CONTROLS ON LANDSLIDE SIZE AND DEFORMATION HISTORY ALONG THE SOUTHERN BIG SUR COAST, CA

Central California’s Big Sur coastal region is home to numerous landslides that pose persistent hazards to life and infrastructure. Geologic observations of the 2017 Mud Creek landslide, one of Big Sur’s most catastrophic landslide events in recent years, reveal it is a part of a much larger and older landslide complex. This observation raises the question of how geologic context and a fuller landslide history may inform future hazard potential for landslides across Big Sur. Here we couple lidar-derived landslide mapping with satellite-derived landslide deformation rates to explore how the distribution of landslides and landslide complexes in southern Big Sur relate to their present kinematics.

We mapped over 3,000 deep-seated landslide deposits and headscarps within three watersheds adjacent to the Mud Creek landslide in southern Big Sur. These watersheds have a very high concentration of landslides, with an average of approximately 40 landslides per km², representing ~38% of the total map area. Along the coast, landslides are larger in area and show complex overlapping relationships, indicating large (~>10,000 m²) initial failures followed by reactivations within the original landslide deposit and in adjacent material.

Approximately 3-4 km inland, landslides form fewer complexes and become smaller and more uniform in size. This morphologic transition suggests a basin-scale control on landslide form and failure mode, potentially driven by changes in fluvial base level that modulate hillslope gradient and relief, and thus stability, throughout the watershed. Only 3.3% of the mapped landslides are actively deforming as measured by interferometric synthetic aperture radar, but these represent some of the largest landslides in the dataset (87^th percentile).

Further, nearly all active landslides are part of larger complexes within the high-relief (300-600 m) coastal zone, suggesting that this morphologic transition zone may also represent a process transition from single-event failures to seasonally deforming, multi-generational complexes. Overall, we find that many of the active landslides along the southern Big Sur coast exhibit complicated, multi-generational histories, and that a systematic variability occurs in landslide size and deformation process potentially driven by changes in fluvial base level that modulate hillslope relief and slope.