Predicting the velocity of slow landslides over multidecadal time scales requires an understanding of the potential drivers of landslide motion, analytical frameworks or models that connect those drivers to kinematic response, and historical records of movement that have sufficient spatial and temporal resolution to test model predictions. Given that detailed long-term records of slow landslide movement are still relatively uncommon, we used a time series of aerial imagery to assemble an 80 yr (1937−2017) record of movement for the Oak Ridge earthflow, a slow-moving landslide in California’s northern Diablo Range. Landslide movement was unsteady and nonuniform during this period. We used this history of movement, visual evidence from imagery and the field, and a record of surface moisture balance (the Palmer Drought Severity Index) to evaluate the relative roles of possible drivers of movement. Drivers considered included: supply of mobile regolith from the landslide source zone, fluvial debuttressing from below, changes in climate forcing over time, and changes in failure plane orientation in space. Specifically, we found that spatial patterns of earthflow velocity within the transport zone, where the bulk of movement occurred, were controlled largely by the slope of the underlying failure plane, whereas temporal patterns were governed largely by climate-driven changes in surface moisture balance (PDSI) at the annual-decadal scale. Declines in sediment supply acted as a secondary control on temporal velocity variations over our study period; however, the influence of this driver likely grows at longer time scales. Evidence for transient waves of motion in response to fluvial debuttressing was limited to the toe, and continued monitoring of Oak Ridge earthflow is required to determine whether those perturbations in sediment flux will migrate further upslope.

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