Landslides are the most common geological event. That's a fact.

They affect millions of people and cause many thousands of deaths. They often occur in countries with poor or inadequate infrastructure — such as Papua New Guinea, where a reported 2,000 people were buried alive following a landslide in May 2024.

Two months later, torrential rains in Indonesia caused a landslide to crash onto a reportedly illegal gold mining operation on Sulawesi island, killing some of the people working there.

But landslides are also reported in high-income areas, like the United States.

Thousands of landslides per year globally

In 2020, the World Bank estimated the average annual number of significant rainfall-triggered landslides between 1980 and 2018, by country.

The World Health Organization says that between 1998 and 2017, landslides affected about 4.8 million people and caused more than 18,000 deaths worldwide: "Climate change and rising temperatures are expected to trigger more landslides, especially in mountainous areas with snow and ice. As permafrost melts, rocky slopes can become more unstable resulting in a landslide."

Four main types of landslide

Landslides are classified by the way the land moves into four rough groups:

• falls,
• topples,
• slides and
• flows.

A fall involves material such as rock that falls through the air from a cliff or other steep slope. The rock may also roll or bounce as it falls.

A topple happens when, for example, a slab of rock moves forward from its base and topples toward the ground.

Slides occur when the material at the base of a slope falls due to a rupture, or what is known as a slip surface.

And flows happen when the slope material gives way and slides down like a liquid. This is usually how mudslides and rock avalanches occur.

Then it depends on the type of geologic material that moves: bedrock, debris or earth.

Debris flows, often called mudflows or mudslides, are among the most common types of landslides. Rock falls are also common.

Why do landslides happen?

Landslides happen when the materials that make up a slope fail to resist the forces acting down upon them.

That pressure can grow when there's excessive rainfall, snowmelt, changes in groundwater, earthquakes, volcanic activity or human activity.

In India, a rainfall-triggered landslide that hit during Cyclone Remal was blamed for the collapse of a quarry in Mizoram state.

And the West Coast of the United States experienced numerous landslides in the first months of 2024, destroying homes and coastlines and blocking roads in California's Big Sur region. Those were caused by extreme weather, rockfall and plain old gravity. 

Can landslides and mudslides be prevented?

Yes, landslides can be prevented.

Research indicates that the simplest way to do this is to counter the very forces that cause landslides and mudslides in the first place. That can be done by improving surface and sub-surface drainage; constructing piles, buttresses and retaining walls that reinforce the base of a slope; or creating paths to divert debris.

The US Geological Survey advises against building "near steep slopes, close to mountain edges, near drainage ways, or natural erosion valleys" because quite often, human activity, including industrial and mining activity, is the main cause. The rest is nature responding.

Edited by: Clare Roth

Sources:

The Global Landslide Hazard Map, The World Bank, June 2020 https://datacatalog.worldbank.org/search/dataset/0037584

Landslides, World Health Organization https://www.who.int/health-topics/landslides#tab=tab_1

Remote sensing data tracks Big Sur landslides of 2024, US Geological Survey https://www.usgs.gov/programs/cmhrp/news/usgs-remote-sensing-data-tracks-big-sur-landslides-2024

How to classify a landslide, British Geological Survey https://www.bgs.ac.uk/discovering-geology/earth-hazards/landslides/how-to-classify-a-landslide/

Prevention and Remediation of Landslides, Kansas Geological Survey https://www.kgs.ku.edu/Publications/pic13/pic13_5.html

Updates: This article was originally published May 25, 2024, and then updated July 8, 2024.