The Puerto Rico Trench is located on the boundary between the Caribbean Sea and the Atlantic Ocean. The oceanic trench is associated with a complex transition between the Lesser Antilles subduction zone to the south and the major transform fault zone or plate boundary, which extends west between Cuba and Hispaniola through the Cayman Trench to the coast of Central America. The trench is 800 kilometres (497 mi) long and has a maximum depth of 8,648 metres (28,373 ft) or 5.373 miles at Milwaukee Deep, which is the deepest point in the Atlantic Ocean and the deepest point not in the Pacific Ocean.
Scientific studies have concluded that an earthquake occurring along this fault zone could generate a significant tsunami.  The island of Puerto Rico, which lies immediately to the south of the fault zone and the trench, suffered a destructive tsunami soon after the 1918 San Fermín earthquake.
The Puerto Rico Trench is located at a boundary between two plates that pass each other along a transform boundary with only a small component of subduction. The Caribbean Plate is moving to the east while the North American Plate is moving to the west. The North American Plate is being subducted by the Caribbean Plate obliquely at the trench while to the southeast, the South American Plate is being more directly subducted along the Lesser Antilles subduction zone. This subduction zone explains the presence of active volcanoes over the southeastern part of the Caribbean Sea. Volcanic activity is frequent along the island arc southeast from Puerto Rico to the coast of South America.
Puerto Rico, the United States Virgin Islands, British Virgin Islands, and the Dominican Republic do not have active volcanoes; however, they are at risk from earthquakes and tsunamis. The Puerto Rico Trench is capable of producing earthquakes greater than magnitude 8.0.
Knowledge of the earthquake and tsunami risks has not been widespread among the general public of the islands located near the trench. Since 1988, the Puerto Rican Seismic Society has been trying to use the Puerto Rican media to inform people about a future earthquake that could result in a catastrophic tragedy.
Following the 2004 tsunami that affected more than forty countries in the Indian Ocean, many more people now fear the consequences that such an event would bring to the Caribbean. Local governments have begun emergency planning. In the case of Puerto Rico and the U.S. Virgin Islands, the United States government has been studying the problem for years.  It is increasing its seismic investigations and developing tsunami warning systems.
On 11 October 1918, the western coast of the island was hit by a major earthquake which caused a tsunami. The 1918 earthquake was caused by an old left-lateral strike-slip fault near the Mona Passage. In 1953, Santo Domingo, Dominican Republic, was affected by the Santo Domingo earthquake. The actual subduction zone (Puerto Rico Trench) has not ruptured in over 200 years, which is a major concern to geophysicists, as they believe it may be due for a major event.
Puerto Rico has always been an area of concern to earthquake experts because, apart from the 1918 episode, there are frequent tremors in and around the island, indicating activity. A 1981 tremor was felt across the island, while another in 1985 was felt in the towns of Cayey and Salinas.
The January 13, 2014 M 6.4 earthquake north of Puerto Rico occurred as a result of oblique-thrust faulting. Preliminary faulting mechanisms for the event indicate it ruptured either a structure dipping shallowly to the south and striking approximately east-west, or a near-vertical structure striking northwest-southeast. At the location of this earthquake, the North America plate moves west-southwest with respect to the Caribbean plate at a velocity of approximately 20 mm/yr, and subducts beneath the Caribbean plate at the Puerto Rico Trench. The location, depth and mechanism of the earthquake are consistent with the event occurring on this subduction zone interface." 
|Puerto Rico Trench|
|Puerto Rico Trench|
Several exploration cruises carried out by USGS in the Puerto Rico Trench have for the first time mapped the entire trench using ship mounted multibeam bathymetry (see references and external links below).
The seafloor was visited for the first time by French bathyscaphe Archimède in 1964  and then by a robotic vehicle in 2012.  The most conspicuous aspect of the footage was the swarm of benthic amphipods. Some of these amphipods were collected by bait bags attached to the vehicle and were brought to the surface for further analysis. The samples recovered were Scopelocherius schellenbergi, a species of lysianassoid amphipod that have so far only been found in ultradeep trenches in the Pacific.  Two invertebrate creatures were also observed in the video. One soft dark individual, estimated to be 10-20 centimeters long, has been identified by Dr. Stace E. Beaulieu of Woods Hole Oceanographic Institution as a sea cucumber, tentatively assigned to genus Peniagone. The other individual, a small crustacean, is tentatively identified as a munnposid isopod, based on morphology and similar walking and jumping movements observed for other hadal munnopsid isopods. Because these individual were not collected, it is not possible to obtain species-level identifications. However, these sightings likely exceed the deepest known records for genus Peniagone and family Munnopsidae.
- ten Brink, Uri. "Puerto Rico Trench 2003: Cruise Summary Results". National Oceanic and Atmospheric Administration. Retrieved 2011-09-11.
- "Earthquakes and Tsunamis in Puerto Rico and the U.S. Virgin Islands" (PDF). U.S. Geological Survey Fact Sheet 141-00. April 2001. Retrieved 2011-09-11.
- M6.4 - 57km N of Hatillo, Puerto Rico, Earthquake Hazard Program, 2014-01-13 04:01:04 UTC, USGS
- Pérès, J.M., 1965, Apercu sur les resultats de deux plongees effectuees dans le ravin de Puerto-Rico par le bathyscaphe Archimède. Deep-Sea Research, Vol. 12: 883-891.
- Søreide, Fredrik, 2012, Ultradeep-sea exploration in the Puerto Rico Trench, Sea Technology Vol. 53, No. 12.
- Lacey, Nichola, Jamieson, Alan & Søreide, Fredrik, 2013, Successful capture of ultradeep sea animals from the Puerto Rico Trench, Sea Technology, Vol. 54, No. 3: 19-21.
- Mapping of the Puerto Rico Trench, the Deepest Part of the Atlantic, is Nearing Completion – United States Geological Survey
- Workshop Addresses Tsunami Hazard to Puerto Rico, the Virgin Islands, and Other Caribbean Islands – United States Geological Survey
- Caribbean Tsunami and Earthquake Hazards Studies – Woods Hole Coastal and Marine Science Center
- Latest Significant Earthquakes – Puerto Rico Seismic Network
- Promare – Promare - Promoting Marine Research and Exploration
- Successful Capture of Ultradeep Sea Animals From the Puerto Rico Trench – Sea Technology