed rock mass. We consider the third scenario the most plausible, in which approximately 6 Mm3 of rock mass, composed of limestone and sandstone, was released from a limestone cliff around 12 ± 2 ka during the Younger Dryas. The collapsed rock mass fell into the ice-free valley floor, ran up the opposite valley side and was deflected towards the northeast following the valley orientation. The rock mass stopped after 2.2 km leaving approximately 6.4 Mm3 deposits spread across the entire valley floor. Subsequently, most of the rock avalanche deposit have been reworked by periglacial activity. We suggest that structural features, lithology and glacial erosion and debuttressing were involved in the weakening of the in-situ bedrock that finally led to the collapse. Our study not only enhances the understanding of rock avalanche mechanisms and their profound impact on Alpine landscape evolution but also elucidates the complex interplay of geological processes that led to the collapse and altered the rock avalanche deposit afterwards.

Large rock and ice avalanches frequently produce cascading processes in High Mountain Asia

Rock and ice avalanches – or combinations thereof – are among the most far-reaching types of hazards in the mountain cryosphere. Here, we report a first complete inventory of rock and ice avalanches in High Mountain Asia (HMA) and assess 60 events predominating from steep glacial and periglacial zones, including 29 cascading processes, resulting in 1366 casualties and >500,000 people displaced. Overall, these hazards are clearly not rare in HMA and are both lethal and destructive.

Engineering geological characteristics and failure mechanics of Jure rock avalanche, Nepal

IntroductionThe rock avalanches are a frequent and disruptive phenomenon in the Himalayas and other mountain chains. To minimize future losses, it is essential to investigate the engineering geological causative factors and mechanism of these mass wasting events.Study areaThe present work is aimed at assessing the failure mechanism of the disastrous 2014 Jure rock avalanche along Araniko Highway, Northern Nepal. The event had blocked the Sunkoshi River and blocked an economically significant route to China.Geotechnical properties and analysisInitially, rockmass characterization and intact strength attribute were determined for the site to classify the failure zone. The parameters measured and obtained from the field and laboratory were integrated into the analytical models to obtain a conclusive interpretation of the failure mechanism. Structural, kinematic, and key block theory analyses have been carried out for decipher the evolution of the failure zone.Results and discussionRock mass was found to be of fair quality, however, the structural instabilities and the presence of water has led to a progressive failure. Movement of the key block and subsequent sliding of wedges and foot failure appears to be a possible failure mechanism.ConclusionThe present research explores the contributory engineering geological aspects of the Jure rock avalanche. The investigation results can be used to tackle similar large scale rock avalanches in similar geological terrains and thus minimizing the losses.

Assessment and Mechanism Analysis of Forest Protection against Rockfall in a Large Rock Avalanche Area

Trees in forests can obstruct falling rocks and serve as a natural barrier to reduce the velocity of falling rocks. Recently, there has been growing interest in utilizing forests to safeguard against potential rockfall. Nevertheless, there is a dearth of research regarding the impact of rock size and forest structure on forest preservation against rockfall. This study takes the Jiweishan rock avalanche that occurred in China in June 2009 as an example to discuss the protection mechanism of forests against rockfall in rock avalanche disasters. Three sizes of rockfalls from the Jiweishan rock avalanche were simulated and analyzed with and without forests using Rockyfor3D software. The findings indicate that forests can mitigate the energy impact of falling rocks. Especially in the debris flow area of rock avalanches, the protective effect of trees on small-sized falling rocks is most obvious, reducing the runout distance and damage range of the debris flow. Moreover, the protective effect of forest structures on rockfall risk was explored. It was found that broad-leaved forests had the best protection against falling rocks, followed by coniferous broad-leaved mixed forests, and coniferous forests had the worst protective effect. Furthermore, increasing forest planting density and tree diameter at breast height (DBH) can result in better protection against rockfall. Thus, rational planning of forest species and planting density in areas of rockfall can effectively reduce the threat of rockfall risk. The research ideas in this study can provide a basis for evaluating the mitigation of rockfall hazards by forests and provide a reference for constructing and planning protective forests in rockfall and rock avalanche hazard areas.

Absolute dating and evolutionary model of large rock avalanches on Mars: Examples from the Hydraotes Chaos and Tiu Valles region

Landslides are geomorphological features observed on many planetary bodies, formed in a wide range of geological, geomorphological and environmental conditions. Hence, study of their morphological and morphometric characteristics, along with their absolute or relative dating, can improve the understanding of geological and environmental conditions at the time of their emplacement. On Mars, landslides are present over a very large part of the planet, and the most spectacular and best studied are those in Valles Marineris where many mass wasting phenomena occurred between Hesperian and Late Amazonian periods. The present study aims at improving the knowledge of Martian geological and geomorphological evolution including environmental conditions through absolute dating and morphological analysis of selected large landslides outside of Valles Marineris. These mass wasting features were used as proxies for revealing how active the relatively recent (Middle to Late Amazonian) geological environment of Mars was through the information of their trigger timings and degradation histories. The five landslides of our study, which have never been analyzed previously, are located in the region of Hydraotes Chaos and Tiu Valles near the dichotomy boundary. The geomorphological and morphometric analyses show that the studied landslides, classifiable as rock avalanches, exhibit morphological features and mobility similar to those of other terrestrial and Martian rock avalanches. The results obtained from the absolute dating reveal that these landslides occurred within a time range spanning from 835 Ma (±290) to 252 Ma (±100) ago, consistent with occurrences of other landslides in the equatorial region of Mars. The most plausible triggering factors include ground shaking resulting from meteoric impacts, marsquakes and volcanic events related to the evolution of Tharsis and Elysium regions during the Amazonian period. Local resurfacing events acting on the studied landslides are probably related to aeolian deflation and deposition, minor mass wasting events and impact cratering. Finally, the geomorphological characteristics of these landslide bodies and their absolute dating reveal a possible recent (< 250–150 Ma) tectonic activity along the flanks of Tiu Valles and the southern Hydraotes Channel.

Slope‐Break Collisions: Comment on “Insight Into Granular Flow Dynamics Relying on Basal Stress Measurements: From Experimental Flume Tests” by K. Li et al.

AbstractNumerical simulations show that the positive correlation observed in laboratory experiments by Li et al. (2022, https://doi.org/10.1029/2021jb022905) between an increase of grain size and particle agitation, on the one hand, and an increase of granular flow mobility, on the other hand, is not a valid cause‐and‐effect relationship. In other words, their mobility differential is not caused by a different energy dissipation rate that results from a different grain size content. Instead, the flows stop because of a head‐on collision with the horizontal flume at the bottom of a steep 40° incline. Essentially, the slope‐break jams the granular movement. Indeed, a combination of laboratory experiments and numerical simulations demonstrated that the mobility of unhindered dense granular flows increases as grain size and clast agitation decrease. Consequently, there is no evidence that the high mobility of large natural rock avalanches is due to an increase of particle agitation.

Glacial lake outburst flood hazard under current and future conditions: worst-case scenarios in a transboundary Himalayan basin

Abstract. Glacial lake outburst floods (GLOFs) are a major concern throughout High Mountain Asia, where societal impacts can extend far downstream. This is particularly true for transboundary Himalayan basins, where risks are expected to further increase as new lakes develop. Given the need for anticipatory approaches to disaster risk reduction, this study aims to demonstrate how the threat from a future lake can be feasibly assessed alongside that of worst-case scenarios from current lakes, as well as how this information is relevant for disaster risk management. We have focused on two previously identified dangerous lakes (Galongco and Jialongco), comparing the timing and magnitude of simulated worst-case outburst events from these lakes both in the Tibetan town of Nyalam and downstream at the border with Nepal. In addition, a future scenario has been assessed, whereby an avalanche-triggered GLOF was simulated for a potential large new lake forming upstream of Nyalam. Results show that large (&gt;20×106 m3) rock and/or ice avalanches could generate GLOF discharges at the border with Nepal that are more than 15 times larger than what has been observed previously or anticipated based on more gradual breach simulations. For all assessed lakes, warning times in Nyalam would be only 5–11 min and 30 min at the border. Recent remedial measures undertaken to lower the water level at Jialongco would have little influence on downstream impacts resulting from a very large-magnitude GLOF, particularly in Nyalam where there has been significant development of infrastructure directly within the high-intensity flood zone. Based on these findings, a comprehensive approach to disaster risk management is called for, combining early warning systems with effective land use zoning and programmes to build local response capacities. Such approaches would address the current drivers of GLOF risk in the basin while remaining robust in the face of worst-case, catastrophic outburst events that become more likely under a warming climate.

Inferring Dynamic Fragmentation Through the Particle Size and Shape Distribution of a Rock Avalanche

AbstractOn 28 August 2017, a large catastrophic rock avalanche occurred in Nayong, Guizhou Province, China. It claimed 35 lives and caused large property losses. The Nayong rock avalanche provides a rare opportunity to infer the dynamic fragmentation of rock masses from deposits. Field surveys combined with an uncrewed aerial vehicle image analysis system were performed to investigate the particle size and shape distribution along the runout path of the Nayong rock avalanche. The Weibull particle size distribution (PSD) model was tested and possesses the highest accuracy and capability over the entire range of the Nayong rock PSD, and it shows that the median size (D50) values of the particles range from 0.16 to 16.31 m along the runout path. Moreover, the evolution of D50 along the runout path confirms that the degree of rock fragmentation varies greatly in different regions. Additionally, the relative breakage (BR) of particles in the deposition area is 0.191. Particle R values increase significantly from 0.55 to 0.78 along the runout path, which indicates that the rock grinding (shape change) process continuously occurs throughout the avalanche path. The image analysis results confirm that the dominance of rock fragmentation and grinding alternates during granular flow transport. Based on these results, the classic fragmentation‐spreading model for long‐runout rock avalanches is optimized by taking into account the method by which grinding rocks increases the speed of avalanches.

A re-examination of the factors controlling mobility of large rock avalanches

A re-examination of the factors controlling mobility of large rock avalanches

Using a Lidar-Based Height Variability Method for Recognizing and Analyzing Fault Displacement and Related Fossil Mass Movement in the Vipava Valley, SW Slovenia

The northern slopes of the Vipava Valley are defined by a thrust front of Mesozoic carbonates over Tertiary flysch deposits. These slopes are characterized by a variety of different surface forms, among which recent and fossil polygenetic landslides are the most prominent mass movements. We used the height variability method as a morphometric indicator, which proved to be the most useful among the various methods for quantifying and visualizing fossil landslides. Height variability is based on the difference in elevations derived from a high-resolution lidar-derived DEM. Based on geologic field mapping and geomorphometric analysis, we distinguished two main types of movements: structurally induced movement along the fault zone and movements caused by complex Quaternary gravitational slope processes. The most pronounced element is the sliding of the huge rotational carbonate massif, which was displaced partly along older fault structures in the hinterland of fossil rock avalanches and carbonate blocks. In addition to the material properties of the lithology, the level of surface roughness also depends on the depositional processes of the individual sedimentary bodies. These were formed by complex sedimentary events and are intertwined in the geological past. The sedimentary bodies indicate two large fossil rock avalanches, while the smaller gravity blocks indicate translational–rotational slides of carbonate and carbonate breccia.

THE MYTH OF THE BATTLE BETWEEN POSEIDON AND POLYBOTES: WAS THE INSPIRATION FROM A LATE HOLOCENE ROCK AVALANCHE AT ZINI, KOS ISLAND?

Scenes of the legendary battle between Poseidon and the giant Polybotes, which occurred on the Greek island of Kos, are depicted on dozens of surviving ceramic objects. Poseidon is shown killing his opponent with his trident, while carrying a huge rock that he had ripped off the island of Kos to bury Polybotes. The legend is interpreted to represent a strong earthquake that caused a large coastal rock fall or rock avalanche. The oldest ceramics representing this disaster date from ca. 540 BCE. The disaster is interpreted to date from this time and was a major event that reverberated throughout the Greek world, triggering the imagination of its artists for several generations. Legend and ancient literary sources suggest that the event took place in southeastern Kos, near the then capital city of Astypalaia, located NW of Zini mountain. Geological studies show a large, relatively recent, rock avalanche on the steep coast on the SE side of Zini mountain. Possible tsunami sand deposits with reworked marsh foraminifera are found 7 m above sea level on NE Zini, only 1 km from the archeological site of the old city of Astypalaia and large boulders are stranded on the opposite rocky coastline up to 6 m above sea level. Noise and dust from a rock avalanche would have been terrifying for the inhabitants of Astypalaia and any tsunami would have overwashed the port at Kamari. In the absence of suitable geological dating methods, ceramics provide the best chronology for the event.

Nyexon Rock Avalanches: A Special Intrusion Restraint Mechanism, Tibet, China

The Nyexon Rock Avalanches in the southern Qinghai-Tibet Plateau is a huge scale earthquake-induced slope disaster in the Holocene, the accumulation area has distinct sedimentological characteristics, which is of great significance for studying the intrusion and restraint mechanism during long-distance transportation of large rock avalanches or debris avalanche. This long-distance transportation induced a series of landform types, such as ridges, hills, and ravines; they are widely distributed in all areas and extensively developed shear zones, jigsaw cracks, and other structures within the sedimentary structure. With the analysis of DEM data and geological survey, two main types of basement structures and their transition relationships are distinguished; they play an essential role in the restraining bottom during rock avalanches. In the sedimentary structure, the block facies and mixing facies occupy the main body of the deposition from the center to the distal area. Under the basement restriction, mixing facies are formed between the bottom of the sedimentary layer and the basement sedimentary structure; the shear band is mainly developed along with the mixing facies and basement facies, which is accompanied by basement liquefaction and rheology. A sedimentary facies model is established based on the sedimentary structure sequence of the Nyexon Rock Avalanches transportation. After analyzing the failure mechanism of the rock avalanches, it is believed that in the initial stage of failure, the rock avalanches is transformed into a particle flow that is similar to the debris avalanche, which is restrained by the basement structure and lateral bound; then, an accumulated obstacle highland is formed in the central area after deceleration, making the transportation of the main fluid to deflect quickly.

Evolution and temporal constraints of a multiphase postglacial rock slope failure

Large rock slope failures are temporal processes which act to modify the landscape after glacial retreat. The slope failure process often shows a lag time of thousands of years after deglaciation, with multiple failure events possible. While global datasets constrain this lag time from extensive mapping and dating of paraglacial rock avalanches, the timeline is poorly refined in northern Norway. We present a case study of multiphase failure at Skredkallen on Vanna, one of a group of coastal islands in Troms, northern Norway. The site contains an actively deforming rock slope above a large rock avalanche deposit. The rock slope deformation (RSD) is a system of fractured and dislocated blocks up to 3 Mm3, and is moving slowly ~5 mm/yr downslope to the north-east. The metasedimentary rock mass contains four pervasive joint sets and a foliation, contributing to a compound structure failure mechanism. The rock mass is further weakened by foliation-parallel sheared mylonite, and the presence of a brittle fault in the immediate area, with evidence of hydrothermal fluid flow though the RSD.The rock avalanche deposit below the slope deformation is calculated to be 3 Mm3, and extends >1 km from the source area, displaying typical mobility for north Norwegian rock avalanches onto undrained sediments. The deposits showcase exceptional lobate morphology with elongated ridge-and-furrow features. Raised shorelines predating and postdating the deposit provide temporal constraints on the deposit and an opportunity to reconstruct a relative timeline for the slope evolution. The postglacial marine limit (>14 cal. ka BP) is obscured by the deposit, while shorelines corresponding to the early Younger Dryas (12.2 cal. ka BP) and the subsequent Tapes transgression maximum (7.6–7.2 cal. ka BP) are prominent across the deposits, implying that the avalanche was emplaced between 15 and 12.2 cal. ka BP. Failure occurred during a time of immense climate instability at the boundary to the early Holocene, consistent with global reports of mountain slope failure following glacial retreat. The avalanche was emplaced into what would have been the marine environment. The anomaly between the rock avalanche source area volume (35 Mm3), and the rock avalanche deposit implies previous failure events, the deposits of which were either removed due to failure of the underlying marine sediments into the fjord, by retreating glacial ice or scour. The initiation of movement at the RSD may be attributed to periods of local climate changes, such as the Holocene Thermal Maximum. Cosmogenic nuclide dating is suggested as the next step to fill gaps in the slope evolution story through the mid to late Holocene.

Brief communication: Detection of glacier surge activity using cloud computing of Sentinel-1 radar data

Abstract. For studying the flow of glaciers and their response to climate change it is important to detect glacier surges. Here, we compute within Google Earth Engine the normalized differences between winter maxima of Sentinel-1 C-band radar backscatter image stacks over subsequent years. We arrive at a global map of annual backscatter changes, which are for glaciers in most cases related to changed crevassing associated with surge-type activity. For our demonstration period 2018–2019 we detected 69 surging glaciers, with many of them not classified so far as surge type. Comparison with glacier surface velocities shows that we reliably find known surge activities. Our method can support operational monitoring of glacier surges and some other special events such as large rock and snow avalanches.

Ambient noise and ERT data provide insights into the structure of co-seismic rock avalanche deposits in Sichuan (China)

The post-seismic history of the 2008 Mw7.9 Wenchuan earthquake shows that marginally stable deposits of large co-seismic landslide dams can pose persistent debris flow hazards for the downstream areas. Here, we combine analyses of single-station recordings of ambient noise with electrical resistivity tomography (ERT) surveys to explore the potential of drawing information on structure and geometry of the deposit of a large rock avalanche triggered by the Mw 7.9 2008 Wenchuan earthquake, which dammed the Yangjia stream in the Sichuan Province (China). The substantial thickness and heterogeneity of this kind of deposits limit the application of standard geophysical techniques, like active seismic surveys, which require highly energetic sources and long linear geophone arrays to reach adequate investigation depths. Passive single-station methods, relying on ambient noise recordings to determine site resonance properties, controlled by the contrast between soft surface layers and a stiffer substratum, offer the opportunity of investigating subsoil properties down to larger depths. In particular, we use a recently developed technique, which isolates the contribution of Rayleigh waves to ambient noise and draws information on sub-soil properties from the inversion of Rayleigh wave ellipticity curves plotted as function of frequency. In this framework, the ERT data can support the ellipticity curve inversion, typically affected by highly non-univocal solutions, by providing constraints for defining of the thickness of the uppermost surficial layers. The results allowed inferring the overlap of different layers within the 2008 rock avalanche deposit, as well as estimating lateral variations in their thickness and S-wave (Vs) velocities.

Stress level effect on mobility of dry granular flows of angular rock fragments

Granular flows of angular rock fragments such as rock avalanches and dense pyroclastic flows are simulated numerically by means of the discrete element method. Since large-scale flows generate stresses that are larger than those generated by small-scale flows, the purpose of these simulations is to understand the effect that the stress level has on flow mobility. The results show that granular flows that slide en mass have a flow mobility that is not influenced by the stress level. On the contrary, the stress level governs flow mobility when granular flow dynamics is affected by clast agitation and collisions. This second case occurs on a relatively rougher subsurface where an increase of the stress level causes an increase of flow mobility. The results show also that as the stress level increases, the effect that an increase of flow volume has on flow mobility switches sign from causing a decrease of mobility at low stress level to causing an increase of mobility at high stress level. This latter volume effect corresponds to the famous Heim’s mobility increase with the increase of the volume of large rock avalanches detected so far only in the field and for this reason considered inexplicable without resorting to extraordinary mechanisms. Granular flow dynamics is described in terms of dimensionless scaling parameters in three different granular flow regimes. This paper illustrates for each regime the functional relationship of flow mobility with stress level, flow volume, grain size, channel width, and basal friction.

High mobility of the channelized ancient Linka rock avalanche within the Bangong - Nujiang suture zone, SE Tibetan Plateau

Abstract Large rock avalanches often result in loss of life and catastrophic damage to infrastructure far from their initial failure positions, predominantly due to their tremendous volumes, high velocities and long runouts. Studies of the surface morphologies and internal structures of rock avalanche deposits can reveal evidence of their movement and deposition mechanisms. Through field survey, satellite image interpretation and particle-size distribution analysis, we studied the ancient, long-runout Linka rock avalanche, which is next to the Nujiang River, southeast Tibetan Plateau. The Linka rock avalanche, with a source volume of ~310 Mm3 originated from a high, steep dip slope and then transformed into a channelized rock avalanche, showing high mobility with a superelevation back-calculated maximal velocity of 42 m/s, a travel distance of ~6200 m and a Fahrboschung of 0.24. The slope failure sequence consisted of an initial failure of an outer limestone slab and a second failure of an inner sandstone slab. The limestone avalanche had 2 or 3 surges, while the sandstone avalanche probably had only one surge. The high mobility of the Linka rock avalanche predominantly resulted from a buckling failure occurred on a high relief source, low-energy dissipation during the channelized flow, intensive rock fragmentation in the basal facies and possible water lubrication when propagating bi-directionally along the Waqu River. Our findings for the Linka rock avalanche support previous knowledge that multiple mechanisms have different roles at different stages in a long runout rock avalanche.

Recurrent rock avalanches progressively dismantle a mountain ridge in Beichuan County, Sichuan, most recently in the 2008 Wenchuan earthquake

Large earthquake-triggered landslides, in particular rock avalanches, can have catastrophic consequences. However, without accelerometer records, the recognition of slopes prone to such failures remains difficult, because slope-specific seismic response depends on many factors including local topography, landforms, structure and internal geology. We address these issues by exploring the case of a rock avalanche of >3 million m3 triggered by the 2008 Mw7.9 Wenchuan earthquake in the Longmen Shan range, China. The failure, denominated Yangjia gully rock avalanche, occurred in Beichuan County (Sichuan Province), one of the areas that suffered the highest shaking intensity and death toll caused by co-seismic landsliding. Even though the Wenchuan earthquake produced tens of large (volume > 1 million m3) rock avalanches, many of which resulted in dangerous landslides, few studies so far have examined the pre-2008 history of the failed slope or reported on the stratigraphic record of mass-movement deposits exposed along local river courses. The presented case of the Yangjia gully rock avalanche shows the importance of such attempts as they provide information on the recurrence of large slope failures and their associated hazards. Our effort stems from recognition, on 2005 satellite imagery, of topography and morphology indicative of a large, apparently pre-historic slope failure and the associated breached landslide dam, both features closely resembling the forms generated in the catastrophic 2008 earthquake. The follow-up reconstruction recognizes an earlier landslide deposit exhumed from beneath the 2008 Yangjia gully rock avalanche by fluvial erosion since May 2008. We infer a seismic trigger also for the pre-2008 rock avalanche based on the following circumstantial evidence: i) the same source area (valley-facing, terminal portion of a flat-topped, elongated mountain ridge) located within one and a half kilometer of the seismically active Beichuan fault; ii) significant directional amplification of ground vibration, sub-parallel to the failed slope direction, detected via ambient noise measurements on the ridge adjacent to the source area of the 2008 rock avalanche and iii) common depositional and textural features of the two landslide deposits. Then, we show how, through consideration of the broader geomorphic and seismo-tectonic contexts, one can gain insight into the spatial and temporal recurrence of catastrophic slope failures in Beichuan County and elsewhere in the Longmen Shan. This insight, combined with local-scale geologic and geomorphologic knowledge, may guide selection of suspect slopes for reconnaissance, wide-area ambient noise investigation aimed at discriminating their relative susceptibility to co-seismic catastrophic failures. We indicate the feasibility of such investigations through the example of this study, which uses 3-component velocimeters designed to register low amplitude ground vibration.

From surface morphologies to inner structures: insights into hypermobility of the Nixu rock avalanche, southern Tibet, China

Large rock avalanches often result in many deaths and catastrophic damage to infrastructure far from source. No consensus has yet been reached on mechanisms causing their extraordinarily long runout since the first study at the Elm Slide, Switzerland. Much current understanding of rock avalanche kinematics and dynamics has been gained from observations of their surface morphologies and inner structures. Based on field surveys, high-resolution satellite image interpretation, and laboratory sieving, we analyzed the ancient, earthquake-induced Nixu rock avalanche in southern Tibet, China, which has unique exposures of both morphological features and inner structures of the deposits. The study revealed that (1) the Nixu rock avalanche probably could be disintegrated into three emplacement stages, transforming from an initial deep-seated rockslide into a large secondary debris avalanche, plus a relatively small third debris avalanche. The spreading of the secondary debris avalanche contributed to the subsequent long-runout propagation. (2) The morphological features observed in successive proximal to distal exposures (such as scarps, flowbands, transverse ridges, grid grooves, hummocks, and splash zones) reflect the change in stress-strain state of debris in motion from tension, to compression, to shear and tension. (3) The complicated inner structures exposed along river banks (such as jigsaw structures, fragmented clasts, inner shear zones, conjunction faults, convoluted lamination, decollements, sand injections, and intrusions) indicate the existence of a thin basal shear zone and a pressurized entrained substrate between the substrate and rock avalanche debris. (4) This suggests turbulent flow with bulldozing in the front, but a laminar flow under a simple shear model for the main body. (5) The analysis suggests that the rock avalanche’s extraordinary mobility involves a combination of momentum transfer, substrate entrainment, liquefied basal shearing, internal shearing, and rock fragmentation.