Variation in Geologic and Topographic Setting Cause Spatial Correlations Between Hillslope Failure Mechanisms in the Ridge Basin, California

A large number of mass movements driven by various failure mechanisms exist within the Ridge Basin region of Southern California. These landslides pose a risk to important transportation and utility corridors as well as residential areas. The field area is also directly within the restraining bend of the San Andreas Fault, and thus subject to high rock uplift rates and seismic activity. Bedrock is composed of terrigeneous clastic sediments deposited in a Late Neogene trans-extensional basin. These units are weakly lithified and have been tilted during syn- and post-depositional folding, creating a steep ridge and valley topography. Previous landslide studies in the area have focused on understanding the mechanics of individual slides or developing static landslide inventories along selected corridors. Studies up to now have not provided a regional synthesis of the how the variable failure mechanisms and triggers could be spatially correlated. In this study, we use a LiDAR derived DEM and low-altitude aerial photography collected in the fall of 2007 to map landslide features in a GIS. The position and morphologic attributes of ~2500 slides are identified. We combine field evidence and topographic form to infer failure mechanism. These mechanisms are then spatially correlated with forcing factors such as lithologic units, soil properties, geologic structures, topographic form and human disturbances. These spatial controls will be used in developing a hillslope stability model for each failure mechanism as well as creating an integrated hillslope stability model for the region.