ACTIVITY LEVELS IN LANDSLIDE-RICH TERRAIN: DECIPHERING MOVEMENT BEHAVIOR USING GPS AND InSAR ALONG THE BIG SUR COAST, CALIFORNIA

Landslide maps often show large regions of potentially hazardous slides, yet assessments of their activity levels typically rely on imprecise interpretation of geomorphic features. One striking example of landslide-rich terrain is the Big Sur coast of California; here, the entire Highway 1 corridor through the town of Lucia crosses a series of contiguous landslides that toe at the beach. A few of these slides demand perennial road maintenance although most are mapped as dormant. The entire coastline is vulnerable to intense winter precipitation and wave attack from large storms; all of the slides are underlain by Franciscan mélange, a mix of argillites, graywackes, greenstones, meta-gabbros, and serpentine. To quantify slide displacement rates and spatial patterns of movement, we have used biannual (spring and fall) repeat static GPS surveys since 2002 and ALOS (L-band) satellite interferometric synthetic aperture radar (InSAR) analysis since 2006. We obtained 3-D GPS positions of 36 monuments to within a few cm using post-processed, short baseline (< 3 km) differential solutions.

GPS measurements, over 8 years, indicate that only a few sites in this corridor are stable; almost all have moved down slope towards the coast at least a few cm and some over 10 m. We divided activity into four groups based on average long-term velocities: slow, very slow, extremely slow, and relatively stable. The highest velocity slides (designated slow) had long-term velocities of 1-2 m/yr, but accelerated to as much as 7m/yr after two consecutive wet years. Very slow slides (commonly 0.02 to 0.2 m/yr) also accelerated during wet years. Although wave attack destabilizes some slides during large winter storms, more slide displacement occurs during the spring-fall interval, suggesting that long-term groundwater patterns control overall motion. Long-term velocities obtained from GPS helped guide the selection of time intervals (ranging from months to years) for InSAR analyses, which delineate the boundaries of the slow and very slow moving slides. The spatial pattern of slide activity likely reflects a complex subsurface distribution of strength, hydraulic properties, and groundwater flow. Only by long-term displacement monitoring through both wet and dry years can we accurately characterize the variability of movement behavior.