Active movement of the Cascade landslide complex in Washington from a coherence-based InSAR time series method

We used interferometric synthetic aperture radar (InSAR) data from the ALOS-1 satellite to investigate the active movement of landslides along the Columbia River Gorge from 2007-2011. To increase the spatial extent of the deformation signal, we developed a coherence-based small-baseline subset (SBAS) method and derived the time series of the landslide movement. We find no significant movement on the previously mapped Washougal, Sakamania, Oregon Shore, and Wind Mountain landslides. However, we document the slow movement of the Redbluff landslide, which is part of the Cascade Landslide Complex. The InSAR data reveal up to approximately 25 cm of line-of-sight displacement over the 4-year observation period. The Redbluff landslide is seasonally activated, with accelerated movement observed during the early winter of each year. The amplitude of the seasonal signal is variable from year-to-year, with the greatest movement observed in early winter of 2009 and 2011. To assess the triggering mechanisms, we compared the displacement time series data with local precipitation data. The deformation correlated well with precipitation, implying the slow movement is controlled by rainfall that infiltrates to the base of the slide.