Preliminary Interpretation of Pre-2014 Landslide Deposits in the Vicinity of Oso, Washington [open pdf - 303KB]
This report offers a preliminary interpretation of pre-2014 landslide deposits in Washington state using maps and figures. "High-resolution topographic surveys allow fairly precise mapping of landslide deposits and their relative ages. Relative ages are determined by cross-cutting relations and the amount of smoothing--more smoothed slide deposits are older--of these deposits. The Tulalip Tribes, in partnership with the Puget Sound Lidar Consortium, acquired a high-resolution lidar (light detection and ranging) survey of the North Fork Stillaguamish River valley in 2013. This report presents a preliminary interpretation of the topography of this area using the lidar data at a scale of 1:24,000. […] These interpretations are preliminary and are based entirely on topography. Where younger events (for example, fluvial erosion or deposition) have eroded or buried landslide deposits, their distinctive morphology will not be evident. The actual extent of landslide deposits is probably greater than shown. In a very few locations, morphology may not be diagnostic. Mapping is only one of several elements in a landslide-hazard assessment. Stratigraphic and chronologic studies are necessary to establish landslide frequencies. Mechanics of past and potential future landslides need to be analyzed to better understand the potential for future failure and the potential for rapid movement. Forecasts of climate change and river meandering may be essential."
Open-File Report No. 2014-1065
U.S. Geological Survey: http://usgs.gov/