Here we present a seismic source model for Puerto Rico, designed for use in probabilistic ground motion hazard analyses. The model consists of characterizations of known on- and offshore faults and their estimated geometries, the magnitudes of maximum earthquakes, one or more recurrence models for each fault, and activity rates for randomly occurring upper-crustal seismicity beneath the island. Slip rates for faults in the Mona and Anegada Passages were estimated by the allocation of GPS-based regional horizontal geodetic vectors onto appropriate groupings of faults. Rates for other sources were based on GPS vectors or empirical fits to Puerto Rico Seismic Network (PRSN) data. Hazard curves for peak horizontal acceleration were computed by using the model for the four corners of the island. The hazard appears to be greatest in the western part of the island, which is closer to the more active sections of the two subduction zones and faults of the Mona Passage extensional regime. To the east the hazard is less because of presumed lower coupling between the North America and Puerto Rico–Virgin Islands microplate, and because of dispersion of relative motion at the Muertos subduction zone onto faults in the Anegada Passage and the Investigator faults south of the island. Recommendations for further work include further development of attenuation parameters, refinement of the local seismicity catalog, and further investigations into faults on and near the island.

Abstract The distribution of small earthquakes recorded during the last 35 years, of large earthquakes during the last 400 years, as well as focal mechanisms determined for moderate earthquakes, have provided insights into the location of, and style of motion on, the margin of the Caribbean plate. In this chapter we review information gained by seismological means to help define the nature of the Caribbean plate and its boundaries. The location of the Caribbean plate boundary is more clearly defined along its eastern, western, and northwestern margins (the Lesser Antilles and Central America); other portions are more complex and still subject to general debate (Fig. 1). The first Caribbean-wide study of seismicity by Sykes and Ewing (1965) delineated several features, including westerly and southerly dipping seismic zones in the Lesser Antilles and eastern Greater Antilles, respectively; a strong source of intermediate-depth events beneath eastern Hispaniola;and shallow-focus earthquakes extending from Central America through the Greater Antilles, as well as along northern South America. Work by Molnar and Sykes (1969) determined the general shape of the easterly dipping seismic zone in Central America, several focal mechanisms consistent with underthrusting of sea floor along the eastern and western margins of the Caribbean plate, and left- and right-lateral motion along segments of the northern and southern margins, respectively. They also estimated 0.5 and 2.0 cm/year as the minimum rates of seismic slip at the Lesser Antilles and Middle America subduction zones. These and other studies clearly demonstrated that the Atlantic Ocean sea floor of

A 50 km long by 11 km wide ridge rises above the Cibao Valley along the southern edge of the eastern Cordillera Septentrional in northeastern Hispaniola. This seismically active feature, the San Francisco Ridge, is interpreted to be a “push-up” because it is a compressed and elevated ridge intimately associated with strike-slip faulting. Deformed and uplifted Middle Miocene to Recent sedimentary units in the ridge provide a stratigraphic window into the later depositional history of the Cibao basin. The depositional environments, paleobathymetry, ages of unconformities, and structural development of the San Francisco Ridge indicate Neogene to recent transpression related to uplift and slip of the eastern Cordillera Septentrional against the eastern Cibao basin. From the city of San Francisco de Macoris eastward, the ridge consists of six to eight west-northwest-trending left oblique-slip faults associated with northwest-trending asymmetric to overturned kink folds. Northeast-striking right-slip faults divide the ridge into domains with contrasting lithologies and structures.