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Thermochemical pressurization is a possible weakening mechanism that could explain the hypermobility of large landslides in carbonate rock. However, the factors controlling the efficiency of this mechanism in generating hypermobility have not been elucidated. Here, we use mass and energy conservation equations—accounting for the thermal decomposition of carbonate and a thermoporoelastic mechanism—to evaluate the efficiency of thermochemical pressurization in the 2009 Jiweishan rockslide event in China. We consider distinct permeability functions for the shear zone and the rock wall, controlled by distinct mechanical processes, and explore the sensitivity of the model to changes in the values of the key parameters. We evaluate that a combination of pore pressure increase and the endothermic nature of the chemical reaction may effectively limit frictional heating. We observe that the increase in pore pressure is controlled by the permeability of the rock wall rather than by that of the shear zone and its thickness. We also find that the permeability of the damaged rock wall is the key factor controlling the energy ratio—a measure of efficiency. We conclude that the comparatively low velocity and small thickness of the sliding body, and the relatively high permeability of the formation in which the Jiweishan landslide developed may have limited the effect of thermal decomposition. Highlights: Shear-zone and wall rock permeabilities evolve differently during a landslide event in carbonate rock. Wall rock permeability controls pore pressure evolution and the efficiency of thermal decomposition. Thermal decomposition in the Jiweishan rockslide was limited but not negligible. [ABSTRACT FROM AUTHOR]

In tectonically active steep mountain belts, such as those in west Sichuan in China, frequent landslide disasters can occur during the rainy season. High-locality landslides often initiate with the failure of fractured rock masses or regolith near the ridge of a high and steep slope. Often, these events occur suddenly in the sense that people downslope are caught by surprise. High-locality landslides can be destructive owing to the large momentum and entrainment potential of the moving mass. Increasing attention has been given to these events since the 2008 Wenchuan earthquake. Indeed, slope ridges are particularly susceptible of topographic amplification of the seismic shaking, resulting in mechanical weakening that predisposes them to enhanced weathering and rain-triggered failures years or decades after the mainshock. Risk reduction in such cases must begin with a systematic surveying of the ridges to identify criticalities. Then, quantitative (numerical) approaches must be used to evaluate volumes and describe the movement. Risk assessments can then be produced, and mitigations suggested. Here, we describe the Tonghua landslide in Li County, which is currently in a creeping stage but threatens critical infrastructures and dwellings. We monitored the site for 3 years and identified additional potentially unstable areas before performing physically based modeling and hazard zonation. Our results could be useful in decision-making for risk reduction. [ABSTRACT FROM AUTHOR]

Major earthquakes in steep orogens can trigger extensive landsliding. Most of the landslide bodies come to rest high on the slopes, but subsequent rainfalls can easily remobilize them. Sharp peaks in landslide rates are systematically observed after major earthquakes, followed by rapid decays to background levels in just several years. Yet, the migration of coseismic debris into tributary channels and rivers remains poorly understood. We collected 12 years of observations of the 2008 Wenchuan earthquake's epicentral region in China and evaluated that debris flows and fluvial transport could only evacuate a small portion of coseismic debris, while over 70% of it had stabilized on the hillslopes. Coseismic debris may remain in the orogen for a long time, but estimates based on initial landslide abundance and sediment export are hampered by uncertainties in the debris fate throughout the earthquake cycle. Plain Language Summary: Strong earthquakes leave a trail of destruction that lasts much longer than the dramatic minutes of strong ground motion. Earthquakes shake the mountains, causing extensive landsliding that delivers huge amounts of debris along the slopes and in drainage channels. This debris is loose and unconsolidated, prone to generating subsequent landslides and debris flows. We have monitored the epicentral area of the magnitude 7.9 Wenchuan Earthquake (2008, China) for 12 years to study the evolution of landslide activity in space, time, and characteristics, and track the fate of the debris from the slopes into channels and rivers. We found that the landslide rates have decreased rapidly. After 10 years, they were already comparable to the pre‐earthquake levels. Most of the debris remained on the slopes, though: it stabilized and never moved again. At current rates, the removal of the earthquake‐generated debris from the landscape through landsliding may take a long time. Key Points: Despite large amounts of erodible coseismic debris, post‐seismic landslide rates decayed to pre‐earthquake levels in less than a decadeOnly a small proportion of debris has left the orogen so far: over 70% of it has stabilized in place, along the hillslopesWith erosion proceeding at this pace, a residence time of several centuries for the debris can be expected [ABSTRACT FROM AUTHOR]

The 2008 Wenchuan earthquake (China, Mw 7.9) highlighted the importance of assessing and mitigating the hazards from co-seismic landslides and landslide dams. The seismic shaking triggered hundreds of thousands of landslides, about 800 of which dammed the course of rivers. To understand whether distinctive factors concurred with the river-damming events, we analyzed the spatial patterns of the river-damming landslides and the non-damming landslides separately, with reference to a number of possible controlling factors. Then, we quantified the significance of these factors using the weight of evidence method, and we used the results to perform a susceptibility assessment in a portion of the earthquake-affected region to verify the effectiveness of the method. We find that the distance to the fault surface rupture, peak ground acceleration (PGA) and lithology play a controlling role for co-seismic landslides of any type. The occurrence of river-damming landslides, rather than by a specific lithology or topography, is more related to hydrological factors, while topographic controls (slope, internal relief and terrain roughness) are more significant for the non-damming landslides. [ABSTRACT FROM AUTHOR]

Rock avalanches represent a serious risk for human lives, properties, and infrastructures. On June 24, 2017, a catastrophic landslide destroyed the village of Xinmo (Maoxian County, Sichuan, China) causing a large number of fatalities. Adjacent to the landslide source area, further potentially unstable masses were identified. Among them, a 4.5-million m3 body, displaced during the landslide event by about 40 m, raised serious concerns. Field monitoring and a reliable secondary risk assessment are fundamental to protect the infrastructure and the population still living in the valley. In this framework, the use of distinct element methods and continuum model methods to simulate the avalanche process was discussed. Various models (PFC, MatDEM, MassMov2D, Massflow) were used with the aim of reproducing the Xinmo landslide and, as predictive tools, simulating the kinematics and runout of the potentially unstable mass, which could cause a new catastrophic event. The models were all able to reproduce the first-order characteristics of the landslide kinematics and the morphology of the deposit, but with computational times differing by several orders of magnitude. More variability of the results was obtained from the simulations of the potential secondary failure. However, all models agreed that the new landslide could invest several still-inhabited buildings and block the course of the river again. Comparison and discussion of the performances and usability of the models could prove useful towards the enforcement of physically based (and multi-model) risk assessments and mitigation countermeasures. [ABSTRACT FROM AUTHOR]

On 8th August 2017, a magnitude Ms 7.0 earthquake struck the County of Jiuzhaigou, in Sichuan Province, China. It was the third Ms ≥ 7.0 earthquake in the Longmenshan area in the last decade, after the 2008 Ms 8.0 Wenchuan earthquake and the 2013 Ms 7.0 Lushan earthquake. The event did not produce any evident surface rupture but triggered significant mass wasting. Based on a large set of pre- and post-earthquake high-resolution satellite images (SPOT-5, Gaofen-1 and Gaofen-2) as well as on 0.2-m-resolution UAV photographs, a polygon-based interpretation of the coseismic landslides was carried out. In total, 1883 landslides were identified, covering an area of 8.11 km2, with an estimated total volume in the order of 25-30 × 106 m3. The total landslide area was lower than that produced by other earthquakes of similar magnitude with strike-slip motion, possibly because of the limited surface rupture. The spatial distribution of the landslides was correlated statistically to a number of seismic, terrain and geological factors, to evaluate the landslide susceptibility at regional scale and to identify the most typical characteristics of the coseismic failures. The landslides, mainly small-scale rockfalls and rock/debris slides, occurred mostly along two NE-SW-oriented valleys near the epicentre. Comparatively, high landslide density was found at locations where the landform evolves from upper, broad valleys to lower, deep-cut gorges. The spatial distribution of the coseismic landslides did not seem correlated to the location of any known active faults. On the contrary, it revealed that a previously-unknown blind fault segment—which is possibly the north-western extension of the Huya fault—is the plausible seismogenic fault. This finding is consistent with what hypothesised on the basis of field observations and ground displacements. [ABSTRACT FROM AUTHOR]

Bedding-controlled landslides in alternated mudstone and sandstone beds in cataclinal slopes are common in the Sichuan basin and in the Three Gorges area in China. They are characterised by very gentle dip angles of the beds (<10°), and usually exhibit slow creep movements throughout the year. However, during the rainy season, they can have sudden accelerations and undergo large displacements. In this work, the influence of saturation degree, displacement rate and stress state on the shear strength of the shear-zone soil was investigated. The results of the displacement-controlled ring-shear tests show a rate-weakening behaviour even at low rates of displacement. Dually, the shear-controlled tests show that the displacement rate can increase dramatically in response to a slight increase of the shear stress. The threshold stress that triggers large displacement is equal to the rate-dependent residual shear strength of the soil. [ABSTRACT FROM AUTHOR]

Thick loess deposits are considered problematic soils due to their susceptibility to collapse suddenly and in large volumes upon increase of water content. In the past decades, a large number of landslides occurred in the Loess Plateau in Gansu Province, north-western China, triggered by the extensive irrigation. The increase of water content has been identified as a key factor controlling the loess strength. However, the role of long-term saturation and pore fluid chemistry changes has not been investigated systematically. This paper reports on an experimental study on loess samples from Heitai loess terrace (Yongjing, Gansu) carried out through chemical analyses, oedometer and ring shear tests. The natural pore fluid was found to be a concentrated saline solution in unsaturated soil condition. Upon saturation, the pore ion concentration drop can contribute to shear strength loss in the clay component and volume decrease. A prolonged exposure to the irrigation water might facilitate cement dissolution, which could result in further weakening. [ABSTRACT FROM AUTHOR]

Earthquakes alter the topography of mountain ranges by shortening and thickening the crust, while also causing erosion via co-seismic landsliding. For single large (>Mw7) earthquakes, co-seismic landslide volumes can exceed the volume of rock uplifted by the earthquake. Hence, large earthquakes are hypothesised to generate rock uplift, but minimal (or no) surface uplift. Surface uplift of mountains is affected by existing regolith limiting the amount of bedrock erosion by landslides, rock uplift by small earthquakes (that do not produce landslides), inter-seismic erosion, and a-seismic rock uplift, hence the role of earthquakes in generating surface uplift of mountains is poorly constrained. Observations of a post seismic multi-temporal landslide inventory of the 2008 Wenchuan earthquake demonstrate that landslide deposits are likely to remain within the Longmen Shan for longer than previously assumed. We demonstrate the importance of persistent regolith and aseismic uplift to surface uplift by using a dimensionless seismic volume balance model. Regolith generated by large earthquakes limits exhumation rates over 102-103 years because smaller landslides remobilise existing regolith rather than eroding bedrock, creating a depth-dependent co-seismic landslide regolith production function (CLRPF). Applying this model to the Longmen Shan in China reveals that 42% of the regolith generation is by co-seismic landsliding. We varied the proportion of rock uplift by earthquakes to a-seismic processes, demonstrating that stochastic rock uplift and CLRPF leads to statistically significant differences in the variability of exhumation rates measured across cosmogenic timescales. To test this model result, we analysed a global database of detrital cosmogenic nuclides from a range of tectonic settings, demonstrating that seismically active compressional landscapes have significantly higher variance and skew. The results demonstrate that the influence of earthquakes on the exhumation of mountains extends beyond the timescale of most dating methods (including cosmogenic radionuclide dating) and that average erosion rates calculated by these methods may significantly underestimate erosion rates. [ABSTRACT FROM AUTHOR]