Oblique aerial photograph of the 2014 landslide in northwest Washington. This image shows the entire extent of the landslide source area and path. This event is commonly named the “Oso Landslide” in many official reports. It is also referred to as the “SR530 Landslide,” as named by Snohomish County and Washington State. Credit: Mark Reid, USGS

Reston, Va. — The Oso landslide, also known as the SR 530 landslide, occurred in northwest Washington state on March 22, 2014, leading to devastating loss of life and destruction of property. Landslide debris blocked the North Fork Stillaguamish River, destroyed about 40 homes and other structures, and buried nearly a mile of State Route 530. Most tragically, it caused 43 fatalities in the community of Steelhead Haven near Oso, Washington.

Oso was emblematic of a worst-case landslide scenario. As such, U.S. Geological Survey scientists have identified it as a key geological-hydrological event that can help explain and inform our understanding of the potential effects of landslides in other settings in the United States and worldwide. USGS scientists have been studying this event since the landslide occurred.

Shortly following the landslide, the USGS assisted state and local agencies by providing emergency landslide monitoring and flood threat information to ensure that if the landslide moved again, warning could be provided to emergency response teams involved in rescue and recovery efforts.

Using helicopters, the USGS deployed three portable instrument packages called “spiders,” specifically developed for monitoring active volcanoes and landslides. These spiders, which were placed on and near the landslide, contained high-precision GPS units for detecting landslide movement as well as geophones for detecting small vibrations. USGS scientists also provided immediate data on water levels and river discharge from an existing permanent streamgage located downstream on the North Fork Stillaguamish River at Arlington. Immediately following the event, the USGS also installed three rapid-deployment gages and three buoys to measure flow, sediment, and lake levels.

A volcano monitoring “spider” was deployed by helicopter to the Oso landslide to track ground movement and seismicity while search-and-rescue operations were ongoing. The spider was equipped with a seismometer (mounted on the far left leg) to track ground shaking and GPS (a dome-shaped instrument on the upper mast) to track subtle ground movement. Data was transmitted via telemetry (tall, white cylindrical antenna mounted on the steel trunk) to USGS field crews. USGS-Cascades Volcano Observatory field crews hiked to the location three months later to retrieve the spider after the search-and-rescue operations had concluded and the landslide had partially stabilized. (Credit: Ben Pauk, USGS. Public domain.)

Five years later

Over the past five years, scientists have examined a long list of factors which led to the landslide, including soil, water, and climatic conditions. Published findings from USGS studies are already being used by planners and emergency response officials to understand the context in which the landslide occurred and the potential impacts of landslides like Oso. USGS scientists continue to study the site to gain new insights that only such a significant, though unfortunate, event can reveal.

USGS published research includes maps showing the relative ages of landslides similar in style and geographically near Oso, models that showed how quickly the landside mobilized, and information about the expected response of the North Fork Stillaguamish River to the ongoing erosion of the millions of tons of material deposited as a result of the landslide. USGS researchers are now nearing the conclusion of a five-year study that mapped the landslide in detail in order to understand the mobility of the event — that is, to understand why the landslide traveled so far.

The USGS has been conducting field work, soils laboratory testing, and additional analyses to identify the likely causes of the large mobility of the Oso landslide. This work entailed making approximately 1,400 on-site observations of the landslide surface to reconstruct the displaced geology of the landslide deposit in order for the mobility of the landslide to be deciphered. The findings of this work will be published in the coming months and are anticipated to help update and develop models that better capture the true behavior of these types of landslides.

USGS civil engineer Brian Collins examines blocks of glacial till that form parts of the Oso landslide deposit. (Credit: Mark Reid, USGS. Public domain.)

What have scientists learned?

Although there is still much to learn about the Oso landslide, USGS and other scientists have unraveled invaluable scientific information that is shedding light on how and why such landslides happen.

The Oso landslide involved a complex sequence of geological and hydrological events that ultimately resulted in a debris-avalanche flow. USGS research indicates that the landslide traveled exceptionally far, crossing the entire one-half-mile wide river valley. Research has also shown that heavy seasonal precipitation likely contributed to destabilizing the slope. In fact, precipitation in the area during February and March of 2014 was 150 to 200 percent of the long-term average.

USGS, working with the University of Washington, University of California, Los Angeles, NOAA, and other partners, published findings that revealed that the time of year in which the seasonal total precipitation had occurred was relevant to triggering the landslide — heavy precipitation occurred toward the end of the rainy season when the ground was already nearly saturated. Another USGS study revealed how models can help explain the travel distance and timing of these types of landslides, which, in turn, can help us improve landslide susceptibility maps for such events in the future.

USGS interviews with eyewitnesses and analyses with dynamic landslide models indicate the landslide’s average speed was about 40 miles per hour. New volume estimates of the landslide using LiDAR-derived maps collected after the landslide reveal that, by the time the event had ended, the landslide had moved about 19 million tons of sand and till, and had covered approximately one-half square mile. That amount of material would cover approximately 700 football fields 10 feet deep.

The slide dammed the North Fork Stillaguamish River to a depth of as much as 25 feet, forming a temporary lake 2.5 miles long, which flooded houses and other structures in Steelhead Haven. In the 6-8 weeks after the landslide, and with some initial assistance from responders using a dredge, the river slowly eroded a channel through the landslide debris. This cut the river bed back to near its pre-landslide elevation and effectively drained the remaining excess water by the middle of May.

The USGS, working in collaboration with the Washington State Department of Transportation and the University of California, Berkeley, performed laboratory testing of soil samples from the landslide to identify the composition of the materials forming the landslide. This work has helped researchers understand how soils may move in other areas that have a similar soil composition.

The most in-depth studies from the USGS indicate that slope failure occurred in two stages over the course of about one minute. During the second stage of movement, the landslide greatly accelerated, crossed the North Fork Stillaguamish River, and mobilized to form a high-speed debris-avalanche. This accelerated movement was caused by the liquefaction of the saturated soils underneath the landslide, allowing the moving soil to quickly hydroplane across the river valley.

USGS hydrologist Mark Reid looks at one of the many grabens (extensional basins) that formed as the Oso landslide swept across the North Fork Stillaguamish River valley. (Credit: Brian Collins, USGS. Public domain.)

The Oso landslide occurred in an area of known landslide activity, but at the time of the slide, scientists had not researched the area to fully understand the geological history and various factors that lead to landslides of this magnitude. Shortly after the slide, USGS scientists identified and published a paper describing the North Fork Stillaguamish River valley and the geologic evidence they had uncovered showing the occurrence of past landslides, some of which traveled in a similar pattern to that of the 2014 landslide. Other researchers have identified the ages of some of these landslides, which range from 500- to 6,000-years old, but there is still a lack of understanding for the overall recurrence time frame for these types of landslides.

The Oso landslide response involved many federal, tribal, state, and local agencies, as well as the private sector. These organizations include Snohomish County; the Washington State Emergency Management Division; the Federal Emergency Management Agency; the Washington State Department of Natural Resources; the Washington State Department of Transportation; NOAA’s National Weather Service; the U.S. Army Corps of Engineers; the Stillaguamish Tribe of Indians; and the USGS.

Landslides occur in all 50 states and U.S. territories and on average cause $1 billion to $2 billion in damages and more than 25 fatalities each year.