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Landslide 101

Scott Elliott
November 30, 2022

Understanding landslides and the risk they pose is extremely important. 

Landslides are more widespread than any other geological event, occurring in all parts of the world. Between 1998 and 2017, landslides affected an estimated 4.8 million people.1 In the United States, between $2 billion and $4 billion worth of landslide-related losses occur annually, with public infrastructure, businesses, homes, and human lives all falling victim.2 

Let's explore what landslides are, how they manifest in the real world, and what lessons they can teach us about managing risk.

What are landslides?

The term “landslide” refers less to a specific type of event and more to a broad range of ground movements, characterized by the downhill movement of rock, soil, or debris. Here are a few different types of landslides:

  • Rockfalls: a descent of loose rocks that have fallen freely from a cliff face.
  • Mudflow: a rapid flow of fluid, liquified debris.
  • Debris flow: a flow of liquified soil and rock.
  • Deep-seated slope failures: a collapse that occurs when the strength of the soil and rock within a slope is exceeded by the weight of the material above.

Just as there are different types of landslides, there are a variety of factors that cause them to occur:3

test 1-1

Landslides have many variables, oftentimes combining to cause landslide events. Three core variables are gravity, friction, and water.4


Most landslides move down a slope because the internal strength of the rock, soil, and sediment comprising an area of the earth becomes less than the strength of gravity above.

gravity 1

Material moves easier along surfaces that have less friction. Whether due to a steeper or more slippery slope, the less friction there is, the easier it is for material to move down a slope.

friction 1

When water is added to the material comprising a slope, the chance of landslide increases; water adds weight to the slope as it seeps into the ground, adding to the gravitational force. Water also lowers the internal strength of the rock, soil, and sediment within the earth, making it less able to withstand the downward pressure of gravity.

Water increases the pore fluid pressure within soil and sediment, separating the grains and decreasing the internal strength. Material with high pore fluid pressure is more likely to shift, move, and slide downhill.

water 1`-1

In summary, landslides are most likely to occur in areas of steep terrain that are saturated by water and prone to other geological occurrences, such as earthquakes.

An area with a bunch of hills and mountains and lots of rain that is also prone to earthquakes... Does that sound like someplace in particular?

Welcome to Washington

Washington state has hundreds of landslides each year, making it one of the most landslide-prone states in the country. And as more residents move into rural parts of the state and the effects of climate change become more acute, the impact of landslide events will become more frequent and severe.

By looking at recent landslide events around the state, we can better understand how landslides occur and the specific types of areas with the most significant risk of experiencing them.

Whidbey Island – 2013

On March 27, 2013, a large landslide occurred in a community outside the town of Coupeville on Whidbey Island.5

The slide sent roughly 200,000 cubic yards of earth downwards toward Puget Sound, leaving a gouge in the side of the island hundreds of feet wide.6 The slide destroyed one house and left several others cut off from the road.

whidbey-2013-landslide-gifBefore and after the Whidbey slide.

The slide occurred in a known slide complex, a site approximately 1.5 miles long and 200 feet high above sea level. DNR geologists believe the slide to be a deep-seated landslide, thought to have been moving slowly since at least 2002 and caused by accumulated groundwater within the slope. Geologists knew the area was landslide-prone but had no idea when or how large the slide would be.

No injuries were reported.

Oso – 2014

On March 22, 2014, a major landslide occurred in Snohomish County, just outside the small town of Oso.7

At 10:37 a.m., an unstable hill partially collapsed, sending debris and mud downwards, ultimately overtaking a remote neighborhood and covering an area of one square mile. As a result of the slide, 43 people lost their lives, and 49 homes and buildings were destroyed. The slide’s total cost is estimated to be close to $10 million.


The land that collapsed and formed into a mudslide was a known landslide hotspot, with documentation of slide activity dating back to the 1930s; the area was known locally as "Slide Hill.” The site was structurally weak, comprised of porous sand and gravel susceptible to water accumulation, increasing the pore pressure and helping to cause the 2014 ground failure.

Up to the present day, the Oso slide is the deadliest single landslide event in the history of the United States.

Seattle – 2022

On January 7, 2022, Seattle fire received a call regarding a house that had slid 15-20 feet off its foundation in the Magnolia neighborhood.8


The event was caused by heavy rainfall on the steep slope area behind the house, triggering a landslide that pushed up against the back of the house and forced it off its foundation.

Two residents were transported to the hospital, both in stable condition.

Bellevue – 2022 

On January 17, 2022, a water main break spurred a landslide in the Somerset neighborhood of south Bellevue, a hilly region within the city.9


Firefighters arrived on the scene early in the morning, responding to a report of minor flooding near the upper half of the neighborhood. They noticed a large amount of water flowing around one of the homes. Upon returning from investigating the source, they observed that the house had slid halfway off its foundation, tilting downhill at an angle of roughly 45 degrees.

No injuries were reported.

Rattlesnake Hills – Ongoing 

The slide was first noticed in October 2017 when large cracks appeared in the ground above a nearby quarry.10

In contrast to the common perception of landslides as sudden and rapid, the Rattlesnake Hills landslide, just outside of Yakima, is moving exceptionally slow; current measurements have determined the rate of the slide to be around 0.7 feet per week, though faster speeds have been recorded.


The slide consists of basalt rock flowing over a layer of sediment. Because the basalt is more stable and the sediment below is less stable, the internal strength of the sediment is less than that of the basalt above, and gravity can pull the basalt layer downwards.

Because of the rate of the slide and the consistent monitoring of its progress, geologists do not believe a “large event,” or large, sudden movement of the land, will occur. Instead, the debris will likely accumulate in the nearby quarry and eventually slow to a halt.

What can we learn?

The historic slides explored above teach us many lessons about landslide risk.

Living on or near a slope can increase risk

The most landslide-prone areas on earth are steep slopes; steeper slopes have less friction, making landslides more common. Because of this, houses and buildings built on top of, in front of, or near steep slopes are at an increased risk of landslide-related damage.

This risk can be compounded by proximity to a body of water and the possibility of increased erosion to nearby slopes.

Historic landslide areas are dangerous

It is essential to be aware of areas that are known slide locations. In the case of the 2014 Oso slide, the area was a well-known slide hotspot, and the surrounding area was not the safest place to live. Similarly, the 2013 Whidbey Island slide took place in an area known to be landslide prone.

Washington state has several resources to pinpoint landslide-prone regions and provide residents with information regarding landslide hazards

Additionally, the United States Geological Survey (USGS) has an interactive map web application showing all historical landslide data and areas where evidence of landsliding has occurred across the US.

Landslide risk increases during wet periods

If the earth becomes saturated, the pore fluid pressure increases, and the internal strength of the earth decreases. The 2022 slide that occurred in Seattle was triggered by a period of significant rainfall, destabilizing the slope behind the house.

Whether sustained rain or snow melt, homeowners who live in landslide-prone areas need to evaluate their properties and nearby areas for signs of movement; leaning trees, cracks in the soil, soil movement, overly-saturated ground, and unusual sounds can all be signs of an impending landslide.

How we can help

Understanding the slope of a property is an integral part of assessing landslide risk. At BuildingMetrix, we can assess risk, not just fire, and can help companies understand their landslide risk and manage their landslide exposure better than ever. 

Reach out to our data experts to learn more. 

More landslide resources

Homeowners Guide to Landslides: Washington and Oregon

King County Landslide Map

USGS Landslide Risk Map Application

More geospatial contentMore earthquake content

[1] World Health Organization, https://www.who.int/health-topics/landslides

[2] American Geosciences Institute, https://www.americangeosciences.org/critical-issues/faq/how-much-do-landslides-cost-terms-monetary-losses 

[3] USGS, https://www.usgs.gov/programs/landslide-hazards/landslides-101 

[4] DNR, https://www.dnr.wa.gov/programs-and-services/geology/geologic-hazards/landslides#why-do-landslides-happen?.3 

[5] Washington DNR Blog, https://washingtondnr.wordpress.com/2013/03/27/whidbey-island-coupeville-landslide/ 

[6] New York Times, https://www.nytimes.com/2013/03/30/us/landslide-in-washington-shows-an-earth-on-the-move.html 

[7] Wikipedia, https://en.wikipedia.org/wiki/2014_Oso_mudslide 

[8] Seattle Fireline, https://fireline.seattle.gov/2022/01/07/landslide-in-magnolia-neighborhood/ 

[9] Bellevue Gov, https://bellevuewa.gov/city-government/departments/fire/emergency-management/prepare-known-hazards/landslides/somerset-slide 

[10] DNR, https://www.dnr.wa.gov/rattlesnake-hills-landslide 

Scott Elliott has 17 years of experience with commercial insurance in claims, underwriting and sales. He joined WSRB to help insurers make better decisions by providing accurate and unbiased information. Scott lives and works in Spokane, Washington.


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