Widespread initiation, reactivation, and acceleration of landslides in the northern California Coast Ranges due to extreme rainfall

Episodically to continuously active slow-moving landslides are driven by precipitation.
Climate change, which is altering both the frequency and magnitude of precipitation world21
wide, is therefore predicted to have a major impact on landslides. Here we examine the
behavior of hundreds of slow-moving landslides in northern California in response to large
changes in annual precipitation that occurred between 2016 and 2018. We quantify the
landslide displacement using repeat-pass radar interferometry and pixel offset tracking
techniques on a novel dataset from the airborne NASA/JPL Uninhabited Aerial Vehicle Synthetic Aperture Radar. We found that 312 landslides were moving due to extreme
rainfall during 2017, compared to 119 during 2016, which was the final year of a historic
multi-year drought. However, with a return to below average rainfall in 2018, only 146
landslides remained in motion. The increased number of landslides during 2017 was primarily accommodated by landslides that were smaller than the landslides that remained
active between 2016 and 2018. Furthermore, by examining a subset of 51 landslides, we
found that 49 had increased velocities during 2017 when compared to 2016. Our results
show that slow-moving landslides are sensitive to large changes in annual precipitation,
particularly the smaller and thinner landslides that likely experience larger basal pore
water pressure changes. Based on climate model predictions for the next century in California, which include increases in annual precipitation and increases in the frequency
of dry-to-wet extremes, we hypothesize that there will be an overall increase in landslide
activity.