CAPTURING 3-D DISPLACEMENTS IN ACTIVE LANDSLIDES USING GPS

Quantifying the timing, location, and velocity of ground displacement is critical for assessing the hazards posed by active landslides. We have used two styles of GPS surveying to capture 3-D ground deformation in landslides: (1) near-real-time monitoring of known active slides to evaluate temporal variations in movement and (2) repeat campaign-style static surveys to distinguish spatial patterns of movement. Both styles use post-processed short base-line (< 3 km) differential measurements to obtain repeat solutions of within mm to a few cm. For near-real-time deformation monitoring, we have developed a low-cost L1-only GPS receiver system designed for automated data acquisition, rapid deployment, and prolonged operation in remote hazardous areas. We tested this system on a large rock mass (~120,000 m ³) that moves episodically in the headscarp of the Mission Peak landslide near Fremont, California. Over the wet winter of 1998, this rock mass accelerated to over 20 mm/day². Since 2000, we have monitored this mass at 30-minute intervals using our GPS system and radio telemetry. During a 4-month period in 2000, cumulative movement of about 70 mm was recorded with velocities ranging up to 2 mm/day. Over the subsequent 4 years, the mass displaced only about 5 mm with small accelerations in the winter or early spring of each year.

To assess long-term movement patterns across large areas sculpted by landslides with differing activities, we have conducted repeat static surveys using L1/L2 GPS receivers. The southern Big Sur coast in California is rich in slow-moving landslides, with an areal slide density of about 53%. Since 2002, we have monitored the 3-D displacement of 34 monuments installed along a 4-km section of this coastline through Lucia, California; this traverse includes both perennially active and currently dormant landslides. Between 2002 and 2004, this region was affected by rainstorms and the nearby M6.5 San Simeon earthquake. Our repeat surveys show that movement ranged from about 2 mm/day for the most active slides, to slight creep in some of the less active slides, to insignificant displacement in other regions. Both our GPS surveying approaches can identify 3-D movement vectors and distributed strain across large areas, and both techniques allow improved landslide hazard assessments.