Bedrock mapping and seismic hazard assessment at Gold Basin landslide, Washington

The Gold Basin landslide is located along the South Fork Stillaguamish River, within the Mount Baker-Snoqualmie National Forest in western Washington State. Recent concerns related to slope stability after the 2014 State Route 530 Landslide near Oso, Washington, forced the closure of the U.S. Forest Service Gold Basin Campground in May of 2014. In addition to safety concerns for National Forest visitors, the landslide-derived sediment pulses shed into the South Fork Stillaguamish River may harm migrant salmon spawning grounds, an important resource for the Stillaguamish Tribe of Indians and for public anglers.

The Gold Basin landslide is composed of three active lobes and has an approximate footprint of 566,560 m². Each lobe consists of steep topographic escarpments contained largely within Pleistocene glacial outwash sediments and debris flow and earth flow deposits at the base. In addition to landslides confined within the Pleistocene glacial strata, bedrock landslides are also apparent on lidar imagery of the study area. Bedrock landslides may pose additional hazard to the area, either during stochastic hillslope failure or during strong ground motion events. Potential seismic sources include the proximal Darrington-Devils Mountain and southern Whidbey Island fault zones, as well as the offshore Cascadia Subduction Zone. Previous analyses of hillslope stability in the Cascade Range suggests that rock mass strength is a useful way of characterizing bedrock and fracture patterns in order to understand potential landslide-prone landscape.

The goals of this investigation are to assess the glacial strata and bedrock geology of the Gold Basin landslide and adjacent areas and to assess how the geology and geomorphology within the study area affect the likelihood of coseismic landsliding.