There have been known various classifications of

landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...

s. Broad definitions include forms of mass movement that narrower definitions exclude. For example, the '' McGraw-Hill Encyclopedia of Science and Technology'' distinguishes the following types of landslides: *fall (by undercutting) *fall (by toppling) * slump *

rockslide A rockslide is a type of landslide caused by rock failure in which part of the bedding plane of failure passes through compacted rock and material collapses ''en masse'' and not in individual blocks. Note that a rockslide is similar to an avalanc ...

* earthflow *

sinkhole A sinkhole is a depression or hole in the ground caused by some form of collapse of the surface layer. The term is sometimes used to refer to doline, enclosed depressions that are locally also known as ''vrtače'' and shakeholes, and to openi ...

s, mountain side *rockslide that develops into rock avalanche Influential narrower definitions restrict landslides to slumps and translational slides in rock and

regolith Regolith () is a blanket of unconsolidated, loose, heterogeneous superficial deposits covering solid rock. It includes dust, broken rocks, and other related materials and is present on Earth, the Moon, Mars, some asteroids, and other terrestri ...

, not involving fluidisation. This excludes falls, topples, lateral spreads, and mass flows from the definition.Varnes D. J., Slope movement types and processes. In: Schuster R. L. & Krizek R. J. Ed., Landslides, analysis and control. Transportation Research Board Sp. Rep. No. 176, Nat. Acad. oi Sciences, pp. 11–33, 1978.Hungr O, Evans SG, Bovis M, and Hutchinson JN (2001) Review of the classification of landslides of the flow type. Environmental and Engineering Geoscience VII, 221-238. The causes of landslides are usually related to instabilities in slopes. It is usually possible to identify one or more

causes and one landslide trigger. The difference between these two concepts is subtle but important. The landslide causes are the reasons that a landslide occurred in that location and at that time and may be considered to be factors that made the slope vulnerable to failure, that predispose the

slope In mathematics, the slope or gradient of a line is a number that describes both the ''direction'' and the ''steepness'' of the line. Slope is often denoted by the letter ''m''; there is no clear answer to the question why the letter ''m'' is used ...

to becoming unstable. The trigger is the single event that finally initiated the landslide. Thus, causes combine to make a slope vulnerable to failure, and the trigger finally initiates the movement. Landslides can have many causes but can only have one trigger. Usually, it is relatively easy to determine the trigger after the landslide has occurred (although it is generally very difficult to determine the exact nature of landslide triggers ahead of a movement event).

Classification factors

Various scientific disciplines have developed taxonomic classification systems to describe natural phenomena or individuals, like for example, plants or animals. These systems are based on specific characteristics like shape of organs or nature of reproduction. Differently, in

classification, there are great difficulties because phenomena are not perfectly repeatable; usually being characterised by different causes, movements and morphology, and involving genetically different material. For this reason, landslide classifications are based on different discriminating factors, sometimes very subjective. In the following write-up, factors are discussed by dividing them into two groups: the first one is made up of the criteria utilised in the most widespread classification systems that can generally be easily determined. The second one is formed by those factors that have been utilised in some classifications and can be useful in descriptions.

A1) Type of movement

This is the most important criterion, even if uncertainties and difficulties can arise in the identification of movements, being the mechanisms of some

landslides Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...

often particularly complex. The main movements are falls, slides and flows, but usually topples, lateral spreading and complex movements are added to these.

A2) Involved material

Rock, earth and

debris Debris (, ) is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, as in geology, large rock fragments left by a melting glacier, etc. Depending on context, ''debris'' can refer t ...

are the terms generally used to distinguish the materials involved in the

process. For example, the distinction between earth and debris is usually made by comparing the percentage of coarse

grain A grain is a small, hard, dry fruit ( caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legu ...

size fractions. If the weight of the particles with a diameter greater than 2 mm is less than 20%, the material will be defined as ''earth''; in the opposite case, it is ''debris''.

A3) Activity

The classification of a landslide based on its activity is particularly relevant in the evaluation of future events. The recommendations of the WP/WLI (1993) define the concept of activity with reference to the spatial and temporal conditions, defining the state, the distribution and the style. The first term describes the information regarding the time in which the movement took place, permitting information to be available on future evolution, the second term describes, in a general way, where the landslide is moving and the third term indicates how it is moving.

A4) Movement velocity

This factor has a great importance in the

hazard A hazard is a potential source of harm. Substances, events, or circumstances can constitute hazards when their nature would allow them, even just theoretically, to cause damage to health, life, property, or any other interest of value. The probab ...

evaluation. A

velocity Velocity is the directional speed of an object in motion as an indication of its rate of change in position as observed from a particular frame of reference and as measured by a particular standard of time (e.g. northbound). Velocity i ...

range is connected to the different type of

, on the basis of observation of case history or site observations.

B1) The age of the movement

Landslide Landslides, also known as landslips, are several forms of mass wasting that may include a wide range of ground movements, such as rockfalls, deep-seated slope failures, mudflows, and debris flows. Landslides occur in a variety of environments, ...

dating is an interesting topic in the evaluation of

. The knowledge of the landslide

frequency Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from '' angular frequency''. Frequency is measured in hertz (Hz) which is ...

is a fundamental element for any kind of

probabilistic Probability is the branch of mathematics concerning numerical descriptions of how likely an event is to occur, or how likely it is that a proposition is true. The probability of an event is a number between 0 and 1, where, roughly speaking, ...

evaluation. Furthermore, the evaluation of the age of the landslide permits to correlate the trigger to specific conditions, as

earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, fr ...

or periods of intense

rain Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water ...

s. It is possible that phenomena could be occurred in past geological times, under specific environmental conditions which no longer act as agents today. For example, in some Alpine areas, landslides of the

Pleistocene The Pleistocene ( , often referred to as the '' Ice age'') is the geological epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was finally confirmed ...

age are connected with particular

tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...

, geomorphological and climatic conditions.

B2) Geological conditions

This represent a fundamental factor of the morphological evolution of a

. Bedding attitude and the presence of discontinuities or faults control the

morphogenesis Morphogenesis (from the Greek ''morphê'' shape and ''genesis'' creation, literally "the generation of form") is the biological process that causes a cell, tissue or organism to develop its shape. It is one of three fundamental aspects of deve ...

B3) Morphological characteristics

As the landslide is a geological volume with a hidden side, morphological characteristics are extremely important in the reconstruction of the technical model.

B4) Geographical location

This criterion describes, in a general way, the location of landsides in the physiographic context of the area. Some authors have therefore identified landslides according to their geographical position so that it is possible to describe " alpine landslides", "landslides in plains", "hilly landslides" or "

cliff In geography and geology, a cliff is an area of rock which has a general angle defined by the vertical, or nearly vertical. Cliffs are formed by the processes of weathering and erosion, with the effect of gravity. Cliffs are common on ...

landslides". As a consequence, specific morphological contexts are referred characterised by slope evolution processes.

B5) Topographical criteria

With these criteria, landslides can be identified with a system similar to that of the denomination of formations. Consequently, it is possible to describe a landslide using the name of a site. In particular, the name will be that of the locality where the landslide happened with a specific characteristic type.

B6) Type of climate

These criteria give particular importance to

climate Climate is the long-term weather pattern in an area, typically averaged over 30 years. More rigorously, it is the mean and variability of meteorological variables over a time spanning from months to millions of years. Some of the meteorologica ...

in the genesis of phenomena for which similar geological conditions can, in different climatic conditions, lead to totally different morphological evolution. As a consequence, in the description of a landslide, it can be interesting to understand in what type of climate the event occurred.

B7) Causes of the movements

In the evaluation of landslide susceptibility, causes of the triggers is an important step. Terzaghi describes causes as "internal" and "external" referring to modifications in the conditions of the stability of the bodies. Whilst the internal causes induce modifications in the material itself which decrease its resistance to

shear stress Shear stress, often denoted by (Greek: tau), is the component of stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel to the material cross section. '' Normal stress'', on ...

, the external causes generally induce an increase of shear stress, so that block or bodies are no longer stable. The triggering causes induce the movement of the mass. Predisposition to movement due to control factors is determining in landslide evolution.

Structural A structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as ...

and geological factors, as already described, can determine the development of the movement, inducing the presence of mass in

kinematic Kinematics is a subfield of physics, developed in classical mechanics, that describes the motion of points, bodies (objects), and systems of bodies (groups of objects) without considering the forces that cause them to move. Kinematics, as a fiel ...

freedom.

Types and classification

In traditional usage, the term landslide has at one time or another been used to cover almost all forms of mass movement of rocks and

at the Earth's surface. In 1978, in a very highly cited publication, David Varnes noted this imprecise usage and proposed a new, much tighter scheme for the classification of mass movements and subsidence processes. This scheme was later modified by Cruden and Varnes in 1996,Cruden, David M., and David J. Varnes. "Landslides: investigation and mitigation. Chapter 3-Landslide types and processes." Transportation research board special report 247 (1996). and influentially refined by Hutchinson (1988)Hutchinson, J. N. "General report: morphological and geotechnical parameters of landslides in relation to geology and hydrogeology." International symposium on landslides. 5. 1988. and Hungr et al. (2001). This full scheme results in the following classification for mass movements in general, where bold font indicates the landslide categories: Under this definition, landslides are restricted to "the movement... of shear strain and displacement along one or several surfaces that are visible or may reasonably be inferred, or within a relatively narrow zone", i.e., the movement is localised to a single failure plane within the subsurface. He noted landslides can occur catastrophically, or that movement on the surface can be gradual and progressive. Falls (isolated blocks in free-fall), topples (material coming away by rotation from a vertical face), spreads (a form of subsidence), flows (fluidised material in motion), and creep (slow, distributed movement in the subsurface) are all explicitly excluded from the term landslide. Under the scheme, landslides are sub-classified by the material that moves, and by the form of the plane or planes on which movement happens. The planes may be broadly parallel to the surface ("translational slides") or spoon-shaped ("rotational slides"). Material may be rock or

(loose material at the surface), with regolith subdivided into debris (coarse grains) and earth (fine grains). Nevertheless, in broader usage, many of the categories that Varnes excluded are recognised as landslide types, as seen below. This leads to ambiguity in usage of the term. The following clarifies the usages of the various terms in the table. Varnes and those who later modified his scheme only regard the slides category as forms of landslide.

Falls

Description: ''" the detachment of soil or rock from a steep slope along a surface on which little or no shear displacement takes place. The material then descends mainly through the air by falling, bouncing, or rolling"'' (Varnes, 1996). Secondary falls: ''"Secondary falls involves rock bodies already physically detached from

and merely lodged upon it"'' (Hutchinson, 1988) Speed: ''from very to extremely rapid'' Type of slope: ''slope angle 45–90 degrees'' Control factor: ''Discontinuities'' Causes: ''Vibration, undercutting, differential

weathering Weathering is the deterioration of rocks, soils and minerals as well as wood and artificial materials through contact with water, atmospheric gases, and biological organisms. Weathering occurs '' in situ'' (on site, with little or no movemen ...

, excavation, or stream erosion''

Topples

Description: ''"Toppling is the forward

rotation Rotation, or spin, is the circular movement of an object around a '' central axis''. A two-dimensional rotating object has only one possible central axis and can rotate in either a clockwise or counterclockwise direction. A three-dimensional ...

out of the slope of a mass of soil or rock about a point or axis below the

centre of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force may ...

of the displaced mass. Toppling is sometimes driven by gravity exerted by material upslope of the displaced mass and sometimes by water or ice in cracks in the mass"'' (Varnes, 1996) Speed: ''extremely slow to extremely rapid'' Type of slope: ''slope angle 45–90 degrees'' Control factor: ''Discontinuities,

lithostratigraphy Lithostratigraphy is a sub-discipline of stratigraphy, the geological science associated with the study of strata or rock layers. Major focuses include geochronology, comparative geology, and petrology. In general, strata are primarily ign ...

'' Causes: ''Vibration, undercutting, differential

, excavation, or stream erosion''

Slides

''"A slide is a downslope movement of

soil Soil, also commonly referred to as earth or dirt, is a mixture of organic matter, minerals, gases, liquids, and organisms that together support life. Some scientific definitions distinguish ''dirt'' from ''soil'' by restricting the former ...

or rock mass occurring dominantly on the surface of rupture or on relatively thin zones of intense shear strain."'' (Varnes, 1996) Mica Dam spillway

Translational slide

Description: ''"In translational slides the mass displaces along a planar or undulating surface of rupture, sliding out over the original ground surface."'' (Varnes, 1996) Speed: ''extremely slow to extremely rapid (>5 m/s)'' Type of slope: ''slope angle 20-45 degrees'' Control factor: ''Discontinuities, geological setting''

Rotational slides

Description: ''"Rotational slides move along a surface of rupture that is curved and

concave Concave or concavity may refer to: Science and technology * Concave lens * Concave mirror Mathematics * Concave function, the negative of a convex function * Concave polygon, a polygon which is not convex * Concave set In geometry, a subset ...

"'' (Varnes, 1996) Speed: ''extremely slow to extremely rapid'' Type of slope: ''slope angle 20–40 degrees'' Control factor: ''morphology and

lithology The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Li ...

'' Causes: ''

Vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The word comes from Latin ''vibrationem'' ("shaking, brandishing"). The oscillations may be periodic, such as the motion of a pendulum—or random, suc ...

, undercutting, differential

, excavation, or stream erosion'' Landslide in Cusco, Peru - 2018

Spreads

"Spread is defined as an extension of a

cohesive Cohesion may refer to: * Cohesion (chemistry), the intermolecular attraction between like-molecules * Cohesion (computer science), a measure of how well the lines of source code within a module work together * Cohesion (geology), the part of shear ...

soil or rock mass combined with a general subsidence of the fractured mass of cohesive material into softer underlying material." (Varnes, 1996). ''"In spread, the dominant mode of movement is lateral extension accommodated by shear or tensile fractures"'' (Varnes, 1978) Speed: ''extremely slow to extremely rapid (>5 m/s)'' Type of slope: ''angle 45–90 degrees'' Control factor: ''Discontinuities,

'' Causes: ''Vibration, undercutting, differential

, excavation, or stream erosion''

Flows

''A flow is a spatially continuous movement in which surfaces of shear are short-lived, closely spaced, and usually not preserved. The distribution of velocities in the displacing mass resembles that in a

viscous The viscosity of a fluid is a measure of its resistance to deformation at a given rate. For liquids, it corresponds to the informal concept of "thickness": for example, syrup has a higher viscosity than water. Viscosity quantifies the inte ...

liquid. The lower boundary of displaced mass may be a surface along which appreciable differential movement has taken place or a thick zone of distributed shear'' (Cruden & Varnes, 1996)

Flows in rock

=Rock Flow

= Description: ''"Flow movements in bedrock include deformations that are distributed among many large or small fractures, or even microfracture, without concentration of displacement along a through-going fracture"'' (Varnes, 1978) Speed: ''extremely slow'' Type of slope: ''angle 45–90 degrees'' Causes: ''Vibration, undercutting, differential

, excavation, or stream erosion''

=Rock avalanche (Sturzstrom)

= Description: ''"Extremely rapid, massive, flow-like motion of fragmented rock from a large rock slide or rock fall"'' (Hungr, 2001) Speed: ''extremely rapid'' Type of slope: ''angle 45–90 degrees'' Control factor: ''Discontinuities,

'' Causes: ''Vibration, undercutting, differential

, excavation or stream erosion''

Flows in soil

=Debris flow

= Description: ''"

Debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented Rock (geology), rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. ...

is a very rapid to extremely rapid flow of saturated non-plastic

in a steep channel"'' (Hungr et al.,2001) Speed: ''very rapid to extremely rapid (>5 m/s)'' Type of slope: ''angle 20–45 degrees'' Control factor: '' torrent

sediments Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sand a ...

, water flows'' Causes: ''High intensity rainfall''

=Debris avalanche

Description: ''"Debris avalanche is a very rapid to extremely rapid shallow flow of partially or fully saturated

on a steep

, without confinement in an established channel."'' (Hungr et al., 2001) Speed: ''very rapid to extremely rapid (>5 m/s)'' Type of slope: ''angle 20–45 degrees'' Control factor: ''morphology,

'' Causes: ''High intensity rainfalls''

=Earth flow

= Description: ''"

Earth flow An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of gravity. It is an intermediate type of mass wasting that is between downhill creep and mudflow. The ty ...

is a rapid or slower, intermittent flow-like movement of plastic, clayey earth."'' (Hungr et al.,2001) Speed: ''slow to rapid (>1.8 m/h)'' Type of slope: ''

angle 5–25 degrees'' Control factor: ''

=Mudflow

= Description: ''"

Mudflow A mudflow or mud flow is a form of mass wasting involving fast-moving flow of debris that has become liquified by the addition of water. Such flows can move at speeds ranging from 3 meters/minute to 5 meters/second. Mudflows contain a significa ...

is a very rapid to extremely rapid flow of saturated plastic debris in a channel, involving significantly greater water content relative to the source material ( Plasticity index> 5%)."'' (Hungr et al.,2001) Speed: ''very rapid to extremely rapid (>5 m/s)'' Type of slope: ''angle 20–45 degrees'' Control factor: '' torrent

sediment Sediment is a naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of wind, water, or ice or by the force of gravity acting on the particles. For example, sa ...

s, water flows'' Causes: ''High intensity

rainfall Rain is water droplets that have condensed from atmospheric water vapor and then fall under gravity. Rain is a major component of the water cycle and is responsible for depositing most of the fresh water on the Earth. It provides water ...

Complex movement

Description: ''Complex movement is a combination of falls, topples, slides, spreads and flows'' April 2011 Heavy Rain Triggers Landslides in Thailand 2

April 2011 Heavy Rain Triggers Landslides in Thailand 2

Causes

Landslide causes include

geological Geology () is a branch of natural science concerned with Earth and other astronomical objects, the features or rocks of which it is composed, and the processes by which they change over time. Modern geology significantly overlaps all other Eart ...

factors, morphological factors, physical factors and factors associated with human activity. Geological causes * Weathered materials * Sheared materials * Jointed or fissured materials * Adversely orientated discontinuities * Permeability contrasts * Material contrasts * Rainfall and snow fall * Earthquakes Morphological causes * Slope angle * Uplift * Rebound * Fluvial erosion * Wave erosion * Glacial erosion * Erosion of lateral margins * Subterranean erosion * Internal erosion * Slope loading * Vegetation change * Erosion Physical causes Topography: * Slope aspect and gradient Geological factors: * Discontinuity factors (dip spacing, asperity, dip and length) * Physical characteristics of the rock (rock strength etc.) Tectonic activity: * Seismic activity (earthquakes) * Volcanic eruption Physical weathering: * Thawing * Freeze-thaw * Soil erosion Hydrogeological factors: * Intense rainfall * Rapid snow melt * Prolonged precipitation * Ground water changes (rapid drawdown) * Soil pore water pressure * Surface runoff Human causes * Deforestation * Excavation * Loading * Water management (groundwater drawdown and water leakage) * Land use (e.g. construction of roads, houses etc.) * Mining and quarrying * Vibration Occasionally, even after detailed investigations, no trigger can be determined - this was the case in the large

Aoraki / Mount Cook Aoraki / Mount Cook is the highest mountain in New Zealand. Its height, as of 2014, is listed as . It sits in the Southern Alps, the mountain range that runs the length of the South Island. A popular tourist destination, it is also a favourite ...

landslide in

New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 List of islands of New Zealand, smaller islands. It is the ...

1991. It is unclear as to whether the lack of a trigger in such cases is the result of some unknown process acting within the landslide, or whether there was in fact a trigger, but it cannot be determined. The trigger may be due to a slow but steady decrease in material strength associated with the

of the rock - at some point the material becomes so weak that failure must occur. Hence, the trigger is the weathering process, but this is not detectable externally. In most cases a trigger is thought as an external stimulus that induces an immediate or near-immediate response in the slope, in this case in the form of the movement of the landslide. Generally, this movement is induced either because the stresses in the slope are altered by increasing shear stress or decreasing the effective normal stress, or by reducing the resistance to the movement perhaps by decreasing the

shear strength In engineering, shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a mater ...

of the materials within the landslide.

Rainfall

In the majority of cases the main trigger of landslides is heavy or prolonged

. Generally this takes the form of either an exceptional short lived event, such as the passage of a

tropical cyclone A tropical cyclone is a rapidly rotating storm system characterized by a low-pressure center, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Dep ...

or even the rainfall associated with a particularly intense

thunderstorm A thunderstorm, also known as an electrical storm or a lightning storm, is a storm characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere, known as thunder. Relatively weak thunderstorms are somet ...

or of a long duration rainfall event with lower intensity, such as the cumulative effect of

monsoon A monsoon () is traditionally a seasonal reversing wind accompanied by corresponding changes in precipitation but is now used to describe seasonal changes in atmospheric circulation and precipitation associated with annual latitudinal osci ...

rainfall in

South Asia South Asia is the southern subregion of Asia, which is defined in both geographical and ethno-cultural terms. The region consists of the countries of Afghanistan, Bangladesh, Bhutan, India, Maldives, Nepal, Pakistan, and Sri Lanka.;;;;; ...

. In the former case it is usually necessary to have very high rainfall intensities, whereas in the latter the intensity of rainfall may be only moderate - it is the duration and existing pore water pressure conditions that are important. The importance of rainfall as a trigger for landslides cannot be overestimated. A global survey of landslide occurrence in the 12 months to the end of September 2003 revealed that there were 210 damaging landslide events worldwide. Of these, over 90% were triggered by heavy rainfall. One rainfall event for example in

Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...

in May 2003 triggered hundreds of landslides, killing 266 people and rendering over 300,000 people temporarily homeless. In July 2003 an intense rain band associated with the annual Asian

tracked across central

Nepal Nepal (; ne, नेपाल ), formerly the Federal Democratic Republic of Nepal ( ne, सङ्घीय लोकतान्त्रिक गणतन्त्र नेपाल ), is a landlocked country in South Asia. It is ma ...

, triggering 14 fatal landslides that killed 85 people. The reinsurance company Swiss Re estimated that rainfall induced landslides associated with the 1997-1998 El Nino event triggered landslides along the west coast of North, Central and South America that resulted in over $5 billion in losses. Finally, landslides triggered by

Hurricane Mitch Hurricane Mitch is the second-deadliest Atlantic hurricane on record, causing over 11,000 fatalities in Central America in 1998, including approximately 7,000 in Honduras and 3,800 in Nicaragua due to cataclysmic flooding from the slow motion ...

in 1998 killed an estimated 18,000 people in Honduras,

Nicaragua Nicaragua (; ), officially the Republic of Nicaragua (), is the largest Sovereign state, country in Central America, bordered by Honduras to the north, the Caribbean Sea, Caribbean to the east, Costa Rica to the south, and the Pacific Ocean to ...

, Guatemala and

El Salvador El Salvador (; , meaning " The Saviour"), officially the Republic of El Salvador ( es, República de El Salvador), is a country in Central America. It is bordered on the northeast by Honduras, on the northwest by Guatemala, and on the south by ...

. Rainfall triggers a large amount of landslides principally because the rainfall drives an increase in pore water pressure within the

. Figure A illustrates the forces acting on an unstable block on a slope. Movement is driven by shear stress, which is generated by the mass of the block acting under gravity down the slope. Resistance to movement is the result of the normal load. When the slope fills with water, the fluid pressure provides the block with buoyancy, reducing the resistance to movement. In addition, in some cases fluid pressures can act down the slope as a result of

groundwater Groundwater is the water present beneath Earth's surface in rock and soil pore spaces and in the fractures of rock formations. About 30 percent of all readily available freshwater in the world is groundwater. A unit of rock or an unconsolidat ...

flow to provide a

hydraulic Hydraulics (from Greek: Υδραυλική) is a technology and applied science using engineering, chemistry, and other sciences involving the mechanical properties and use of liquids. At a very basic level, hydraulics is the liquid counter ...

push to the landslide that further decreases the

stability Stability may refer to: Mathematics * Stability theory, the study of the stability of solutions to differential equations and dynamical systems ** Asymptotic stability ** Linear stability ** Lyapunov stability ** Orbital stability ** Structural st ...

. Whilst the example given in Figures A and B is clearly an artificial situation, the mechanics are essentially as per a real landslide. Slopesyst

In some situations, the presence of high levels of

fluid In physics, a fluid is a liquid, gas, or other material that continuously deforms (''flows'') under an applied shear stress, or external force. They have zero shear modulus, or, in simpler terms, are substances which cannot resist any shea ...

may destabilise the slope through other mechanisms, such as: *Fluidization of debris from earlier events to form debris flows; *Loss of

suction Suction is the colloquial term to describe the air pressure differential between areas. Removing air from a space results in a pressure differential. Suction pressure is therefore limited by external air pressure. Even a perfect vacuum cannot ...

forces in silty materials, leading to generally shallow failures (this may be an important mechanism in residual soils in tropical areas following

deforestation Deforestation or forest clearance is the removal of a forest or stand of trees from land that is then land conversion, converted to non-forest use. Deforestation can involve conversion of forest land to farms, ranches, or urban area, urban ...

); *Undercutting of the toe of the slope through river erosion. *Destabilizing of non-lithified earth materials through soil-piping. Considerable efforts have been made to understand the triggers for landsliding in natural systems, with quite variable results. For example, working in

Puerto Rico Puerto Rico (; abbreviated PR; tnq, Boriken, ''Borinquen''), officially the Commonwealth of Puerto Rico ( es, link=yes, Estado Libre Asociado de Puerto Rico, lit=Free Associated State of Puerto Rico), is a Caribbean island and Unincorporated ...

, Larsen and Simon found that storms with a total precipitation of 100–200 mm, about 14 mm of rain per hour for several hours, or 2–3 mm of rain per hour for about 100 hours can trigger landslides in that environment. Rafi Ahmad, working in

Jamaica Jamaica (; ) is an island country situated in the Caribbean Sea. Spanning in area, it is the third-largest island of the Greater Antilles and the Caribbean (after Cuba and Hispaniola). Jamaica lies about south of Cuba, and west of Hispan ...

, found that for rainfall of short duration (about 1 hour) intensities of greater than 36 mm/h were required to trigger landslides. On the other hand, for long rainfall durations, low average intensities of about 3 mm/h appeared to be sufficient to cause landsliding as the storm duration approached approximately 100 hours. Corominas and Moya (1999) found that the following thresholds exist for the upper basin of the Llobregat River, Eastern

Pyrenees The Pyrenees (; es, Pirineos ; french: Pyrénées ; ca, Pirineu ; eu, Pirinioak ; oc, Pirenèus ; an, Pirineus) is a mountain range straddling the border of France and Spain. It extends nearly from its union with the Cantabrian Mountains to ...

area. Without antecedent rainfall, high intensity and short duration rains triggered

debris flow Debris flows are geological phenomena in which water-laden masses of soil and fragmented Rock (geology), rock rush down mountainsides, funnel into stream channels, entrain objects in their paths, and form thick, muddy deposits on valley floors. ...

s and shallow slides developed in colluvium and weathered rocks. A rainfall threshold of around 190 mm in 24 h initiated failures whereas more than 300 mm in 24-48 h were needed to cause widespread shallow landsliding. With antecedent rain, moderate intensity precipitation of at least 40 mm in 24 h reactivated mudslides and both rotational and translational slides affecting clayey and silty-clayey formations. In this case, several weeks and 200 mm of precipitation were needed to cause landslide reactivation. A similar approach is reported by Brand et al. (1988) for Hong Kong, who found that if the 24-hour antecedent rainfall exceeded 200 mm then the rainfall threshold for a large landslide event was 70 mm·h⁻¹. Finally, Caine (1980) established a worldwide threshold: I = 14.82 D - 0.39 ''where: I is the rainfall

intensity Intensity may refer to: In colloquial use * Strength (disambiguation) *Amplitude *Level (disambiguation) *Magnitude (disambiguation) In physical sciences Physics *Intensity (physics), power per unit area (W/m2) * Field strength of electric, ma ...

(mm·h⁻¹), D is duration of rainfall (h)'' This threshold applies over time periods of 10 minutes to 10 days. It is possible to modify the formula to take into consideration areas with high mean annual precipitations by considering the proportion of mean annual precipitation represented by any individual event. Other techniques can be used to try to understand rainfall triggers, including: • Actual rainfall techniques, in which measurements of rainfall are adjusted for potential

evapotranspiration Evapotranspiration (ET) is the combined processes by which water moves from the earth’s surface into the atmosphere. It covers both water evaporation (movement of water to the air directly from soil, canopies, and water bodies) and transpir ...

and then correlated with landslide movement events • Hydrogeological balance approaches, in which pore water pressure response to rainfall is used to understand the conditions under which failures are initiated • Coupled rainfall - stability analysis methods, in which pore water pressure response models are coupled to slope stability models to try to understand the complexity of the system • Numerical slope modelling, in which

finite element The finite element method (FEM) is a popular method for numerically solving differential equations arising in engineering and mathematical models, mathematical modeling. Typical problem areas of interest include the traditional fields of struct ...

(or similar) models are used to try to understand the interactions of all relevant processes

Seismicity

The second major factor in the triggering of landslides is

seismicity Seismicity is a measure encompassing earthquake occurrences, mechanisms, and magnitude at a given geographical location. As such, it summarizes a region's seismic activity. The term was coined by Beno Gutenberg and Charles Francis Richter in 1 ...

. Landslides occur during earthquakes as a result of two separate but interconnected processes: seismic shaking and pore water pressure generation.

Seismic shaking

The passage of the

earthquake An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

wave In physics, mathematics, and related fields, a wave is a propagating dynamic disturbance (change from equilibrium) of one or more quantities. Waves can be periodic, in which case those quantities oscillate repeatedly about an equilibrium (r ...

s through the rock and soil produces a complex set of accelerations that effectively act to change the gravitational load on the slope. So, for example, vertical accelerations successively increase and decrease the normal load acting on the slope. Similarly, horizontal accelerations induce a shearing force due to the

inertia Inertia is the idea that an object will continue its current motion until some force causes its speed or direction to change. The term is properly understood as shorthand for "the principle of inertia" as described by Newton in his first law o ...

of the landslide mass during the accelerations. These processes are complex, but can be sufficient to induce failure of the slope. These processes can be much more serious in mountainous areas in which the seismic waves interact with the terrain to produce increases in the magnitude of the ground accelerations. This process is termed '

topographic Topography is the study of the forms and features of land surfaces. The topography of an area may refer to the land forms and features themselves, or a description or depiction in maps. Topography is a field of geoscience and planetary sci ...

amplification'. The maximum acceleration is usually seen at the crest of the slope or along the ridge line, meaning that it is a characteristic of seismically triggered landslides that they extend to the top of the slope.

Liquefaction

The passage of the earthquake waves through a granular material such as a soil can induce a process termed

liquefaction In materials science, liquefaction is a process that generates a liquid from a solid or a gas or that generates a non-liquid phase which behaves in accordance with fluid dynamics. It occurs both naturally and artificially. As an example of t ...

, in which the shaking causes a reduction in the pore space of the material. This densification drives up the pore pressure in the material. In some cases this can change a granular material into what is effectively a liquid, generating 'flow slides' that can be rapid and thus very damaging. Alternatively, the increase in pore pressure can reduce the normal stress in the slope, allowing the activation of translational and rotational failures.

The nature of seismically-triggered landslides

For the main part seismically generated landslides usually do not differ in their morphology and internal processes from those generated under non-seismic conditions. However, they tend to be more widespread and sudden. The most abundant types of earthquake-induced landslides are rock falls and slides of rock fragments that form on steep slopes. However, almost every other type of landslide is possible, including highly disaggregated and fast-moving falls; more coherent and slower-moving slumps, block slides, and earth slides; and lateral spreads and flows that involve partly to completely liquefied material (Keefer, 1999). Rock falls, disrupted rock slides, and disrupted slides of earth and debris are the most abundant types of earthquake-induced landslides, whereas

earth flow An earthflow (earth flow) is a downslope viscous flow of fine-grained materials that have been saturated with water and moves under the pull of gravity. It is an intermediate type of mass wasting that is between downhill creep and mudflow. The ty ...

s, and

avalanche An avalanche is a rapid flow of snow down a slope, such as a hill or mountain. Avalanches can be set off spontaneously, by such factors as increased precipitation or snowpack weakening, or by external means such as humans, animals, and ear ...

s of rock, earth, or debris typically transport material the farthest. There is one type of landslide that is essential uniquely limited to earthquakes -

failure, which can cause fissuring or subsidence of the ground. Liquefaction involves the temporary loss of strength of sands and silts which behave as viscous fluids rather than as soils. This can have devastating effects during large earthquakes.

Volcanic activity

Some of the largest and most destructive landslides known have been associated with volcanoes. These can occur either in association with the eruption of the volcano itself, or as a result of mobilisation of the very weak deposits that are formed as a consequence of volcanic activity. Essentially, there are two main types of volcanic landslide: lahars and debris avalanches, the largest of which are sometimes termed flank collapses. An example of a lahar was seen at Mount St Helens during its catastrophic eruption on May 18, 1980. Failures on volcanic flanks themselves are also common. For example, a part of the side of Casita Volcano in Nicaragua collapsed on October 30, 1998, during the heavy precipitation associated with the passage of Hurricane Mitch. Debris from the initial small failure eroded older deposits from the volcano and incorporated additional water and wet sediment from along its path, increasing in volume about ninefold. The lahar killed more than 2,000 people as it swept over the towns of El Porvenir and Rolando Rodriguez at the base of the mountain. Debris avalanches commonly occur at the same time as an eruption, but occasionally they may be triggered by other factors such as a seismic shock or heavy rainfall. They are particularly common on strato volcanoes, which can be massively destructive due to their large size. The most famous debris avalanche occurred at Mount St Helens during the massive eruption in 1980. On May 18, 1980, at 8:32 a.m. local time, a magnitude 5.1 earthquake shook Mount St. Helens. The bulge and surrounding area slid away in a gigantic rockslide and debris avalanche, releasing pressure, and triggering a major pumice and ash eruption of the volcano. The debris avalanche had a volume of about , traveled at , and covered an area of , killing 57 people.

Snowmelt

In many cold mountain areas,

snowmelt In hydrology, snowmelt is surface runoff produced from melting snow. It can also be used to describe the period or season during which such runoff is produced. Water produced by snowmelt is an important part of the annual water cycle in many part ...

can be a key mechanism by which landslide initiation can occur. This can be especially significant when sudden increases in temperature lead to rapid melting of the snow pack. This water can then infiltrate into the ground, which may have impermeable layers below the surface due to still-frozen soil or rock, leading to rapid increases in pore water pressure, and resultant landslide activity. This effect can be especially serious when the warmer

weather Weather is the state of the atmosphere, describing for example the degree to which it is hot or cold, wet or dry, calm or stormy, clear or cloudy. On Earth, most weather phenomena occur in the lowest layer of the planet's atmosphere, the ...

is accompanied by precipitation, which both adds to the groundwater and accelerates the rate of thawing.

Water-level change

Rapid changes in the groundwater level along a slope can also trigger landslides. This is often the case where a slope is adjacent to a water body or a river. When the water level adjacent to the slope falls rapidly the groundwater level frequently cannot dissipate quickly enough, leaving an artificially high water table. This subjects the slope to higher than normal shear stresses, leading to potential instability. This is probably the most important mechanism by which river bank materials fail, being significant after a

flood A flood is an overflow of water ( or rarely other fluids) that submerges land that is usually dry. In the sense of "flowing water", the word may also be applied to the inflow of the tide. Floods are an area of study of the discipline hydrol ...

as the river level is declining (i.e. on the falling limb of the hydrograph) as shown in the following figures. Falling river level causing landslide 1

It can also be significant in

coastal The coast, also known as the coastline or seashore, is defined as the area where land meets the ocean, or as a line that forms the boundary between the land and the coastline. The Earth has around of coastline. Coasts are important zones in n ...

areas when sea level falls after a storm tide, or when the water level of a reservoir or even a natural lake rapidly falls. The most famous example of this is the Vajont failure, when a rapid decline in lake level contributed to the occurrence of a landslide that killed over 2000 people. Numerous huge landslides also occurred in the Three Gorges (TG) after the construction of the TG dam.

Rivers

In some cases, failures are triggered as a result of undercutting of the slope by a river, especially during a flood. This undercutting serves both to increase the gradient of the slope, reducing stability, and to remove toe weighting, which also decreases stability. For example, in Nepal this process is often seen after a glacial lake outburst flood, when toe erosion occurs along the channel. Immediately after the passage of flood waves extensive landsliding often occurs. This instability can continue to occur for a long time afterwards, especially during subsequent periods of heavy rain and flood events.

Colluvium-filled bedrock hollows

Colluvium-filled bedrock hollows are the cause of many shallow earth

in steep mountainous terrain. They can form as a U or a V shaped trough as local

bedrock In geology, bedrock is solid rock that lies under loose material ( regolith) within the crust of Earth or another terrestrial planet. Definition Bedrock is the solid rock that underlies looser surface material. An exposed portion of be ...

variations reveal areas in the bedrock which are more prone to

than other locations on the slope. As the weathered bedrock turns to

, there is a greater elevation difference between the soil level and the hard bedrock. With the introduction of water and the thick soil, there is less cohesion and the soil flows out in a landslide. With every landslide more bedrock is scoured out and the hollow becomes deeper. After time, colluvium fills the hollow, and the sequence starts again.

Classification factors

A1) Type of movement

A2) Involved material

A3) Activity

A4) Movement velocity

B1) The age of the movement

B2) Geological conditions

B3) Morphological characteristics

B4) Geographical location

B5) Topographical criteria

B6) Type of climate

B7) Causes of the movements

Types and classification

Falls

Topples

Slides

Translational slide

Rotational slides

Spreads

Flows

Flows in rock

=Rock Flow

=Rock avalanche (Sturzstrom)

Flows in soil

=Debris flow

=Debris avalanche

=Earth flow

=Mudflow

Complex movement

Causes

Rainfall

Seismicity

Seismic shaking

Liquefaction

The nature of seismically-triggered landslides

Volcanic activity

Snowmelt

Water-level change

Rivers

Colluvium-filled bedrock hollows

See also

References

Further reading