Landslides/Mass Movements – Introduction

La Conchita, CA Landslide of 2005[4]
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Landslides are sudden, short-lived geomorphic events that involve the rapid-to-slow descent of soil or rock in sloping terrains. The term landslide includes a wide range of ground movement, such as rock falls, failure of slopes, and debris flows.

Although gravity acting on an over-steepened slope is the primary reason for a landslide, there are other contributing factors:

  • erosion by rivers, glaciers, or ocean waves create oversteepened slopes;
  • rock and soil slopes are weakened through saturation by snowmelt or heavy rains;
  • earthquakes create stresses that make weak slopes fail;
  • earthquakes of magnitude 4.0 and greater have been known to trigger landslides;
  • volcanic eruptions produce loose ash deposits, heavy rain, and debris flows;
  • excess weight from accumulation of rain or snow, stockpiling of rock or ore, from waste piles, or from man-made structures may stress weak slopes to failure and other structures.

Slope material that becomes saturated with water may develop a debris flow or mud flow. The resulting slurry of rock and mud may pick up trees, houses, and cars, thus blocking bridges and tributaries causing flooding along its path.[1]

Large muddy lahars commonly begin as volcanic landslides. The collapse or “flank failure” of a volcano will generate a fast-moving landslide that usually transforms into a lahar after traveling a few kilometers. Depending on the size of the landslide, its water content, and extent to which the volcano’s rocks have been weakened and turned into clay by a hydrothermal system, the resulting lahar may travel more than 100 km downstream. Such large muddy lahars are extremely dangerous.[2]

On average, landslides in the United States cause $1 to $2 billion in property damage and more than 25 fatalities per year. Posing threats to settlements and structures, landslides often result in catastrophic damage to highways, railways, waterways, and pipelines.[3]

Where do Landslides occur?

Landslides occur in every state and U.S. territory. The Appalachian Mountains, the Rocky Mountains and the Pacific Coastal Ranges and some parts of Alaska and Hawaii have severe landslide problems. Any area composed of very weak or fractured materials resting on a steep slope can and will likely experience landslides.

Although the physical cause of many landslides cannot be removed, geologic investigations, good engineering practices, and effective enforcement of land-use management regulations can reduce landslide hazards.[5]

How do landslides cause tsunamis?

A landslide is often triggered by an earthquake – which on impact creates a tsunami. As the rapidly moving mass enters the water and as water displaces behind and ahead of a rapidly moving underwater or above water landslide – a tsunami is generated.

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.[6]

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.[7]

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.[8]



  1. USGS Landslides hazards Programs – Landslides 101 – What is a Landslide?:
  2. USGS Landslides Hazards Program – Lahars Triggered by Sudden Landslides at Volcanoes:
  3. NASA Earth Observatory – Landslides:
  4. Image Source: [Accessed January 2, 2014]
  5. USGS Landslides Hazards Program – Landslides 101 – What is a Landslide?:
  6. USGS FAQs:
  7. USGS FAQs:
  8. USGS FAQs: