UC Santa Cruz

The persistent movement of deep-seated, slow-moving landslides is a common phenomenon with important scientific and practical implications. These landslides, often referred to as earthflows, can dominate erosion in mountainous landscapes, control the long-term evolution of catchment geometry, and cause progressive damage to infrastructure and property over time. The role of earthflows in shaping landscapes is amplified by their longevity. Historical observations demonstrate that they are commonly active for decades or centuries, while geologic and geochronologic evidence further suggests that some may persist in intermittently-active states for millennia. Slow, persistent motion over these timescales requires complementary mechanisms to both repeatedly reactivate previously failed materials and to limit velocity during periods of activity. More than 30 years of robust scientific investigations into the nature of these mechanisms has yielded tremendous insights. These include the sensitivity of earthflows to rainfall-driven changes in pore-water pressure, the existence of internal feedbacks that limit earthflow velocity after the onset of motion, and the recognition that external factors, like climate and base-level, help set the periodicity of intermittent earthflow movement. A common goal of all these investigations has been to generate data and models that will allow for the prediction of future earthflow behavior, which remains challenging despite many advances. Predicting the velocity of slow landslides over multi-decadal timescales requires an understanding of the potential drivers of earthflow motion, analytical frameworks and models to connect those drivers to kinematic response, and historical records of movement with sufficient spatial and temporal resolution to test model predictions. Predicting velocity on shorter timescales, such as the onset of motion in a given wet season, requires high-temporal resolution, time-series measurements of the hydro-mechanical properties of the earthflow in question. And lastly, scaling up predictions of future velocity at one earthflow to many will require the use of high-spatial resolution, remotely-sensed imagery capable of monitoring large regions with repeat measurements. This dissertation aims to contribute to each of these requirements through an extended study of Oak Ridge earthflow, a long-lived, slow-moving landslide in California’s northern Diablo Range. In Chapter 1, we used a time series of aerial imagery to assemble an 80-year kinematic history for Oak Ridge earthflow. We find that spatial patterns of earthflow velocity 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 timescales. In Chapter 2, we use 10 years of high-spatial resolution, multispectral satellite imagery to assess the long-accepted, but relatively untested, hypothesis that hydro-mechanical isolation of earthflows from surrounding, stable slopes contributes to their ability to fail persistently over time. In Chapter 3, we use of 2.5 years of high-temporal resolution records of rainfall, pore-water pressure, and displacement to argue that the initial onset of earthflow acceleration in response to seasonal rainfall is likely to be more closely related to the timescales of vadose zone processes than pore-water pressure diffusion through saturated soil, as has been previously suggested. Collectively, these studies provide new observations and approaches towards understanding earthflow behavior at multiple scales.