Tracking differential landslide motion on the Big Sur coast with SfM topography

Active mountain-scale rockslides on the rugged Big Sur Coast in Central California sculpt the western escarpment of the transpressional Santa Lucia Mountains and routinely damage the Pacific Coast Highway (PCH) transportation route. We test the utility of repeat fixed-wing photogrammetric surveys for evaluating the style and pace of three-dimensional (3-D) landslide displacements in response to heavy rainfall and episodic intense wave erosion of heterogenous Franciscan Complex rocks. We construct a sequence of high-resolution topographic models using 4-D Structure-from-Motion (SfM) techniques to co-align oblique image sets (20 total) collected by the U.S. Geological Survey from January 2017-June 2018 and California Coastal Records Project from 2002-2015. The resulting point clouds (with density up to 23 points/m²) capture spectacular details in the evolving landscape along the PCH, including the Paul's Slide / Hermitage Hill complex and the Dani Creek landslide near Lopez Point, and the Mud Creek landslide located 19 km to the south. Digital elevation models (DEMs) of difference show areas of progressive ground-surface change resembling deep-seated deformation. Basal slip surfaces appear to sole near the beach, consistent with field and borehole observations. Secondary deformation of the nested Paul's Slide, however, is visible only upslope of the PCH in highly weathered bedrock. We track the changing position of rocks and other visible features on landslide surfaces to reconstruct a time series of 3-D ground-surface displacement fields for the January-July 2017 period. Repeat differential GPS surveys corroborate observed velocity patterns and complex trends computed from SfM point clouds. Averaged velocities for Hermitage Hill ( 1 m/mo) and Paul's Slide ( 3 m/mo) indicate well-coupled deformation with irregular rates over the study period. Downslope motion of Paul's Slide typically outpaces the larger underlying Hermitage Hill by a factor of 2-4. Our results suggest two stages of acceleration following peak annual rainfall, and the latter is roughly contemporaneous with failure of the Mud Creek landslide on May 20, 2017. This displacement timing is substantially out-of-phase with patterns of cumulative rainfall and suggests a broadly synchronous delay in elevated pore-fluid pressures.