Surface roughness dating of long-runout landslides near Oso, Washington (USA), reveals persistent postglacial hillslope instability

Establishing regional landslide chronologies is necessary to advance from hazard recognition to risk assessment, and to understand the evolution of landslide-prone terrain. Despite recent advances in landslide mapping due to the availability of high-resolution lidar imagery, estimating the timing of slope failures remains a challenge. Here we present a new integrated approach to dating landslides on a regional scale by augmenting quantitative surface roughness analysis with radiocarbon dating and numerical landscape modeling. We calibrate a roughness-age curve, which we use to date 25 deep-seated landslides in glacial sediment surrounding the catastrophic A.D. 2014 Oso landslide along the North Fork Stillaguamish River in Washington State (USA). Lidar bare-earth images show a high density of long-runout landslides in this region. Using our roughness-age curve, we estimate an average Holocene landslide frequency of 1 every 140-500 yr, and show that the 2014 Oso landslide was the latest event in an active history of slope failures throughout the Holocene. With each landslide, substantial sediment is delivered to the North Fork Stillaguamish River, driving shifts in the active channel that ultimately affect the pattern of landslides across the valley. The high frequency of landslides in this area, where river incision and isostatic uplift rates have dropped dramatically since peaking soon after ice retreated from the region, shows that landscapes inundated by glacial sediment do not require dramatic changes in base level to remain highly unstable for tens of thousands of years.