TIMING OF LONG-RUNOUT LANDSLIDES NEAR OSO, WA REVEALS PERSISTENT POSTGLACIAL HILLSLOPE INSTABILITY

On March 22, 2014, 8 million m³ of glacial sediment collapsed and surged across the North Fork Stillaguamish River near Oso WA, killing 43 people. Widespread landsliding in this region is evident in LiDAR bare-earth imagery, but ages of these events remain unknown. Establishing a landslide chronology is the first step in advancing from hazard recognition to risk assessment, and is crucial to understanding landscape evolution in this region. In this study, we integrate radiocarbon dating, quantitative roughness analysis, and numerical modeling to constrain the timing of these long-runout landslides. Using a roughness-age curve, we estimate an average Holocene landslide recurrence frequency of 140 - 500 years, and show that the Oso Landslide was the latest event in an active history of slope failures that likely began soon after glacial retreat ~16,000 years ago. These landslides deliver substantial sediment to the North Fork Stillaguamish River, shifting the active channel dramatically, and limiting long-term river incision. The frequency of landslides here, where both uplift and incision rates are low, shows that regions inundated by glacial sediment do not require tectonic or climatic forcing to remain highly unstable for thousands of years after ice retreat. Our results also demonstrate that surface roughness, when combined with even a limited set of absolute ages, is a reliable indicator of landslide age that can be used to construct regional landslide chronologies.