Deadly Washington Landslide Was Not Caused By An Earthquake: USGS
March 27, 2014

Deadly Washington Landslide Was Not Caused By An Earthquake: USGS

[ Watch the Video: Landslide Hazards ]

Lawrence LeBlond for - Your Universe Online

A large landslide in northwest Washington that buried some 30 houses and killed at least two dozen people on March 22, 2014 may have been erroneously blamed on a minor earthquake that reportedly struck the area several days prior to the disaster.

Shortly after a press briefing by Snohomish County Emergency Management’s John Pennington was issued on Tuesday announcing that a 1.1 magnitude earthquake on March 10 near the site of the mudslide, numerous news agencies began reporting that the mudslide was caused by the earthquake. SCEM officials at the time said they were eyeing the earthquake as a possible cause of the landslide, but had not confirmed that.

The USGS, which has been working with local and state officials to provide the latest up-to-date information on the landslide and relief efforts, is saying that no earthquake is associated with the landslide.

Seismograph readings taken from the area show no indication that an earthquake is associated with the landslide near Oso, Washington on March 22, 2014, according to the USGS. The readings do show two wave signals, both being attributed to two landslides that occurred about four minutes apart. The data show no earthquake or other local seismic events around the time of the landslide.

“The seismic signals are of long period surface waves, with no clear high-frequency P or S phases that we would expect to see if a local earthquake occurred at the time of the event. The landslides generated elevated levels of local ground shaking for over an hour,” reads a report statement by the USGS.

The seismic readings were taken by the University of Washington Pacific Northwest Seismic Network, which is operated in cooperation with the USGS.

The USGS said in a recent report that it is continuing to support state and county agencies to provide assistance, assess the situation, and alleviate impacts as the hazard is not yet over. There is still concern that future rains or snowmelt from nearby mountains could exacerbate the situation. Experts are currently gathering LiDAR imagery and aerial photographs to help map the extent of the landslide.

This particular valley has been prone to large, sudden landslides in the past. In fact, large landslides are normal in many parts of the western foothills of the North Cascades. The Nooksack Valley in Whatcom County, for example, has seen at least five major catastrophic landslides in the past 12,000 years, according to the USGS.

There are many types of landslides, and the Oso event was a rotational slide complex.

“The term “slide” refers to mass movements where there is a distinct zone of weakness that separates the slide material from more stable underlying material and movement occurs mainly by slipping or sliding (rather than flowing) along this zone. The two major types of slides are rotational slides and translational slides. A rotational slide is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide,” explained the USGS in a report.

“Material along the lower edges of the slide broke up and transformed to a “debris flow,” also commonly referred to as a “mud slide” or “mud flow.” A debris flow is a flowing mixture of water-saturated debris that moves downslope under the force of gravity. Debris flows consist of material varying in size from clay to blocks several tens of meters in maximum dimension. When moving, they resemble masses of wet concrete and tend to flow downslope along channels or stream valleys,” added the Survey.

Landslides such as this one occur in all 50 states and cause upwards of $2 billion and more than 25 fatalities on average each year. The most common and perhaps deadliest hazards associated with mudslides are falling rocks, mud and debris flows.

In order to learn more about how and when events like the Oso landslide occur, the USGS has produced maps of the areas susceptible to landslides and what type of rainfall conditions are needed to lead to such events. This data will also help researchers understand how fast mudslides travel and how far they can move.

Images Below: Photograph from an aerial survey showing the extent and impacts from the landslide in northwest Washington that occurred on March 22, 2014. The survey was conducted by the Washington State Department of Transportation, Washington State Department of Natural Resources, USGS, and King County Sheriff's Office. Credit: Air Support Unit, King’s County Sheriff’s Office