Holocene Surface Ruptures On The Salinas Fault And Southeastern Great Southern Puerto Rico Fault Zone, South Coastal Plain Of Puerto Rico

We analyzed light detection and ranging (lidar) data and aerial photography to locate active faults near the south coast of Puerto Rico and excavated paleo-seismic trenches across the Salinas fault and the Great Southern Puerto Rico fault zone (GSPRFZ). We document evidence for two Holocene surface-rupturing earthquakes along both faults. Two earthquakes on the Salinas fault occurred after the deposition of sediments that are 7400-10,400 yrs old. No quantitative ages constrain the timing of the two earthquakes on the southeast GSPRFZ, but we interpret both events to have occurred during the Holocene or latest Pleistocene, based on the similarities in the characteristics of the faulted sediment and the development of soils exposed in both trenches. Stratigraphic and geomorphic evidence suggests components of both vertical and lateral slip on both faults. These results show that onshore active faults in Puerto Rico are more common than previously recognized and highlight the need for additional study to search for Holocene active faults and for an updated seismic hazard analysis for the island.