HR: 1340h
AN: S23B-0251    [Abstracts]
TI: Late Quaternary Activity on the Cerro Goden Fault, Puerto Rico and Limitations of High-resolution Seismic Reflection/refraction Data for Trench-scale Investigations
AU: * Zachariasen, J
EM: Judy_Zachariasen@urscorp.com
AF: URS Corporation, 1333 Broadway Suite 800, Oakland, CA 94612
AU: Prentice, C S
EM: cprentice@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94035
AU: Schweig, E
EM: schweig@usgs.gov
AF: U.S. Geological Survey, 3876 Central Avenue, Suite 2, Memphis, TN 38152
AU: Abreu, R
EM: RCAbreuParis@netscape.net
AF: University of Puerto Rico, PO Box 9000 Calle Post, Mayaguez, PR 00681
AU: Rymer, M J
EM: mrymer@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94035
AU: Catchings, R D
EM: catching@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Rd, Menlo Park, CA 94035
AB: The Cerro Goden Fault runs along the base of the La Cadena de San Francisco range north of Añasco in western Puerto Rico, and joins with the Great Southern Puerto Rico Fault zone. This fault zone disrupts Eocene and older rocks but its Quaternary history is unclear. It bounds the northern Añasco Valley, which is seismically active. Offshore investigations suggest the fault cuts the sea floor (Grindlay et al., 2005) and an onshore reconnaissance study by Mann et al. (2005) found geomorphic evidence of Quaternary displacement. We gathered high-resolution seismic reflection/refraction data along two lines that crossed the projection of the fault in the Añasco Valley to help identify possible fault strands in the near subsurface. The seismic data were gathered using a shoot-through acquisition method with a BETSY-Seisgun shooting source consisting of 8-gauge, 400 grain shotgun blanks set in holes drilled about 30 cm below the ground surface. Both lines had shot and geophone spacings of 5 m. Common Depth Point (CDP) spacings for both lines were 2.5 m in the shallow section of the profiles. We used the results from the seismic lines to site six trenches where the data revealed discontinuities, possibly attributable to faulting, within the upper 2-3 meters. Our trenches ranged in depth from 2 - 5 meters, but in none of the trenches did we find evidence of surface faulting. A mountain-front-facing subsurface scarp, which may have been produced by older Quaternary faulting, appeared in several trenches. Holocene fluvial sediments have been deposited against this scarp and have buried it, but are unfaulted. Our results suggest either that the Cerro Goden Fault has not been active since the deposition of the Holocene fluvial sediments or that the high-resolution seismic data were not adequate to define the location of fault traces to the precision necessary for trench investigations. Both the seismic data and the buried scarps suggest there may be faults at depth, but these structures do not, as the seismic data suggest, break the upper 2-3 meters of sediment at our trench sites.
DE: 7221 Paleoseismology (8036)
SC: Seismology [S]
MN: Fall Meeting 2005