Landslides
Landslides are a major geologic hazard. They are widespread, resulting in $1-2 billion in damages and 25-50 fatalities in the US each year. Landslides are frequently, but not always, triggered by strong ground motions like those created during an earthquake.
The term “landslide” includes a wide range of ground movements, such as rock falls, deep failure of slopes, mudflows and shallow debris flows. The primary reason for all landslides is the effects of gravity on a steep slope, but there can be a number of other contributing factors, including:
Erosion by rivers, glaciers, or ocean waves creating over-steepened slopes over time;
Rock and soil slopes weakening through saturation by snowmelt or heavy rains;
Excess weight from accumulation of rain or snow, stockpiling of rock or ore, creation of waste piles, or development of man-made structures adding stress to slopes.
Earthquakes and volcanoes can also contribute significantly to landslide hazards. For example:
Earthquakes create stresses that make weak slopes fail;
Volcanic eruptions produce loose ash deposits, heavy rain, and debris flows.
Strong earthquake ground shaking greatly increases the likelihood of landslides where the landscape is susceptible to these types of ground failure. If the ground is saturated with water, particularly following heavy rainfall, the shaking will result in more landslides than normal.
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 and causing flooding along its path.
Landslides into and under bodies of water can also generate tsunamis or seiche waves (Tsunamis) that can be locally quite destructive.
Significant Landslides in the Pacific Northwest
Landslides triggered by earthquake shaking are a major concern in the Pacific Northwest. Earthquake Point in Chelan County, Washington, is named after the Magnitude 7, 1872 earthquake (1872 North Cascades Earthquake) that struck nearby. According to the plaque at Earthquake Point, erected by the Washington State Department of Transportation and Washington State Parks and Recreation Commission, “The shaking split the mountain, forming the cliff to the west and causing a huge rock slide which stopped the flow of the Columbia River for several hours.”
A major landslide occurred in Oso, Washington in 2014, killing 43 people and destroying dozens of buildings. Lieutenant Governor Brad Owen declared a state of emergency, and President Barack Obama declared a major disaster. The landslide blocked the North Fork of the Stillaguamish River, causing the river to back up and flow into a new channel. The Oso slide was not triggered by an earthquake, but PNSN scientists were able to analyze the ground motion in the area before, during, and after the landslide.
Large and small landslides occur frequently in the Puget Sound area, where glacial geology dominates, and evidence has been uncovered that details large block landslides into Lake Washington and Lake Sammamish about the time of the last Seattle Fault earthquake approximately 1100 years ago. A 2013 study suggests the next big quake on the Seattle Fault may cause devastating damage from landslides, greater than previously thought and beyond the areas previously defined as prone to landslides.
Landslide Hazard Maps
Landslide hazard maps provide planners and engineers a picture of where landslides are likely to occur. Closer analysis is necessary before development can safely occur in high risk areas.
Landslides tend to occur where landslides have happened previously. Where records have been kept, landslide inventory maps provide a useful first-level view of where the risk is highest. Slope angle, subsurface geology, and rainfall data, added to this knowledge base, can inform landslide predictions and mitigation priorities.
In Seattle, shallow landslides are common on coastal bluffs overlooking Puget Sound. Hundreds of landslides occurred on the bluffs in late December 1996 and early January 1997 as heavy rainfall accompanied by abrupt warming caused rapid melting of recent heavy snow. Smaller numbers of landslides occur every few years during extended rainy periods and intense storms. This landslide hazard map shows past landslides in Seattle and high risk zones of future landslides. Learn more about landslide hazard mapping in King County here.
Some parts of Portland, Oregon are also prone to landslides, particularly the West Hills and high-elevation areas on the east side of the city. An abnormally high rain event, followed by deep freezing and rapid thawing, led to extensive flooding and landslides throughout Western Oregon in the winter of 1996. An earthquake on the Portland Hills Fault or the Cascadia Subduction Zone, especially if it took place during the wetter winter months, could cause landslides as well. Landslide maps for the Portland, Oregon, area are available here.
More Information About Landslides
The Washington State Department of Ecology has published information about landslides, historic events, and mitigation. The City of Seattle offers valuable landslide hazard, preparedness, and policy information.
The Oregon Department of Geology and Mineral Industries (DOGAMI) has also published fact sheets and maps about landslide risks and historical landslides.
Read a Homeowner’s Guide to Landslides for Washington and Oregon here.