Frequently Asked Questions

What is a landslide, and what causes them?
What was the biggest landslide in the world?
How much do landslides cost in terms of monetary losses?
How many deaths result from landslides?
What was the most expensive landslide to fix in the United States?
Is there a list of the largest landslides in the world?
Is there a list of the largest landslides in the United States?
Where can I go to find landslide information for my area?
Is there reference material available about landslides?
What is a landslide inventory map?
What is a landslide susceptibility map?
What is a landslide hazard map?
What is a landslide risk map?
How do landslides cause tsunamis?
What should I know about wildfires, debris flows, and other types of landslides?
Where can I find information about Southern California landslides?

1. Q: What is a landslide, and what causes them?

A: A landslide is defined as "the movement of a mass of rock, debris, or earth down a slope". (Cruden, 1991). Landslides are a type of "mass wasting" which denotes any down slope movement of soil and rock under the direct influence of gravity. The term "landslide" encompasses events such as rock falls, topples, slides, spreads, and flows, such as debris flows commonly referred to as mudflows or mudslides (Varnes, 1996). Landslides can be initiated by rainfall, earthquakes, volcanic activity, changes in groundwater, disturbance and change of a slope by man-made construction activities, or any combination of these factors. Landslides can also occur underwater, causing tsunami waves and damage to coastal areas. These landslides are called submarine landslides.

Failure of a slope occurs when the force that is pulling the slope downward (gravity) exceeds the strength of the earth materials that compose the slope. They 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 of speeds up to 200 miles per hour (more commonly, 30 - 50 miles per hour), depending on the slope angle, water content, and type of earth and debris in the flow. These flows are initiated by heavy, usually sustained, periods of rainfall, but sometimes can happen as a result of short bursts of concentrated rainfall in susceptible areas. Burned areas charred by wildfires are particularly susceptible to debris flows, given certain soil characteristics and slope conditions.

Information on debris flows can be found in our Publications section.

Sources of Information for FAQ #1:

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2. Q: What was the biggest landslide in the world?

A: 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 km³.

The world's biggest prehistoric landslide, discovered so far, 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 N, 47.65 degrees E. 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! For a current aerial view of the landslide, please see:

Sources of Information for FAQ #2:

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3. Q: How much do landslides cost in terms of monetary losses?

A: In the United States, it is estimated that the total dollar losses from landslides is between one and two billion dollars ($1.6 billion and $3.2 billion, year 2000 dollars). This figure is a conservative estimate, as there is no uniform method or overall agency that keeps track of or reports landslide losses. Landslides result in extremely high monetary losses in other countries, but there is no overall estimate as to the exact amount. The El Niño event in the San Francisco Bay Area caused landslide damage estimated to total approximately $140.9 million (1998 dollars; $154.4 million in 2003 dollars).

Sources of Information for FAQ #3:

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4. Q. How many deaths result from landslides?

A: It is estimated that 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 much higher. Most people are killed by rock fall, debris-flows, or volcanic landslides (called lahars). Debris flows occurring in December, 2003 killed 16 people in the San Bernardino, California area.

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5. Q. What was the most expensive landslide to fix in the United States?

A: The Thistle, Utah, landslide cost in excess of $200 million dollars (1984 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.

Eventually, the Thistle landslide reached a state of equilibrium across the valley, but fears of reactivation caused the railway to construct a tunnel through the bedrock around the slide zone at a cost of one million dollars. Also, the highway had to be realigned around the landslide. When the lake caused by the landslide was drained, the residual sediment partially buried the town and virtually no one returned to Thistle. This landslide is still moving at present, although at a fairly slow rate. State officials continue to monitor this landslide.

Source for FAQ #5:

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6. Q: Is there a list of the largest landslides in the World?

A: Yes - click here for a list - Worldwide Overview of Large Landslides of the 20th and 21st Centuries

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7. Q: Is there a list of the largest landslides in the United States?

A: The list of Significant Landslides in the United States is available at this link.

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8. Q: Where can I go to find landslide information for my area?

A: Each of our 50 states has a State Geological Survey. Most State Surveys have some landslide information. State Geologists Contact Information provides links to the State Geologist for every state.

Also, your City and/or County may have a public works engineer and/or a geologist on staff, who would be best able to answer questions about landslides in your local area.

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9. Q: Is there reference material available about landslides?

A: Yes - the USGS Hazards Team website has an interactive bibliographic database which can help you find landslide information.

For more information contact Lynn Highland at:

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10. Q: What is a landslide inventory map?

A: This type of map shows the locations and outlines of landslides. A landslide inventory is a data set that may present a single event, a regional event, or multiple events. Small-scale maps may show only landslide locations whereas large-scale maps may distinguish landslide sources from deposits and classify different kinds of landslides and show other pertinent data.

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11. Q: What is a landslide susceptibility map?

A: This type of map ranks slope stability of an area into categories that range from stable to unstable. Susceptibility maps show where landslides may form. Many susceptibility maps use a color scheme that relates warm colors (red, orange, and yellow) to unstable and marginally unstable areas, and cool colors (blue and green) to stable areas.

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12. Q: What is a landslide hazard map?

A: A landslide hazard map indicates the possibility of landslides occurring throughout a given area. A hazard map may be as simple as a map that uses the locations of old landslides to indicate potential instability, or as complex as a quantitative map incorporating probabilities based on variables such as rainfall thresholds, slope angle, soil type, and levels of earthquake shaking. An ideal landslide hazard map shows not only the chances that a landslide may form at a particular place, but also the chance that it may travel downslope a given distance.

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13. Q. What is a landslide risk map?

A: This type of map shows the expected annual cost of landslide damage throughout an area. Risk maps combine the probability information from a landslide hazard map with an analysis of all possible consequences (property damage, casualties, and loss of service).

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14. Q. How do landslides cause tsunamis?

A: Tsunamis are large, potentially destructive sea waves, most of which are formed as a result of submarine earthquakes, but which 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. Environmental damage by these tsunamis include coral reef destruction, contamination of wells and other sources of fresh water by salt water, denudation of trees and other types of dry-land vegetation, accelerated beach erosion, and fish and other marine life fatalities due to abnormal wave action. The flooding and powerful wave action of the tsunami may potentially cause damage to man-made containment vessels of petroleum products, chemicals, and garbage landfills, resulting in toxic leakage, which in turn has the potential to pollute both coastal land and ocean environment.

Tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps. Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that the 1998 tsunami, which killed thousands of people and destroyed coastal villages along the northern coast of Papua-New Guinea, was generated by a large underwater slump of sediments, triggered by an earthquake.

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. For an eyewitness account of the tsunami caused by the movement and landslides of the 1964 Alaska earthquake, please see:

Current research in the Canary Islands concludes that there have been at least five massive volcano landslides that occurred in the past, and that these same large events may occur in the future. These 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.

Source of Information for FAQ #14:

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15. Q. What should I know about wildfires, debris flows, and other types of landslides?

A: Here are a list of articles and publications which tell you what you need to know:

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16. Q. Where can I get my questions answered about Southern California landslides?

A: There is a separate set of Southern California landslides FAQs.

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