We have determined focal mechanisms for the largest earthquake (MS=7.8) recorded instrumentally in the Caribbean Basin, the August 4, 1946, Hispaniola earthquake, and three of its large-magnitude (MS≥6.1) aftershocks. We also relocated 63 aftershocks and one foreshock of the event series. The aftershock series is elongate, trends WNW, and is centered on the Saman¿ Peninsula of northeast Hispaniola. Shallow aftershocks are in a 75-km-wide linear zone, and intermediate depth (70 to 130 km) aftershocks apparently delineate a moderately south or SSW dipping slab. It is not clear, however, whether these events indicate active subduction of North American Atlantic Ocean lithosphere or are strike-slip events on the interface between subducted but no longer sinking slab and Caribbean mantle. We constrained focal mechanisms of the main shock and three aftershocks by combining observed P and S polarities and amplitude ratios and also by waveform modeling. The two methods yield consistent results. The mechanisms include strike-slip and thrust displacements on NW striking nodal planes. Fault dip is variable, NE or SW. The NW striking fault planes parallel mapped terrane boundaries and faults in the North America (NA)-Caribbean (Ca) plate boundary zone and are also parallel to the aftershock series trend. We interpret the events to be motions on a WNW trending restraining bend segment of the NA-Ca plate boundary in eastern Hispaniola. We have calculated magnitudes for eight of the earthquakes in the series; for the three events (including the main shock) for which data are available, our magnitudes are systematically less than the previously published magnitude estimates. Given the high magnitude and large aftershock area of the August 4, 1946, event, these earthquakes probably represent the true long-term interplate motions between North America and the terranes in this portion of the plate boundary zone. ¿ American Geophysical Union 1995