Puerto Rico actually is on a plate boundary. Most of those earthquakes outline the Puerto Rico Trench -- a southward-dipping subduction zone at the edge of the

Puerto Rico actually is on a plate boundary. Most of those earthquakes outline the Puerto Rico Trench -- a southward-dipping subduction zone at the edge of the Caribbean Plate (that wraps around to connect with both the strike-slip plate boundary to the west and the weastward-dipping Lesser Antilles subduction zone to the southeast).