PREDICTING THE ONSET AND LIMITS OF SEASONAL REACTIVATION IN EARTHFLOWS FROM VADOSE ZONE HYDROLOGY

Earthflows in the Franciscan mélange of northern California are large, slow-moving landslides that seasonally reactivate from rainfall. This reactivation occurs as rainfall infiltrates through the vadose zone and drives up destabilizing pore water pressures at the landslide base. Prediction of earthflow reactivation is complicated by these highly variable unsaturated fluxes that depend on both soil hydraulic properties and available moisture storage. Furthermore, the consistent creeping of Franciscan earthflows sets them apart from other landslides in mélange that can surge or fail catastrophically. Here we seek to understand the extent to which the hydrologic characteristics of earthflows both define their reactivation timescale and limit their deformation. Comparing the seasonal rainfall flux to the moisture-dependent hydraulic conductivity of a landslide, which we call the soil moisture pulsivity (P), we predict that earthflows in Franciscan mélange define an endmember behavior where the slow hydraulic conductivity of the vadose zone accumulates water from many storms into a single wetting front. This impacts seasonal reactivation as the wetting front should nearly saturate the landslide upon its arrival, creating a bimodal saturated-unsaturated system where motion directly follows wetting front arrival. Limited basal pore pressure range in the saturated phase should therefore act to throttle landslide velocity. To test these hypotheses, we use a five-year record of rainfall and piezometer records from the Oak Ridge earthflow in California’s Diablo Range. Time series of pore-water pressure consistently show a single large spike midway through the rainy season that immediately precedes reactivation, followed by small-scale concurrent rises in both pore pressure and motion tied to individual storms. This suggests that the timescale of seasonal activity for earthflows relates directly to the time needed to saturate the vadose zone, and we illustrate this with a seasonal rainfall intensity-duration curve based on an analytical unconfined aquifer model. The seasonally creeping and self-throttling nature of earthflows therefore directly results from the low-pulsivity characteristics of their unsaturated zone hydrology.