Velocity modeling and earthquake relocation are used to examine the structure and seismicity of an area south of the Puerto Rico trench in the northeast Caribbean, and to explore whether and how the North American plate is currently subducting under the Caribbean plate along the Puerto Rico trench. A velocity model is obtained by inverting data for hypocenters, velocities, and station delays from P-wave arrival times for 160 earthquakes with depths of 0 to 85 km recorded by a local network of 18 stations. In the depth interval 0 to 24 km, three layers can be resolved for the region between 64° and 66°W, and 18°20′ and 19°40′N. An extension of the structural investigation south to 18°N, by including 67 additional events with hypocenters as deep as 150 km, shows four well-resolved layers from 0 km to approximately 75 km depth. The velocity model probably represents an average of the overriding Caribbean plate and the underthrust North American plate. A cross-section of the relocated events shows a shallowly dipping seismic zone near 15 to 25 km depth which smoothly changes to a fairly steeply dipping seismic zone at intermediate depth. When coupled with faultplane solutions of events in this region, these data provide evidence that the North American plate is presently involved in active oblique subduction. Two possible faults in the overthrust Caribbean plate have also been found. The two most southern, consistently reporting stations show high residuals for certain P waves that originate north of 18°55′N. Two more nothern stations give high residuals for phases throughout the region, probably indicating their underlying limestone geology.

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