USING GPS AND GIS TO PRODUCE A LANDSLIDE FRACTURE MAP: OBSERVED DEFORMATION OF LANDSLIDE SURFACES AS INDICATORS OF MOVEMENT PROCESSES

The Salmon Falls landslide with an area of ~0.25 km2 is an active, deep-seated, canyon rim slide located west of Buhl, Idaho on the western Snake River Plain. Movement of the landslide has produced a myriad of fractures on the ground surface. This research project has produced a landslide fracture map using GPS and GIS. The fracture map provides documentation of the distribution, orientation and growth rates of surface fractures on the slide, which can provide insight into landslide kinematics and stability analysis. The types of deformation that are currently monitored are the distribution and growth of individual cracks, as well as the displacement of points on the landslide surface measured with both GPS and laser theodolite surveying.

The motion of landslide fractures is analogous to that of tectonic faults. There are extensional fractures in the toe area of the landslide, which is impinging on Salmon Falls Creek, a major tributary to the Snake River. Extension is occurring because the toe is rising due to rotational movement of the landslide and subsequently collapsing into Salmon Falls Creek. Fractures with strike-slip motion mark the southern boundary of the landslide. The main landslide block is breaking away from the canyon wall, subsiding and forming large fractures resembling normal faults.

Results from the fracture map combined with the theodolite and GPS surveys indicate that the landslide resembles a rotational style failure. GPS monitoring results, gathered since February 2003, confirm that the main headwall block is slowly subsiding at a rate of 20 cm/yr, and that the toe is slowly rising at the rate of 8 cm/yr. Prior to the deployment of the GPS units, laser theodolite results indicate that the landslide was moving more rapidly, at an average rate of 200 cm/yr between 2001 and 2002. The motion of the landslide is complex and the slide may contain elements of both rotational and translational failure, as well as multiple failure planes. A continuation of the theodolite and GPS surveys combined with observed surface deformation will elucidate the geometry of landslide motion.