The 22 March 2014 Oso Landslide, Snohomish County, Washington: Findings of the GEER...

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2014 Fall Meeting Section: Natural Hazards Session: The Oso Disaster and Other New Perspectives on High-Mobility Landslides and Debris Flows II Title: The 22 March 2014 Oso Landslide, Snohomish County, Washington: Findings of the GEER Reconnaissance Investigation Authors: Wartman, J, University of Washington, Seattle, WA, United States Montgomery, D R, University of Washington, Seattle, WA, United States Gilbert, R, University of Texas at Austin, Austin, TX, United States delaChapelle, J, Golder Associates, Inc., Redmond, WA, United States Keaton, J R, AMEC, Los Angeles, CA, United States Scott, A, Federal Highway Administration, Denver, United States Benoit, J, University of New Hampshire Main Campus, Durham, NH, United States Abstract: We report the findings of the NSF-supported Geotechnical Extreme Events Reconnaissance (GEER) investigation of the Oso Landslide. Our findings are principally based on data collected during a four-day team reconnaissance across the entire landslide area, but also draw upon other data sources including lidar surveys, high-resolution imagery, geologic mapping, precipitation data, and seismic records. The Oso Landslide claimed 43 lives, making it the deadliest landslide disaster in U.S. history. The landslide occurred within a thick sequence of glacial sediments that were deposited into the North Fork Stillaguamish River valley during the last glacial advance. Geomorphic evidence suggests that the valley in the vicinity of Oso Landslide has experienced multiple large landslides over at least the past 6,000 years. Intense three-week rainfall that immediately preceded the event very probably played an important role in triggering the landslide; however, many other factors likely contributed to destabilization of the landslide mass. These include: (i) alteration of the local groundwater recharge and hydrogeological regime due to previous landsliding and, possibly, land use practices, (ii) weakening and alteration of the landslide mass due to previous landsliding and other natural geologic processes, and (iii) changes in stress distribution resulting from removal and deposition of material from earlier landsliding. During our field reconnaissance we identified six distinctive landslide zones and several subzones that are characterized by different geomorphic expression resulting from deformation styles, geologic materials, vegetation, and sequence of deposition. Based on the reconnaissance observations and other available data, we hypothesize that the landslide occurred in two major stages. The first stage of movement is interpreted to be a remobilization of the 2006 slide mass and headward extension that included part or all of the forested slope of an ancient landslide. The second stage occurred in response to the unloading and the redirection of stresses within the landslide mass. We additionally approximated the risk for fatalities due to landslides in the region and found that it to be relatively high compared to guidelines for landslides in other developed countries and for large dams in the U.S. Cite as: Author(s) (2014), Title, Abstract NH53C-04 presented at 2014 Fall Meeting, AGU, San Francisco, Calif., 15-19 Dec. Learn more here: http://abstractsearch.agu.org/meetings/2014/FM/NH53C-04