Variation in dip of the Caribbean Plate along the Muertos Trough

The M 7.0 Haiti earthquake of 2010 is a reminder that the northeast Caribbean is at high risk for seismic and tsunami hazards. The islands of Puerto Rico and Hispaniola are located between two subduction zones, with the Puerto Rico trench to the north and the Muertos trough to the south. The Puerto Rico trench is caused by southward-directed subduction of the North American Plate, and the Muertos trough is the northern boundary of the Caribbean Plate. There is no active volcanism on Puerto Rico, however earthquake depths and seismic tomography imply that the slab of Caribbean plate continues beneath Puerto Rico. Puerto Rico overlies these two slabs with extensional stress to both the east (Mona Passage) and west (Anacapa Passage). The cause of the extension is unknown, but GPS measurements show that Puerto Rico has anti-clockwise rotation, and the extensional Anacapa Passage may be produced by the rotation. To the west, it is debated whether the Mona Passage is a boundary between two micro-plates. By combining interpreted profiles of gravity data with the locations of earthquakes within the subducting slab on both sides of Mona Passage, we interpret that the subducting slab has a variation in dip along-strike of the Muertos Trough. The subducting slab east of the Mona Passage has a shallow dip angle, whereas the subducting slab west of the Mona Passage has a steep dip angle. This change in dip implies a north to south boundary beneath the Mona Passage, and that this passage is the micro-plate boundary between Puerto Rico and Hispaniola. The shallow dip of the slab to the east of the Mona Passage may cause the rotation of the microplate and the lateral extension.