Earthflow kinematics

Slow-moving landslides (earthflows) are common in hilly areas with fine-grained mechanically weak rock. These landslides are an important mechanism of hillslope lowering, can transport large amounts of sediment from hillslopes to river systems, and present a persistent natural hazard. Despite their importance, what controls the spatial distribution of earthflows remains poorly constrained. We use airborne interferometric synthetic aperture radar (InSAR) to document 150 active earthflows along the central, creeping portion of the San Andreas Fault, California, USA. The earthflows display seasonal movement in response to winter rainfall as is characteristic of slow-moving landslides. While our data extends up to 10 km from the fault, approximately 75% of detected landslides occur within 2 km of the trace of the active fault. Topographic, precipitation, seismic, and lithologic metrics alone are not enough to explain the observed spatial distribution of earthflows. Instead, we hypothesize that earthflow occurrence proximate to the creeping San Andreas Fault is limited by the width of the fault damage zone, where rock is weak and highly fractured. We suggest that north of the creeping section, increased precipitation promotes earthflow occurrence at distances further from fault damage zones where there is suitable rock type, but the spatial distribution of earthflows may be partially modulated by large magnitude earthquakes. Such earthquakes episodically trigger co-seismic catastrophic landslides, removing material that could potentially develop into earthflows. Our analysis suggests that the necessary conditions for earthflow formation in central California include some combination of reduced rock strength, threshold precipitation, southwest facing aspects, fine-grained sedimentary rock, and possibly the absence of large magnitude earthquakes.