Slow-moving landslides over the U.S. West Coast: InSAR-derived inventory and geologic controls

Slow-moving landslides, often with nearly imperceptible creeping motion, are an important landscape shaper and a dangerous natural hazard across the globe, yet their spatial distribution and geologic controls are still poorly known owing to a paucity of detailed, large-area observations. Here we applied InSAR (Interferometric Synthetic Aperture Radar) processing on approximately 7,000 scenes of L-band satellite SAR images that cover the populous U.S. West Coast consisting of Washington, Oregon, and California states, and revealed 617 spatially large (410413106 m2) and presently active (20072019) slow-moving landslides. Only 4.6% of these revealed landslides are included in the currently existing, non-systematically mapped landslide database of the United States, potentially because many slow-moving landslides lack readily identifiable features (e.g., fresh headscarps) or occur in rarely accessed lands. Additionally, we found that slow-moving landslides are generally larger and more spatially frequent in homogeneous bedrock with low rock strength, and they are preferentially located on hillslopes with geologically recent uplift. Notably, landslide size and spatial density in the relatively weak metamorphic rocks and mélange, which have pervasive tectonically sheared discontinuities, foliation, and abundant clay minerals, were two times larger than those in sedimentary and igneous rocks. The hillslopes with landslides were found to be uplifting approximately three times faster than the average for the whole region. About 75% of the identified landslides were distributed over the mountainous terrains with relatively abundant precipitation; however, the landslide occurrence cannot be accurately predicted by a fixed rainfall threshold across variable regions. To summarize, these results suggest that slow-moving landslides can be effectively uncovered by the L-band InSAR and their occurrence and character may be anticipated from vertical land uplift and bedrock lithology.