Why do landslides happen?



What Is a Landslide?

Why do landslides happen?

Types of landslides

Detection and Prevention



Causes: Natural and Human

 Landslides are not individual events, they occur in conjunction with other factors Landslides occur when the hill or mountain side is unstable. Factors that can allow gravity to overcome the resistance of earth material are:


Erosion (the natural wearing away of rocks and soil) by rivers, glaciers or oceans can overly steepen slopes. (USGS)


Addition of moisture - Moisture equals weight and water from heavy rainfall, flooding, rapid snow melting, glacier melting, and an increased water table can all saturate the hillside and cause a landslide. El Nino, the weather phenomenon that can increase precipitation, led to thousands of landslides in 1982-83 and 1997-1998. (USGS)

Above landslide in Oregon caused by massive flooding (Photo courtesy of www.lcd.state.or.us/ coast/hazards.html)


Shocks and Vibrations - Earthquakes, typically those of 4.0 magnitude and above, can can create stresses that weaken slopes. Earthquakes tend to produce the largest and most destructive landslides. The earthquake-induced Alaskan landslide of 1964  happened when the vibrations disrupted the clay soil particles and the water contained in them rose to the surface.  The picture below shows one of the 1964 landslides that struck an elementary school. (Landslides Triggered by Earthquakes)

Photo credit: U.S.G.S., Menlo Park, CA


Volcanic eruptions can produce loose ash deposits, heavy rain, and debris flows. The eruption of Mount St. Helens on May 18, 1980 triggered massive landslides, including the largest landslide in the world, which moved 2.8 km^3 of earth material. (USGS)

Click on the above picture to see a short informational movie clip about the Mount St. Helens landslides!

The Human Effect on Landslides

Overdevelopment - Human activities such as construction, building, transportation, building dams and canals, and mining can disturb large volumes of earth materials. In fact, landslide damage is increasing every year as our population expands further into hilly regions. The San Francisco Bay region is turbulent enough on its own, but as more people overdevelop the land becomes less and less stable. The picture to the right displays the devastating effects of building too close to a hill with landslide potential. (photo credit: R.L. Schuster, U.S.G.S)

Deforestation - As the human population grows the demand for clear land for crops and housing grows too. So forests, with trees that hold the soil in place, are logged, burned, and developed. And while this alone will usually not cause a landslide, the land becomes much more susceptible to heavy rains and floods and landslides can occur with much less rain than if a forest was still there. Wildfires, either natural or manmade, also have the same effect. (Socioeconomic and environmental...)