CREEPING, CHARGING, AND CESSANT HILLSLOPES: INVESTIGATING ENVIRONMENTAL CONTROLS ON EARTHFLOW OCCURRENCE AND ACTIVITY NEAR HOLLISTER, CA

In tectonically active mountainous regions with weak and clay-rich bedrock and seasonally varying precipitation, earthflows are common occurrences. Such conditions are prevalent along the creeping section of California’s San Andreas Fault, which includes our study area south of Hollister, CA. It is well understood that the amount of annual rainfall controls the rate of displacement of a slide, however, the absolute activity of one slide to another can vary significantly. This research aims to use remote sensing, field survey methods, and geospatial software, such as image correlation techniques like IMCORR in order to resolve the degree of influence of geological and environmental variables on landslide movement and sediment delivery to stream networks that lead to this difference in absolute activity. Using repeat LiDAR and drone photogrammetry, volumetric surface changes resulting from slide motion and gullying can be revealed, allowing us to quantify sediment transfer by earthflows to streams. Research on two earthflows in the Hollister Hills SVRA indicates a strong relative correlation between slide motion and extreme wet and dry seasons at the decadal scale, and significant variance in absolute sediment delivery and displacement was observed between the two earthflows, likely due to different drainage structures. Preliminary results from comparing movement vectors generated with IMCORR to physical ground pegs shows promise in IMCORR’s ability to quantify earthflow movement, permitting the analysis of more earthflows. By expanding our dataset, we aim to enhance our ability to investigate factors that contribute to variable behavior of earthflows as a whole and their sedimentological connectivity to stream channels.