Gravitational Mass Movement (‘Landslide’)

Primary reference(s)

Cruden, D.M. and Varnes, D.J.,1996, Landslide Types and Processes, Transportation Research Board, U.S. National Academy of Sciences, Special Report, 247: 36-75

Annotations

Additional scientific description

Gravitational mass movements occur when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope (Varnes, 1978). Such movements are commonly known as 'landslides'; however, many slope hazards that are referred to as such are driven by processes other than sliding. For this reason, usage of the term Gravitational Mass Movement is preferred as a broad term to encompass the wider group of slope failure mechanisms defined by Cruden and Varnes (1996). It covers movement by:

• Rock, Debris and Earth Fall (GH0301)
• Rock, Debris and Earth (mud) Spread (GH0302)
• Rock, Debris and Earth (mud) Flow (GH303)
• Rock, Debris and Earth (mud) Slide (GH0304)
• Rock, Debris and Earth Topple (GH0305)

Sub-categories of these are defined by the type of material (e.g., rock, soil, debris, earth, vegetation, ice), by the velocity of the mass movement (Cruden and Varnes, 1996; Hungr et al., 2014), and whether terrestrial or submarine.

Earthquake triggered gravitational mass movements typically affect steep slopes and slopes underlain by sediments that are prone to liquefaction. Within a given region, it is possible to discriminate earthquake-triggered gravitational mass movements from those initiated by other triggering processes. For example, Lee (2012) reported that earthquake-induced gravitational mass movements in Taiwan are mostly located on steeper, longer slopes and at a higher position of the slope when compared to storm-induced shallow gravitational mass movements, suggesting that topographic amplification plays an important role in earthquake-induced gravitational mass movements. In hard rock terrains, earthquakes trigger a higher proportion of rockfalls Zhang et al. (2014) compared earthquake-triggered gravitational mass movements with rainfall-triggered gravitational mass movements in the Wenchuan area of China and found that the earthquake gravitational mass movements were triggered on steeper slopes, larger gravitational mass movements dominated in areas underlain by harder bedrock compared with areas underlain by alluvium. In contrast, the rainfall-induced gravitational mass movements were characterised by a greater volume of channelled deposits and were of a higher density, but smaller area and were characterised by debris slides and debris flows. In areas that are underlain by weak bedrock that is saturated, strong earthquake-induced ground shaking will result in more gravitational mass movements than normal (Fan et al., 2019).

Volcano triggered gravitational mass movements range in size from less than 1 km3 to more than 100 km3 in area (USGS, no date). They comprise masses of rock, soil and snow that are mobilised when the flank of a volcano collapses and moves downslope. The mobilised rock and sediment can be very destructive and entrain more sediment (as well as vegetation or structures) along its path. The high velocity and momentum allow them to cross valleys and run up slopes several hundred metres high. Larger landslides are generally more deep-seated, involving weak hydrothermal and magmatic systems in the volcano. Gravitational mass movements are common on volcanic cones because they are tall, steep, and weakened by the rise and eruption of molten rock. Magma releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering minerals to clay (USGS, no date). The gravitational mass movements leave a hummocky terrain that reflects the initial structure of the edifice (de Vries and Davies, 2015). The sediment largely comprises unsorted and unstratified angular-to-subangular debris (Siebert, 1996). Runout lengths are commonly many times the height of the volcano. Many gravitational mass movements contain or incorporate water that leads to secondary debris flow and lahar generation. Runout varies with the extent of air or fluid entrainment; however, the physical basis of the long runouts is not fully understood. Most are the result of several factors, including volcanic flank failures. Gravitational mass movements on volcanic islands such as Hawaii, Reunion and Tristan da Cunha are characterised by long runout distances and volumes exceeding 1000 km3 (Hürlimann et al., 2000).

Key relevant UN convention / multilateral treaty

Sendai framework for Disaster Risk Reduction 2015-2030.

Drivers

Gravitational mass movements, whether terrestrial or submarine, can be triggered by multiple sources including earthquakes, volcanic activity, storms, high precipitation, differential weathering (e.g., freeze/thaw) and human activity. These mass movements can in turn trigger tsunamis and can have multiple associated hazards including ground fracturing and ground gases (HIP ref). 

The fundamental geological, geomorphological, and hydrological conditions are significant underlying factors contributing to gravitational mass movements. Soil and rock types, such as weak rock strata (e.g., shale and mudstone) or loose sediments (e.g., clay and loess), are more prone to sliding. Tectonic activities render slopes with well-developed faults and joints more unstable. Prolonged rainfall or intense downpours can increase pore water pressure and soften the soil and rock, thereby facilitating landslides. 

Gravitational mass movements are usually triggered in steep slopes but can also occur in shallow slopes. These slopes are commonly underlain by sediments such as weakly cemented rocks, more indurated rocks with pervasive discontinuities, residual and colluvial sand, volcanic soils with sensitive clays (e.g., Iburi–Tobu earthquake, Hokkaido; Kameda et al., 2019), loess, alluvium and deltaic deposits.

Impacts

Gravitational mass movements can be extremely destructive, especially when failure is large, sudden and (or) the velocity is rapid. According to the World Health Organisation, between 1998-2017 gravitational mass movements affected an estimated 4.8 million people and caused over 18,000 deaths (WHO, 2025). Deaths are caused by burial of towns or villages or disruption of foundations and subsequent collapse of buildings. 

The Mount St. Helens eruption was triggered by gravitational mass movement as a consequence of structural instability of the volcano. The eruption caused the death of 57 people, 53 through direct impacts including asphyxiation, thermal injuries, and trauma. Snowmelt led to extensive river flooding (Oregon State University, 2020). 

Gravitational mass movement impacts can cascade to dam rivers and impound lakes, which can collapse days to centuries later. They can cause extensive mountain valley flooding and leave a geomorphology that may be prone to remobilisation during heavy rainfall, potentially evolving as debris flows. Cracks and fractures can form and widen on mountain crests and flanks, conditioning the landscape for an increased frequency of mass movements that lasts for decades. Increased debris load delivery to rivers can cause bank erosion and floodplain accretion as well as stream channel switching that affect flooding frequency, settlements, ecosystems, and infrastructure (Fan et al., 2019). In coastal and offshore regions, submarine mass movement impacts threaten submarine installations such as oil platforms, pipelines, cables, and wind installation, through destabilization of the seabed or breakage of cables. 

While the physical damage of slope hazards is well documented, health impacts are complex. The risk of an increase in infectious diseases is of concern during the response and recovery phase after any major disaster. Displacement of people due to the destruction of their homes and other infrastructure can place them in unfamiliar surroundings which, if they conflict with traditional beliefs and practices with regard to water supply and hygiene, can result in unsafe behaviours. The medium- to long-term effects of changes to the environment caused by mass movements, such as deforestation, and changes to river courses, can increase the risk of vector-borne diseases, and as a result, the health impacts can extend long after the initial disaster is over. Disruption of soil can also increase exposure to infectious organisms (Kennedy et al., 2015). The psychosocial and mental health impacts on survivors and rescue personnel from mass movements are increasingly recorded. The prevalence of psychiatric disorders and wider support needed to reduce misuse of substances has been identified (Kennedy et al., 2015; Dell’Aringa et al., 2018).

Multi-hazard context

The figure below summarises common interactions between gravitational mass movement (landslide) and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.

Multi-hazard diagram

Risk Management

In terms of natural hazards, the Encyclopaedia of the Environment reported two categories of protection: active protections to remove the hazard itself and passive protections, which do not seek to oppose natural phenomena only to limit their harmful consequences for developments (buildings, communication routes) (Dennis and Didier, 2019). 

• Active protections are diverse and include (i) general methods, such as surface or deep drainage, and slope vegetation that limits erosion due to runoff (gully excavation) and infiltration that alters mechanical properties (friction, cohesion of rock joints); (ii) supports, such as retaining walls, rock bolts or anchored mesh covered with sprayed concrete; (iii) wire mesh (draped or pinned), i.e., metal structures designed to contain the massif and prevent the spread of falling rocks and blocks; and (iv) scaling/mining, which are radical solutions that comprise removing unstable elements, although these solutions are not always as definitive as expected (the continuous alteration and the blast vibrations are often harmful to the stability of the surrounding massifs) (Dennis and Didier, 2019).
• Passive protections are also diverse: (i) barriers and dikes are gabions or concrete blocks placed at the foot of unstable slopes; their purpose is to stop the propagation of rock elements before reaching the stakes; their location, which requires sufficient space and their dimensioning, takes into account the properties of the materials that constitute them, but first of all numerical simulations that are made to model the propagation of blocks (trajectory circulations); (ii) the diverters are also embankments; installed on the slope, they divert the flow of the elements towards a space without stakes; (iii) protection galleries, similar to avalanche tunnels, are likely to protect communication routes when crossing corridors; (iv) rigid fences are placed on steep slopes as close as possible to the starting areas; their installation is often difficult to achieve; and (v) deformable wire-mesh can be placed lower on the slopes until they are close to the issues at stake; the most well-known case is the use of ‘submarine’ type nets (used during the Second World War to prevent the penetration of ports by submarine vehicles) stretched between rigid poles and maintained by fusible carabiners; this device is thus calculated to resist an impact energy previously determined in the study of randomness and its propagation (Dennis and Didier, 2019). 

The appropriate responses to reduce the risks associated with rock instabilities are effective but often require a large budget. It is not viable to consider eliminating risk wherever it exists. The best protection is always based first on geological reconnaissance, planning and use of preventative actions such as drainage or regular purging of unstable elements and on monitoring based on measurements when movements are detected. Such monitoring often triggers an alert with a road closure or evacuation of an inhabited area (Dennis and Didier, 2019).

Monitoring

The section and the table below offer an overview of monitoring gravitational mass movement (landslide). This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.