Field and Remote-Sensing Evidence for Hydro-mechanical Isolation of a Long-Lived Earthflow in Central California

Abstract

Persistent motion of slow-moving landslides has been linked to entrapment of water in slide masses by weak, low-permeability shear zones at their basal and lateral margins. This so-called “bathtub effect” should have remotely sensible effects on soil moisture and vegetation health. Here we assess this effect at a seasonally active earthflow in northern California with analysis of soil properties and nine years (2009–2018) of multispectral satellite imagery. The shear zone has low hydraulic conductivity, and the earthflow shows elevated water content relative to its surroundings. Spectral indices suggest that the earthflow is perennially wetter, and its vegetation thrives for longer into the dry season as a result. The magnitude of the bathtub effect is correlated with cumulative multiyear rainfall, and its surface expression disappears during the driest year of the 2012–2015 drought. These findings demonstrate a new method for identifying and monitoring mechanical and hydrological interactions that enable persistent landslide motion.