Use of seismic refraction techniques for investigating the morphology of recent landslides in a tropical rain forest in Puerto Rico

U.S. Geological Survey GSA Center 651 Federal Drive Guaynabo, Puerto Rico 00965-5703, USA

Abstract

Surveys were conducted on two recent rotational failures (earth slumps) that damaged a highway in the Caribbean National Forest. Seismic refraction surveys on the recent landslide surfaces permitted a rapid and less expensive investigation than test drilling of the dimensions and depth of failure planes. The accuracy of refraction data was verified with field observations.

Seismic velocities of overlying colluvium and saprolitic soils at a one- year-old slump are 590 meters per second. Underlying weathered bedrock velocities are 1888 meters per second. The thickness of colluvium ranged from 3 to 5 meters on the toe of the slump, and the thickness of saprolitic soil above the head scarp ranged from 4 to 8 meters. An approximately 10-year old slump has a velocity of 790 meters per second in the colluvium and saprolitic soil. Seismic velocity in the weathered bedrock is 1646 meters per second. Quartz-diorite bedrock, reached only at this site, had a seismic velocity of 6380 meters per second. The depth to bedrock from seismic data was calculated to be 12 meters, which compared well with bedrock exposure at 11 meter depth in an adjacent slump.

The morphology of the 10-year-old slump, which is still active, but has been filled in to repair the highway, was revealed in a U-shaped velocity contact, which appears to be a potential failure surface. This surface is approximately 7 meters deep; the width tapers from 11 meters at the top down to 2 meters. This U-shaped morphology, manifested by a seismic velocity contrast, matches the lateral dimensions of an earlier failure at the same site evidenced by distinct differences in age and type of vegetation. Additionally, it lies below, and is of the same lateral extent as downslope-curving cracks in the road surface. These surveys, involving only a few days of field time, demonstrate the economy and usefulness of seismic refraction techniques in landslide investigations.

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