THE 22 MARCH 2014 OSO LANDSLIDE, SNOHOMISH COUNTY, WA: GEER REPORT SUMMARY

The 22 March 2014 Oso Landslide in Snohomish County, Washington, is among the most significant geologic disasters in recent U. S. history. The 2014 landslide occurred on a valley slope with documented history of intermittent landslide movement dating back to the 1940s; slope movements in 2006 blocked the North Fork of the Stillaguamish River and caused shallow flood damage to some nearby homes. The Oso Landslide is one of many landslides that have occurred on valley slopes above the river. It generated vibrations at 10:37 A.M. local time that were recorded on nearby seismograph stations and became a rapidly moving, unchannelized debris flow that spread out as it travelled about 1 km across the valley, damming the North Fork of the Stillaguamish River, destroying and carrying away nearly 50 homes, killing 43 people, and burying about 1.5 km of State Highway 530. Geologic and geomorphic features suggest that the slide developed in two stages: the first mobilized into a fluidized debris flow that did most or all of the damage, whereas the second slid into the space created by the first and ran up onto the trailing part of it. Preliminary landslide frequency estimates span 500 to 5000 years in the valley segment around the Oso landslide. Rainfall in the three weeks prior to the Oso landslide was very heavy, yet no other slides occurred in the valley. The National Science Foundation-supported Geotechnical Extreme Events Reconnaissance (GEER) Association (www.geerassociation.org) assembled a team to investigate the landslide with a primary focus to document short-lived geotechnical features and make the findings available through GEER’s website. The team’s report describes precipitation, geologic, groundwater, and geotechnical factors, as well as impacts to infrastructure and hazard-communication aspects of the disastrous landslide event.