The Puerto Rico Trench, the deepest point anywhere in the Atlantic Ocean, runs along the northern edge of Puerto Rico and sits close enough to shore that its earthquakes have reached land within minutes. About 4 million U.S. citizens live in Puerto Rico and the U.S. Virgin Islands, all of them within range of a subduction zone where the North American plate slides beneath the Caribbean plate. A new seafloor map of the trench, published by the U.S. Geological Survey, gives hazard scientists the clearest picture yet of a fault system that has already produced a fatal tsunami and an estimated magnitude-8 earthquake.
Unlike the well-known subduction zones off the Pacific Northwest or Alaska, the Puerto Rico Trench sits directly beside a mid-sized population of American citizens rather than a sparsely settled coastline, which is part of why USGS researchers keep returning to remap it. The trench itself plunges to depths that make it the deepest place in the entire Atlantic basin, deeper than any point in the Gulf of Mexico or the open ocean off the U.S. East Coast. Older nautical charts left much of the seafloor along the trench only roughly sketched, which is the exact gap the newest survey was built to close.
A magnitude-8 earthquake in 1787 that predates any instrument record
USGS researchers, drawing on historical accounts and modern seafloor mapping, estimate that an earthquake north of Puerto Rico and Hispaniola in 1787 reached roughly magnitude 8, causing widespread damage across both islands more than two centuries before seismographs existed to record it. Because the event predates instrumental seismology entirely, scientists have had to reconstruct its size from written damage reports rather than any waveform, the same method used to size up the 1886 Charleston earthquake on the U.S. mainland.
That reconstruction matters because the 1787 rupture appears to have occurred on the same broad fault system that later produced a fatal tsunami in the 20th century, giving researchers a second, older data point for how large this subduction zone is capable of shaking. Geophysicist Uri ten Brink and colleagues at the Woods Hole Coastal and Marine Science Center, who led the new seafloor survey, have spent years piecing together exactly where along the trench such a rupture could occur again, since no single fault segment has been definitively tied to either historical event.
A 1918 tsunami that killed 40 people in northwest Puerto Rico
In October 1918, a magnitude-7.3 earthquake on the Puerto Rico Trench triggered a tsunami that killed 40 people along the island’s northwest coast, the deadliest tsunami in Puerto Rico’s recorded history. Waves reportedly reached close to 20 feet in some locations, arriving fast enough along a coastline packed with fishing towns that there was little time to react before the water came ashore.
The Puerto Rico Seismic Network, run out of the University of Puerto Rico at Mayagüez since the USGS transferred it there in 1987, now serves as the island’s official tsunami focal point under United Nations standards, coordinating directly with the Pacific Tsunami Warning Center. The network traces its origin to 1974, when it was first installed to assess seismic risk for a planned nuclear power plant, and it has since grown into 25 seismic stations spread across Puerto Rico and the Virgin Islands.
Seafloor landslides and a modern sequence that is still active
The new mapping effort also identified large landslide escarpments carved into the seafloor north of Puerto Rico, features whose exact ages remain undetermined but that point to a history of underwater slope failures capable of generating their own local tsunamis independent of any earthquake. Those escarpments sit within a region where the two plates converge at roughly 20 millimeters a year, a slow but steady rate that has produced frequent seismic activity for as long as written records exist.
The most recent reminder came in December 2019, when a sequence of earthquakes near Guayanilla, on Puerto Rico’s south coast, built toward a magnitude-6.4 mainshock on January 7, 2020, triggering landslides and damaging a boardwalk in Ponce. That sequence forced the Puerto Rico Seismic Network to install additional monitoring stations on short notice, work the USGS says continued for months after the mainshock as aftershocks kept arriving. USGS scientists have documented at least 4,484 deaths from tsunamis in the broader Caribbean region over the past 500 years, a toll that spans centuries of earthquakes most residents today have never heard of, let alone the trench sitting a short distance offshore that produced several of them.
Every coastal municipality in Puerto Rico now participates in a Tsunami Ready program built around recognizing natural warning signs, since a trench this close to shore leaves little time for an official alert to travel before waves arrive. The seafloor map that prompted renewed USGS attention to the trench does not predict when the next large earthquake will strike; it simply gives 4 million residents of Puerto Rico and the Virgin Islands a more detailed picture of the fault system running just off their coast.
This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.
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