EVIDENCE FOR LATE QUATERNARY FAULTING OF THE SOUTH-CENTRAL COASTAL PLAIN OF PUERTO RICO

As a result of a seismic hazard assessment for the Puerto Rico Electric Power Authority, a number of west- and northwest-trending lineaments on the south-central coastal plain of Puerto Rico have been identified as late Quaternary active faults. LiDAR imagery was collected along an 11-km-wide and 60-km-long strip from just east of Punta Arenas continuing west past Juana Diaz. The imagery shows two zones of lineaments that cut across topography and are coincident with topographic scarps on Quaternary deposits. To the north, the relatively straight, more northwest-trending lineaments are coincident with large bends in streams suggestive of right-lateral strike-slip offset. Where these lineaments cross generally southward flowing streams there are dramatic changes in the channel geometry (width and slope), the depth of channel incision, thickness of channel alluvium, the preservation of stream terraces, and the evolution of some drainages against the regional slope. This zone of lineaments coincides closely with the mapped trace of the Great Southern Puerto Rico Fault Zone (GSPRFZ).

To the south, the more westerly-trending lineaments form a sinuous ~0.5-km-wide zone composed of two or more sub-parallel lineaments. This zone is coincident with the Salinas fault zone (SFZ) and continues to the east and appears to merge with the mapped trace of the GSPRFZ. The northernmost lineament is a nearly continuous and consistently south-facing topographic scarp. The southernmost lineaments are discontinuous, with both north- and south-facing scarps. Topographic profiles constructed across the zone consistently display up to 2 to 3 m of displacement. Excavations across the scarps expose displace late Quaternary deposits. Evidence points to at least two late Quaternary surface rupturing events with both vertical and lateral motion.