ThePuerto Rico Trench is located on the boundary between theNorth Atlantic Ocean andCaribbean Sea, parallel to and north ofPuerto Rico, where theoceanic trench reaches the deepest points in theAtlantic Ocean. The trench is associated with a complex transition from theLesser Antillesfrontal subduction zone between theSouth American plate andCaribbean plate to theoblique subduction zone and thestrike-slip transform fault zone between theNorth American plate and Caribbean plate, which extends from the Puerto Rico Trench at thePuerto Rico–Virgin Islands microplate through theCayman Trough at theGonâve microplate to theMiddle America Trench at theCocos plate.^{[1][2][3][4][5][6]}

Constituting the deepest points in theAtlantic Ocean, the trench is 810 kilometres (503 mi) long^[7] and has a maximum documented depth between 8,376 metres (27,480 ft)^[8] and 8,740 metres (28,675 ft).^[7][9] The deepest point is commonly referred to as theMilwaukee Deep, with theBrownson Deep naming the seabed surrounding it.^[10] However, more recently, the latter term has also been used interchangeably with the former to refer to this point.^[11][12][13] The exact point was identified by theDSSV Pressure Drop using a state-of-the-art Kongsberg EM124multibeam sonar in 2018, and then directly visited and its depth verified by the crewed submersible Deep-Submergence VehicleDSV Limiting Factor (a Triton 36000/2 model submersible) piloted byVictor Vescovo.^[14][15][16]

Scientific studies have concluded that an earthquake occurring along this fault zone could generate a significanttsunami.^[17] The island ofPuerto Rico, which lies immediately to the south of the fault zone and the trench, suffered a destructive tsunami soon after the1918 San Fermín earthquake.

The Puerto Rico Trench is located at a boundary between twoplates that pass each other along a transform boundary with only a small component ofsubduction. TheCaribbean plate is moving to the east relative to theNorth American plate. 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 activevolcanoes over the southeastern part of the Caribbean Sea. Volcanic activity is frequent along theLesser Antillesisland arc southeast from Puerto Rico to the northern coast ofSouth America.

Although originally part of avolcanic arc, theVirgin Islands,Puerto Rico,Hispaniola,Cuba, andJamaica do not have activevolcanoes. The Virgin Islands and Puerto Rico do not have active volcanic activity since approximately 30 million years ago,^[18] while the last active volcanoes in Hispaniola,Thomazanue andMorne la Vigie, became extinct within 1.5 million years ago. However, the islands are at risk ofearthquakes andtsunamis. The Puerto Rico Trench has produced earthquakes greater than magnitude 8.0 and is considered capable of continuing to do so.^[19][20]

According toNASA, beneath the trench is a mass so dense it deflects gravitational pull on the surface of the ocean, causing it to dip somewhat. It also has a negative effect on the accuracy of navigational instruments.^[21]

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 thePuerto Rican media to inform people about a future earthquake that could result in a catastrophic tragedy.

Following the2004 tsunami that affected more than forty countries in theIndian 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, theUnited States government has been studying the problem for years.^[22] It is increasing its seismic investigations and developingtsunami warning systems.

On 11 October 1918, the western coast of Puerto Rico was hit by amajor earthquake which caused atsunami. The 1918 earthquake was caused by an old left-lateral strike-slip fault near theMona 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 ofCayey andSalinas.

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."^[23]

Several exploration cruises carried out byUSGS in the Puerto Rico Trench have for the first time mapped the entire trench using ship mounted multibeam bathymetry.

The seafloor was visited for the first time by French bathyscapheArchimède in 1964^[24][25] and then by a robotic vehicle in 2012.^[26] The most conspicuous aspect of the footage was the swarm ofbenthicamphipods. 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 wereScopelocheirus schellenbergi, a species oflysianassid amphipod that have so far only been found in ultradeep trenches in the Pacific.^[27]

Twoinvertebrate creatures were also observed in the video. One soft dark individual, estimated to be 10–20 cm (3.9–7.9 in) long, has been identified by Dr. Stace E. Beaulieu ofWoods Hole Oceanographic Institution as asea cucumber, tentatively assigned to genusPeniagone. The other individual, a small crustacean, is tentatively identified as amunnopsid isopod, based on morphology and similar walking and jumping movements observed for otherhadal munnopsid isopods. Because these individuals were not collected, it is not possible to obtain species-level identifications. However, these sightings likely exceed the deepest known records for genusPeniagone and family Munnopsidae.

The American explorerVictor Vescovo dived to the deepest point of the Puerto Rico Trench and therefore the Atlantic Ocean on 19 December 2018, as part of theFive Deeps Expedition. He reached a depth of 8,376 m (27,480 ft) ±5 m (16 ft) at 19°42'49" N, 67°18'39" W by directCTD pressure measurements with the Deep-Submergence VehicleDSV Limiting Factor (a Triton 36000/2 model submersible) and thus became the first person to reach the bottom of the Atlantic Ocean while also making the second-deepest recorded solo dive in history at that time.^[28] Many media outlets referred to the deep asBrownson Deep,^[11][12][13] in opposition to past references to the area, where the termMilwaukee Deep was used instead.

The operating area was surveyed by the support ship, the Deep Submersible Support VesselDSSV Pressure Drop, with a Kongsberg SIMRAD EM124 multibeam echosounder system. The gathered data will be donated to theGEBCO Seabed 2030 initiative.^[29][30] The dive was part of theFive Deeps Expedition. The objective of this expedition was to thoroughly map and visit the deepest points of all five of the world's oceans by the end of September 2019.^[31][32]

• Mariana Trench
• Plate tectonics
• List of oceanic trenches

• 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

19°50′9″N66°45′16″W / 19.83583°N 66.75444°W /19.83583; -66.75444