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

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. Plain Language Summary The ability of some landslides to move slowly and persistently for many years without accelerating catastrophically may be linked to entrapment of rain water in slide masses by slow-draining boundaries at their sides and bottoms. The existence of this so-called bathtub effect can be tested for directly with soil analysis but should also have visible effects on soil moisture and vegetation health that can be measured remotely using satellite imagery. We examined a seasonally active landslide in northern California with slow-draining boundaries and elevated water content relative to its surroundings. Nine years (2009-2018) of satellite imagery suggest that the earthflow is perennially wetter and its vegetation thrives for longer into the dry season as a result. The magnitude of this effect is correlated with cumulative multiyear rainfall. These findings demonstrate a new method for identifying and monitoring the hydrology and movement of persistent landslides.