The death toll rose to 24 on Wednesday (March 26, 2014) in the devastating mudslide that demolished the village of Oso in Washington state this past weekend. In that event, a 1,500-foot-wide segment of hillside suddenly cut away and began sliding. Hundreds of emergency responders and volunteers on the scene all week, in the search for survivors and bodies. In the wake of this tragedy, Deanna Connors explores the natural forces at work to create landslides and mudslides.
The term landslide refers to the downward movement of a large mass of rocks, soil, mud and organic debris. Areas with steep slopes, for example mountainous regions, are particularly susceptible to landslide hazards. Most landslides are caused by multiple factors that act together to destabilize the slope.
The primary cause of a landslide is the influence of gravity acting on weakened materials that make up a sloping area of land. While some landslides occur slowly over time (e.g., land movement on the order of a few meters per month), the most destructive ones happen suddenly after a triggering event such as heavy rainfall or an earthquake.
Water can trigger landslides because it’s heavy and adds a lot of extra weight to the land. The extra weight makes it more likely that slope materials (soil, rock, etc.) will succumb to the forces of gravity. Excessive water is thought to be one of the most common triggers for landslides.
Other factors that weaken slope materials also contribute to the occurrence of landslides. These factors include both natural events such as geological weathering and erosion and human-related activities such as deforestation and changes made to the flow of groundwater. Vegetation removal after droughts, fires, and logging has been associated with an increased risk for landslides.
Landslides are classified according to the type of material that falls and how that material moves downslope. For example, there are rock falls, mud slides, and debris flows. All of these terms represent a type of landslide.
One particularly destructive type of landslide is known as a lahar. Lahars are volcanic mud flows or debris flows that are capable of traveling at very fast speeds down the slope of a volcano.
The Landslide Handbook—A Guide to Understanding Landslides is a great resource from the U.S. Geological Survey for those who want to learn more about landslides.
Bottom line: Landslides are mainly caused by gravity acting on weakened rocks and soil that make up a sloping area of land. Both natural and human-related activities can increase the risk for landslides. Water from heavy rainfall is a frequent trigger for landslides.