HR: 0830h
AB: The island of Puerto Rico is located within a diffuse and complex plate boundary zone between the North American and Caribbean plates. The relative motion along this boundary is dominantly left-lateral strike slip, but compression and extension are locally significant. Although tectonic models proposed for the region predict the presence of onshore active faults in Puerto Rico, no faults with Holocene displacement have previously been documented on the island, and current seismic hazard assessments only consider the impact of distant, offshore seismic sources. Our new mapping and trenching studies demonstrate that repeated Holocene surface rupture has occurred on a previously unrecognized fault in southwestern Puerto Rico. The Lajas Valley of southwestern Puerto Rico is an E-W trending, 30-km-long, linear depression bounded by abrupt mountain fronts on its northern and southern edges. This geomorphology and the closed drainage of this valley suggest fault control, and therefore we chose this region to focus our studies. From analysis of aerial photographs and field reconnaissance, we identified a roughly east-west trending scarp crossing an alluvial fan on the southern side of the Lajas Valley. We excavated a 2-meter-deep trench across this scarp that exposed two fault zones, about 1m apart, disrupting the alluvial deposits. Structural relations indicate normal fault slip, valley-side down, with a component of strike-slip motion, though we could not determine whether the horizontal displacement is right or left lateral. The two fault zones terminate at different stratigraphic horizons, indicating at least two surface-rupturing events. Radiocarbon analyses of organic material collected from the sediments suggest that these two earthquakes occurred during the last 7500 years. The earlier event occurred between 5650 and 7550 Cal. years B.P. The younger event occurred post 5040 Cal. years B.P., but no minimum age has yet been established. This fault may be part of a system of faults that extends from the western end of the Lajas Valley toward Ponce, the second largest city in Puerto Rico. Additional studies are needed to assess the seismic hazard associated with this fault.
AN: T11B-10
TI: Prehistoric Earthquakes Associated with a Late Quaternary Fault in the Lajas Valley, Southwestern Puerto Rico
AU: * Prentice, C S
EM: cprentice@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd MS 977, Menlo Park, CA 94025 United States
AU: Mann, P
AF: Institute for Geophysics, University of Texas, Austin, TX 78713 United States
AU: Burr, G
AF: NSF Accelerator Facility, University of Arizona, Tucson, AZ 85721 United States
DE: 7221 Paleoseismology
SC: T
MN: Fall Meeting 2000