Geologists, engineers, and other professionals often rely on unique and slightly differing definitions of landslides. This diversity in definitions reflects the complex nature of the many disciplines associated with studying landslide phenomena.
For our purposes, landslide is a general term used to describe the downslope movement of soil, rock, and organic materials under the effects of gravity and also the landform that results from such movement (please see figure 1 for an example of one type of landslide).
Figure 1. This landslide occurred at La Conchita, California, USA, in 2005. Ten people were killed. (Photograph by Mark Reid, U.S. Geological Survey.)
Varying classifications of landslides are associated with specific mechanics of slope failure and the properties and characteristics of failure types; these will be discussed briefly herein.
Figure 2. A simple illustration of a rotational landslide that has evolved into an earthflow. Image illustrates commonly used labels for the parts of a landslide (from Varnes, 1978, Reference 43).
There are a number of other phrases/terms that are used interchangeably with the term “landslide” including mass movement, slope failure, and so on. One commonly hears such terms applied to all types and sizes of landslides.
Regardless of the exact definition used or the type of landslide under discussion, understanding the basic parts of a typical landslide is helpful.
Figure 2 shows the position and the most common terms used to describe the unique parts of a landslide.
What are the Effects and Consequences of Landslides?
Landslide effects occur in two basic environments: the built environment and the natural environment. Sometimes there is intersection between the two; for example agricultural lands and forest lands that are logged.
Effects of Landslides on the Built Environment
Landslides affect manmade structures whether they are directly on or near a landslide. Residential dwellings built on unstable slopes may experience partial damage to complete destruction as landslides destabilize or destroy foundations, walls, surrounding property, and above-ground and underground utilities. Landslides can affect residential areas either on a large regional basis (in which many dwellings are affected) or on an individual site basis (where only one structure or part of a structure is affected). Also, landslide damage to one individual property’s lifelines (such as trunk sewer, water, or electrical lines and common-use roads) can affect the lifelines and access routes of other surrounding properties. Commercial structures are affected by landslides in much the same way residential structures are affected. In such a case, consequences may be great if the commercial structure is a common-use structure, such as a food market, which may experience an interruption in business due to landslide damage to the actual structure and (or) damage to its access roadways.
Fast-moving landslides such as debris flows are the most destructive type of landslide to structures, as they often occur without precursors or warnings, move too quickly for any mitigation measures to be enacted, and due to velocity and material are often very powerful and destructive. Fast-moving landslides can completely destroy a structure, whereas a slower moving landslide may only slightly damage it, and its slow pace may allow mitigation measures to be enacted. However, left unchecked, even slow landslides can completely destroy structures over time. Debris avalanches and lahars in steep areas can quickly destroy or damage the structures and lifelines of cities, towns, and (or) neighborhoods due to the fact that they are an extremely fast-moving, powerful force.
The nature of landslide movement and the fact that they may continue moving after days, weeks, or months preclude rebuilding on the affected area, unless mitigative measures are taken; even then, such efforts are not always a guarantee of stability.
One of the greatest potential consequences from landslides is to the transportation industry, and this commonly affects large numbers of people around the world. Cut and fill failures along roadways and railways, as well as collapse of roads from underlying weak and slide-prone soils and fill, are common problems. Rockfalls may injure or kill motorists and pedestrians and damage structures. All types of landslides can lead to temporary or long-term closing of crucial routes for commerce, tourism, and emergency activities due to road or rail blockage by dirt, debris, and (or) rocks . Even slow creep can affect linear infrastructure, creating maintenance problems.
Figure 1. A landslide on the Pan American Highway in El Salvador, Central America, near the town of San Vicente, in 2001. (Photograph by Ed Harp, U.S. Geological Survey.)
Figure 1 shows a landslide blocking a major highway. Blockages of highways by landslides occur very commonly around the world, and many can simply be bulldozed or shoveled away. Others, such as the one shown in figure 1, will require major excavation and at least temporary diversion of traffic or even closure of the road.
As world populations continue to expand, they are increasingly vulnerable to landslide hazards. People tend to move on to new lands that might have been deemed too hazardous in the past but are now the only areas that remain for a growing population. Poor or nonexistent land-use policies allow building and other construction to take place on land that might better be left to agriculture, open-space parks, or uses other than for dwellings or other buildings and structures. Communities often are not prepared to regulate unsafe building practices and may not have the legitimate political means or the expertise to do so.
Effects of Landslides on the Natural Environment
Landslides have effects on the natural environment:
The morphology of the Earth’s surface—mountain and valley systems, both • on the continents and beneath the oceans; mountain and valley morphologies are most significantly affected by downslope movement of large landslide masses;
The forests and grasslands that cover much of the continents; and
The native wildlife that exists on the Earth’s surface and in its rivers, lakes, • and seas.
Figures 2, 3, and 4 show the very large areal extent of some landslides and how they may change the face of the terrain, affecting rivers, farmland, and forests.
Figure 2. The active volcano, Mount Shasta in California, USA. Note the landforms in the foreground, caused by a debris avalanche that occurred about 300,000 years ago. The debris avalanche traveled great distances from the volcano and produced lasting landform effects that can still be seen today. (Photograph by R. Crandall, U.S. Geological Survey.)
Figure 3. View looking downstream at the confluence of the Río Malo (flowing from lower left) and the Río Coca, northeastern Ecuador, in South America. Both river channels have been filled with sediment left behind by debris flows triggered by the 1987 Reventador earthquakes. Slopes in the area had been saturated by heavy rains in recent days before the earthquake. Debris/earth slides, debris avalanches, debris/mudflows, and resulting floods destroyed about 40 kilometers of the Trans-Ecuadorian oil pipeline and the only highway from Quito. (Photograph by R.L. Schuster, U.S. Geological Survey; information from Reference 32.)
Figure 4. The Slumgullion landslide, Colorado, USA. This landslide (formally referred to also as an earthflow) dammed the Lake Fork of the Gunnison River, which flooded the valley and formed Lake Cristobal. (Photograph by Jeff Coe, U.S. Geological Survey.)
Forest, grasslands, and wildlife often are negatively affected by landslides, with forest and fish habitats being most easily damaged, temporarily or even rarely, destroyed. However, because landslides are relatively local events, flora and fauna can recover with time. In addition, recent ecological studies have shown that, under certain conditions, in the medium-to-long term, landslides can actually benefit fish and wildlife habitats, either directly or by improving the habitat for organisms that the fish and wildlife rely on for food.
The following list identifies some examples of landslides that commonly occur in the natural environment:
Submarine landslide is a general term used to describe the downslope mass movement of geologic materials from shallower to deeper regions of the ocean. Such events may produce major effects to the depth of shorelines, ultimately affecting boat dockings and navigation. These types of landslides can occur in rivers, lakes, and oceans. Large submarine landslides triggered by earthquakes have caused deadly tsunamis, such as the 1929 Grand Banks (off the coast of Newfoundland, Canada) tsunamis.
Coastal cliff retreat , or cliff erosion, is another common effect of landslides on the natural environment. Rock-and-soil falls, slides, and avalanches are the common types of landslides affecting coastal areas; however, topples and flows also are known to occur. Falling rocks from eroding cliffs can be especially dangerous to anyone occupying areas at the base of cliffs, or on the beaches near the cliffs. Large amounts of landslide material can also be destructive to aquatic life, such as fish and kelp, and the rapid deposition of sediments in water bodies often changes the water quality around vulnerable shorelines.
Landslide dams can naturally occur when a large landslide blocks the flow of a river, causing a lake to form behind the blockage. Most of these dams are short-lived as the water will eventually erode the dam. If the landslide dam is not destroyed by natural erosional processes or modified by humans, it creates a new landform—a lake. Lakes created by landslide dams can last a long time, or they may suddenly be released and cause massive flooding downstream.
There are many ways that people can lessen the potential dangers of landslide dams, and some of these methods are discussed in the safety and mitigation sections of this volume. Figure 32 shows the Slumgullion landslide one of the largest landslides in the world—the landslide dam it has formed is so large and wide, that it has lasted 700 years.
Source : The Landslide Handbook—A Guide to Understanding Landslides
A landslide is defined as the movement of a mass of rock, debris, or earth down a slope due to gravity. The materials may move by falling, toppling, sliding, spreading, or flowing.
Landslide Animation:
What causes a landslide?
Almost every landslide has multiple causes. Slope movement occurs when forces acting down-slope (mainly due to gravity) exceed the strength of the earth materials that compose the slope. Landslides can be triggered by rainfall, snowmelt, changes in water level, stream erosion, changes in ground water, earthquakes, volcanic activity, disturbance by human activities, or any combination of these factors.
What are submarine landslides?
Earthquake shaking and other factors can also induce landslides underwater. These landslides are called submarine landslides. Submarine landslides sometimes cause tsunamis that damage coastal areas.
Where do landslides occur?
Landslides in the United States occur in all 50 States. The primary regions of landslide occurrence and potential are the coastal and mountainous areas of California, Oregon, and Washington, the States comprising the intermountain west, and the mountainous and hilly regions of the Eastern United States. Alaska and Hawaii also experience all types of landslides.
How fast do landslide travel?
Landslides can move slowly, (millimeters per year) or can move quickly and disastrously, as is the case with debris flows. Debris flows can travel down a hillside at speeds up to 200 miles per hour (more commonly, 30 – 50 miles per hour), depending on the slope angle, water content, volume of debris, and type of earth and debris in the flow. These flows are initiated by heavy periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall or other factors in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions.
Why study landslides?
Landslides are a serious geologic hazard. It is estimated that in the United States they cause in excess of $1 billion in damages and from about 25 to 50 deaths each year. Globally, landslides cause billions of dollars in damages and thousands of deaths and injuries each year.
Who is most at risk for landslides?
As people move into new areas of hilly or mountainous terrain, it is important to understand the nature of their potential exposure to landslide hazards, and how cities, towns, and counties can plan for land-use, engineering of new construction and infrastructure, and other measures which will reduce the costs of living with landslides. Although the physical causes of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.
Do human activities cause landslides?
Yes, in some cases human activities can be a contributing factor in causing landslides. Many human-caused landslides can be avoided or mitigated. They are commonly a result of building roads and structures without adequate grading of slopes, of poorly planned alteration of drainage patterns, and of disturbing old landslides.
Where can I find landslide information for my area?
The USGS National Landslide Information Center (NLIC) is a part of the U.S. Geological Survey Landslide Hazards Program that collects and distributes all forms of information related to landslides. The NLIC is designed to serve landslide researchers, geotechnical practitioners engaged in landslide stabilization, and anyone else concerned in any way with landslide education, hazard, safety, and mitigation. Every state in the US has a geoscience agency and most have some landslide information. The Association of American State Geologists provides links to the State Geologist for every state.
What was the most expensive landslide to fix in the United States?
The Thistle, Utah, landslide cost in excess of $200 million dollars to fix. The landslide occurred during the spring of 1983, when unseasonably warm weather caused rapid snowmelt to saturate the slope. The landslide destroyed the railroad tracks of the Denver and Rio Grande Western Railway Company, and the adjacent Highway 89. It also flowed across the Spanish Fork River, forming a dam. The impounded river water inundated the small town of Thistle. The inhabitants of the town of Thistle, directly upstream from the landslide, were evacuated as the lake began to flood the town, and within a day the town was completely covered with water. Populations downstream from the dam were at risk because of the possible overtopping of the landslide by the lake. This could cause a catastrophic outburst of the dam with a massive flood downstream. Eventually, a drain system was engineered to drain the lake and avert the potential disaster.
How many deaths result from landslides?
An average of between 25 and 50 people are killed by landslides each year in the United States. The worldwide death toll per year due to landslides is in the thousands. Most landslide fatalities are from rock fall, debris-flows, or volcanic debris flows.
What should I know about wildfires and debris flows?
Wild land fires are inevitable in the western United States. Expansion of human development into forested areas has created a situation where wildfires can adversely affect lives and property, as can the flooding and landslides that occur in the aftermath of the fires. There is a need to develop tools and methods to identify and quantify the potential hazards posed by landslides produced from burned watersheds. Post-fire landslide hazards include fast-moving, highly destructive debris flows that can occur in the years immediately after wildfires in response to high intensity rainfall events, and those flows that are generated over longer time periods accompanied by root decay and loss of soil strength. Post-fire debris flows are particularly hazardous because they can occur with little warning, can exert great impulsive loads on objects in their paths, and can strip vegetation, block drainage ways, damage structures, and endanger human life. Wildfires could potentially result in the destabilization of pre-existing deep-seated landslides over long time periods.
How do landslides cause tsunamis?
Tsunamis are large, potentially deadly and destructive sea waves, most of which are formed as a result of submarine earthquakes. They may also result from the eruption or collapse of island or coastal volcanoes and the formation of giant landslides on marine margins. These landslides, in turn, are often triggered by earthquakes. Tsunamis can be generated on impact as a rapidly moving landslide mass enters the water or as water displaces behind and ahead of a rapidly moving underwater landslide.
What are some examples of landslides that have caused tsunamis?
The 1964 Alaska earthquake caused 115 deaths in Alaska alone, with 106 of those due to tsunamis generated by tectonic uplift of the sea floor, and by localized subareal and submarine landslides. The earthquake shaking caused at least 5 local slide-generated tsunamis within minutes after the shaking began. An eyewitness account of the tsunami caused by the movement and landslides of the 1964 Alaska earthquake.
Research in the Canary Islands concludes that there have been at least five massive volcano landslides that occurred in the past, and that similar large events may occur in the future. Giant landslides have the potential of generating large tsunami waves at close and also very great distances and would have the potential to devastate large areas of coastal land as far away as the eastern seaboard of North America.
Rock falls and rock avalanches in coastal inlets, such as those that have occurred in the past at Tidal Inlet, Glacier Bay National Park, Alaska, have the potential to cause regional tsunamis that pose a hazard to coastal ecosystems and human settlements. On July 9, 1958, a magnitude M 7.9 earthquake on the Fairweather Fault triggered a rock avalanche at the head of Lituya Bay, Alaska. The landslide generated a wave that ran up 524 m on the opposite shore and sent a 30-m high wave through Lituya Bay, sinking two of three fishing boats and killing two persons.
How soon does the danger of landslides end after the rain stops?
It’s not possible to exactly predict the number of days or weeks that landslides remain a danger after heavy rain. Residents near mountain slopes, canyons, and landslide prone areas should stay alert even after heavy rain subsides.
Why is southern California vulnerable to landslides?
Areas that have been burned by recent wildfires are highly susceptible to debris-flow activity that can be triggered by significantly less rainfall than that which triggers debris flows from unburned hill slopes.
What was the biggest landslide in the world?
The world’s biggest historic landslide occurred during the 1980 eruption of Mount St. Helens, a volcano in the Cascade Mountain Range in the State of Washington, USA. The volume of material was 2.8 cubic kilometers (km).
What was the biggest prehistoric landslide?
The world’s biggest prehistoric landslide, discovered so far on land, is in southwestern Iran, and is named the Saidmarreh landslide. The landslide is located on the Kabir Kuh anticline in Southwest Iran at 33 degrees north latitude, 47.65 degrees east longitude. The landslide has a volume of about 20 cubic kilometers, a depth of 300 m, a travel distance of 14 km and a width of 5 km. This means that about 50 billion tons of rock moved in this single event!
Some single landslide events have killed numbers in excess of the populations of small countries.
Landslides are life-threatening events that can make it seem as though the world we live upon is crumbling around us. Those landslides listed below are some of the deadliest in recorded human history, each taking away human life by the thousands.
10. Diexi Slides, Sichuan, China, August 1933 (3,000+ deaths)
On August 5, 1933, a strong earthquake triggered a massive landslide in Diexi, Mao County, Szechwan, China. The event, known as the Diexi Slides, claimed more than 3,000 lives, and destroyed many villages within the affected region. The old town of Diexi suffered the worst fate of all as it sank into the landslide-created dam below.
9. Khait Landslide, Tajikstan, July 1949 (4,000 deaths))
For centuries, the mountainous belt running through Central Asia has witnessed a large number of disasters involving earthquake-triggered landslides. One such natural catastrophe occurred in July of 1949, when the 7.4 magnitude Khait Earthquake triggered hundreds of landslides near the southern limits of the Tien Shan ranges in central Tajikistan. The adjacent valleys of Yasman and Khait were the most affected by these earthquake-induced landslides. The Khait Landslide involved rockslides with saturated loess travelling at an estimated average velocity of around 30 meters per second. Approximately 4,000 people were killed in this tragic natural disaster.
8. 62 Nevado Huascaran Debris Fall, Ranrahirca, Peru, January 1962 (4,500 deaths)
Mount Huascarán is a famous Peruvian mountain with a snowcapped peak that rises to a height of 22,205 feet. In January of 1962, a thaw triggered the breaking off of a portion of the north summit of the mountain, leading to a landslide/avalanche that led to the tragic death of nearly 4,500 people. The avalanche, locally referred to as ‘Huayco’, involved a massive ice sheet that was estimated to be about 1 kilometer wide and 40 feet high. As the ice sheet moved rapidly down the slopes, it gathered rock and debris from the mountain and strengthened in force, completely burying several villages in Ranrahica underneath it.
7. Huaraz Debris Flows, Ancash, Peru, December 1941 (5,000 deaths)
In December of 1941, the residents of Huaraz, a Peruvian city in the Ancash region, were completely unaware that a retreating glacier tongue above their city would soon be responsible for wreaking havoc its people and claim thousands of the lives living within. Just before dawn on December 13, 1941, disaster struck the Peruvian city when a landslide resulted in glacial ice crashing down into Lake Palcacocha, generating huge waves that completely destroyed the dam on the lake. This released large volumes of water, itself laden with mud, rock, and ice, into the valley below with an unimaginably high force. Another dam in the nearby Lake Jircacocha was also broken by the flowing glacial water, resulting in the furious waters of both of the two lakes emptying themselves onto the city of Huaraz, claiming more than 5,000 lives in the process.
6. Kelud Lahars, East Java, Indonesia, May 1919 (5,000+ deaths)
Mount Kelud, in Eastern Java, Indonesia, is quite infamous as an extremely active, hazardous volcano, and one which has erupted about 30 times in the past killing thousands of people in its volcanic disasters. One of the deadliest eruptions of this volcano occurred on May 19, 1919, when over 38 million cubic meters of water were expelled from the crater lake of the volcano, which had accumulated large amounts of sediment and volcanic material to form lethal lahars. The lahars moved down the mountains with high velocity and swept away and drowned all that were unfortunate enough to be in its path.
5. North India Flood mudslides, Kedarnath, India, June 2013 (5,700 deaths)
One of the worst natural disasters in the history of India occurred in June of 2013, when powerful flash floods killed around 5,700 people in the Himalayan state of Uttarakhand. Consistent cloudbursts and incessant monsoon rainfall were primarily held responsible for the disaster, which has been officially termed as a natural calamity. However, a section of environmentalists, scientists, and the educated public think otherwise. According to them, thoughtless human intervention in the Himalayan mountain ecosystem had rendered the ecosystem extremely fragile and prone to disaster. The unchecked tourism in the region had promoted the rapid growth of hotels, roads, and shops throughout the region without paying heed to the environmental laws and demands of the ecosystem. The mushrooming of hydroelectric dams in Uttarakhand was also another important factor held responsible for the environmental damage. Heavy rainfall had been previously recorded in the region which had also led to flash floods, but the devastation produced in 2013 was comparable to no earlier data. It is believed that floodwaters had no outlets this time, as most of the routes taken by the water previously were now blocked by sand and rocks. Hence, the lethal waters, laden with debris from dam construction and large volumes of mud and rocks, inundated towns and villages and buried all forms of life that came in its way.
4. 70 Nevado Huascaran Debris Fall, Yungay, Peru, May 1970 (22,000 deaths)
In May of 1970, an earthquake triggered a massive series of landslides and avalanches of rock and snow that buried the towns of Yungay and Ranrahirca. Nearly 22,000 people perished in this natural disaster. The avalanche travelled a distance of 16.5 kilometers. It ended up carrying 50-100 million cubic meters of water, mud, and rocks, which reached the village of Yungay and smothered all life forms therein under its deadly cover.
3. Armero Tragedy, Tolima, Colombia, November 1985 (23,000 deaths)
A dormant volcano, the Nevado del Ruiz in Tolima, Colombia, suddenly came to life on November 13, 1985, wreaking havoc on the nearby villages and towns, and killing as many as 23,000 people. A pyroclastic flow from the crater of the volcano had melted the glaciers in the mountain and sent deadly lahars, saturated with mud, ice, snow, and volcanic debris, rushing down the mountain at killer speeds towards the residential areas directly below it. The lahars soon engulfed the town of Armero, killing thousands there, while casualties were also reported in such other towns as Chinchiná
2. Vargas Tragedy, Vargas, Venezuela, December 1999 (30,000 deaths)
The Winter of 1999 witnessed unusually heavy rainfall in the Vargas State of Venezuela. The rainfall triggered a series of large and small flash floods and debris flows that claimed around 30,000 lives in the region. As per estimates, approximately 10% of the population of Vargas perished in the disaster. The entire towns of Carmen de Uria and Cerro Grande completely vanished under the mud bed, and a large number of homes were simply swept away into the nearby ocean.
1. Haiyuan Flows, Ningxia, China, December 1920 (100,000+ deaths)
The 8.5-magnitude Haiyuan Earthquake was the world’s second deadliest earthquake of the 20th Century. It generated a series of 675 major loess landslides causing massive destruction to lives and property. The natural calamity which struck the rural district of Haiyuan on the evening of December 16, 1920 claimed over 100,000 lives, and severely damaged an area of approximately 20,000 square kilometers. The worst affected areas included the the epicenter of the earthquake in the Haiyuan County in what is now the Ningxia Hui Autonomous Region, as well as the neighboring provinces of Gansu and Shaanxi. Haiyuan County alone lost more than 50% of its population in the disaster. One of the landslides buried an entire village in Xiji County as well.
