Prentice, Carol S.¹, Mann, Paul², Weber, John³, Crosby, Christoper⁴, and Peña, Luis R.⁵

¹U.S. Geological Survey, 345 Middlefield Rd, MS 977, Menlo Park, CA 94025, cprentice@usgs.gov
²Institute for Geophysics, University of Texas, Austin, TX, 78713
³Department of Geology, Grand Valley State University, Allendale, MI 49401
⁴Arizona State University, Tempe, AZ
⁵Avenue Cuesta Colorada, #2, Santiago, Dominican Republic

Throughout much of the Caribbean region, the major seismogenic structures are offshore, and inaccessible to traditional paleoseismic techniques. However, where onshore faults occur, excavations reveal faulted Holocene sediments that record paleoseismic events. In addition, Quaternary landforms reveal evidence for active tectonics. Over the the last decade, we have conducted paleosesimic research in the Domincan Republic, Puerto Rico, Trinidad, and most recently in Jamaica.

Our studies of the Septentrional Fault zone (SFZ) in the Dominican Republic show that the most recent rupture on the SFZ east of Santiago, in the central part of the Cibao Valley, occurred about 800 years ago, and was associated with a minimum of about 4 m of left-lateral strike-slip displacement. The penultimate event occurred after AD30, suggesting a recurrence interval of 800 to 1200 years. Studies of offset Holocene stream terraces suggest a SFZ slip rate of 6-12 mm/yr, indicating that this structure accommodates about half of the geodetically determined total plate-boundary motion of approximately 19 mm/yr.

The major structures of the North American-Caribbean plate boundary are offshore north of Puerto Rico. However, our mapping and paleoseismic studies demonstrate that repeated Holocene surface rupture has occurred on a previously unrecognized fault scarp in the Lajas Valley of southwestern Puerto Rico. Several excavations across this scarp exposed faulted alluvium and evidence for two, and possibly three earthquakes. Radiocarbon ages suggest that at least two earthquakes occurred on this fault in the last 5000 years.

Trinidad is located along the plate boundary between the South American and Caribbean plates. Geodetic studies suggest that a significant fraction of the total plate-boundary motion is being accommodated across the Central Range Fault in central Trinidad. Our excavations across the Central Range Fault indicate that the most recent surface rupture occurred within the last 2700 years, and prior to 550 years ago.