LIQUEFACTION AND HUMMOCK FORMATION IN THE 2014 OSO LANDSLIDE EVENT

Liquefaction of wet sediment dramatically decreases shear resistance and thereby promotes landslide mobility. Mechanisms that can generate liquefaction include cyclical loading, as during ground shaking, rapid loading, or shearing of loose, contractive sediment. The 2014 landslide near Oso, Washington created a variety of deposits, including an extensive debris-avalanche hummock field extending across much of the North Fork Stillaguamish River (NFSR) valley. Hummock formation entails ubiquitous extension in multiple directions, and typically involves more coherent materials sliding on a weak layer. Using field observations, we documented liquefaction features and their relation to hummocks.

We used pre- and post-slide aerial lidar to quantify landslide deposit thickness and hummock morphology. Hummocks varied from >20m tall near the pre-slide NFSR at the base of the slope to more widely spaced, <1m high blocks near the distal debris-flow deposit across the valley plain. Our field observations showed the hummocks were not liquefied; some contained intact pieces of glacial stratigraphy and rafted forest ferns on their surfaces. We also mapped >350 sand boils, classic indicators of liquefaction, as both isolated vents and fields of multiple vents. Boil diameters ranged from decimeters to >1m. Evidence of sand boils was transient; those identified during 2014 were destroyed by the following year. We also mapped slosh pits, where deposits of various lithologies formed “bath tub” rings in depressions between hummocks. These rings were often elevated in the direction of sliding, likely caused by sloshing during slide deceleration. Slosh pits commonly contained fields of sand boils.

We found that sand boils formed both subaerially and under ponds in depressions between hummocks, typically where deposits were <5m thick. Sand boils were scattered throughout the hummock field, but common near the transition between hummocks and debris-flow deposits. Numerous boils also erupted through >7m thick deposits, especially in distal areas with back-reflected debris-flow deposits, indicating that basal pore pressures remained elevated far from the landslide source. For this slide, basal liquefaction enhanced hummock formation and transport.