An examination of seasonal deformation at the Portuguese Bend landslide, southern California, using radar interferometry

The Portuguese Bend landslide, located on the Palos Verdes peninsula in southern California, is a slow-moving landslide whose activity is closely tied to infiltration from rainfall. We use radar interferometry to constrain the seasonal kinematics of the landslide from 1995 to 2000. Traditional interferometric synthetic aperture radar methods estimate an average downslope displacement rate of 1.00 m/yr with an uncertainty range of 0.39-2.57 m/yr during the summer months. Interferograms show a consistent loss in phase coherence over the landslide for epochs greater than 6 months, especially in the winter. Interferograms become incoherent as a result of large displacement, and decorrelation is used to map the spatial extent of slide movement through time. The displacement rate increases sharply several weeks after the beginning of the rainy season as rainwater percolating into the slide elevates pore pressures. Phase coherence is lost when the slide velocity surpasses similar to 2.6 m/yr for interferograms with durations of a few months. A physically based model based on rainfall-driven landslide movement incorporates these observations to constrain hydraulic diffusivity to between 0.9 and 3.5 x 10(-6) m(2)/yr.