Overview

According to the U.S. Geological Survey (USGS), landslides are a “movement of a mass of rock, debris or earth down a slope.”

“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 five modes of slope movement: falls, topples, slides, spreads and flows. These are further subdivided by the type of geologic material (bedrock, debris or earth). Debris flows (commonly referred to as mudflows or mudslides) and rockfalls are examples of common landslide types.”

Gravity is the primary factor in landslides, but erosion, saturated ground, earthquakes, volcanic eruptions and excess weight from rain or snow contribute to ground movements (rockfalls, slopes’ deep failure and shallow debris flows).

The majority of landslides do not affect humans, but the ones that do are devastating to homes, businesses and infrastructure.

At least 167 people died in landslides and floods in Leyte Province, Philippines, after Tropical Storm Megi hit the country in April 2022. In some cases, human activity can contribute to causing landslides. For example, heavy rains in the town of Retamas, Peru, in March 2022, triggered a landslide that killed at least two people. The affected area is heavily impacted by mining, including informal mining, which may explain the environmental damage seen in satellite images.

Most major landslides are so-called “secondary hazards” triggered by another disaster, like an earthquake, volcanic eruption or erosion after a wildfire. Underwater landslides trigger the majority of tsunamis. Experts believe that such a landslide likely caused a September 2018 tsunami in Palu, Indonesia, following an earthquake.

According to United Nations Office for Disaster Risk Reduction (UNDRR), “In most developed countries with high landslide hazard, landslide events rarely end up as disasters. This is mainly due to the low exposure in the most landslide-prone areas, as well as the increasing ability to identify the landslide-prone areas and to implement appropriate landslide risk management actions.”

Key Facts

  • The USGS estimates that in the U.S., landslides cause more than $1 billion in damages annually.
  • In the U.S., 25 to 50 people are killed in landslides annually, but worldwide that number is in the thousands.
  • The largest landslide ever is related to the 1980 eruption of Mount St. Helens volcano in Washington state in the U.S. The landslide velocity was 70-150 mph (112-240 kph).
  • Studies and assessment of the reported occurrence of landslide disasters in the CRED EM-DAT database suggest that the most exposed countries to landslide risk are in south Asia, along the Himalayan belt, in east Asia, south-eastern Asia and in Central and South America.

How to Help

  • Support planning to reduce catastrophic loss. By investing in hazard analysis and mapping to guide land-use decisions, the potential for catastrophic loss can be reduced. Land use planning, including zoning certain areas as unsafe for development, will not always reduce the chance of a landslide but can minimize damage.
  • Physical barriers can prevent damage. In cases where potential landslides could affect existing structures, physical controls can be used including installing retaining walls and rerouting surface and underwater drainage. This type of intervention is often expensive and constrained by landslide magnitude and frequency, and the size of settlements at risk, but may be necessary in some contexts. Depending on the scale and cost, government investment may be required, and philanthropy can provide support through convening, advocacy and collaboration.
  • Invest in early warning systems. Effective landslide risk mitigation should be carried out at the local or regional level. On the local scale, an early warning system is an example of a risk mitigation measure that can be effective. Landslide advisories make communities aware that rainfall may lead to debris-flow activity, and they should take precautions if there is heavy rain. Landslide watches indicate that landslide activity is possible and preparedness activities should be undertaken, whereas a landslide warning means activity is occurring.
  • Address root causes of social vulnerability to reduce risk. Building on unstable hillsides can put lives at risk, but this may be the only available or affordable housing option for many poor and marginalized people. Strengthening institutions, addressing poverty, and improving access to safe and affordable housing can help reduce risk.

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(Photo: Civil Defense Colombia carries out search and rescue operations after a landslide in Cundinamarca, Colombia, April 2022. Source: Civil Defense Colombia via Twitter)

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