MODELING 3D SURFACE DISPLACEMENTS AND FAILURE PLANE SURFACE OF EARTHFLOWS: THE MILL GULCH EARTHFLOW, CA

Earthflow movement rates span over many orders of magnitudes. The pore pressure on their failure planes, influenced by seasonal wet weather, are one of many factors that control their motion. Specific conditions at the failure plane, where the balance of driving stress and frictional resistance control motion, remain largely unknown due to lack of direct access. We document 3D surface displacement fields for the Mill Gulch Landslide, California to infer its 3D failure plane geometry over several-year time scales. We derived displacements for the landslide from 2003 to 2013 at 3-4 year intervals with normalized cross correlation of high resolution LiDAR. The landslide’s source zone tends to move uniformly and continuously at ~1 m/yr while the transport zone moves faster at ~5 m/yr and is more variable. Movement at the toe of the landslide is highly variable, and one lobe stopped completely then reactivated. We used the principle of conservation of mass to invert the two most recent displacement fields for the landslide’s failure plane. The source zone failure plane was located at a consistent depth in both time periods and its location and extent changed in the transport zone and toe regions. While inferring failure plane geometry qualitatively is standard engineering geology practice, using repeat LiDAR through a 6-year period on one site is a significant advance in quantifying failure plane geometry at dramatically improved spatial and temporal resolutions. This improved resolution revealed that surface morphology doesn’t always reflect considerable differences in displacement rates, and large, active patches of slow-moving landslides can vary from yearly.