Study pinpoints likely date for a massive earthquake on the Puerto Rico Trench -- and highlights an underappreciated regional hazard

Anegada is a postcard-worthy Caribbean island in the British Virgin Islands. But hundreds of meters inland from its white-sand beaches is something unexpected and decidedly out of place: hundreds of large coral boulders, their grayish exteriors a testament to centuries of weathering. Researchers have now determined they probably washed ashore in the late 14th century, after a major earthquake triggered a tsunami. The findings, published this month in Geophysical Research Letters, suggest the nearby Puerto Rico Trench poses an underappreciated tsunami hazard.

"If there was an event like this that happened today, it would have devastating effects on the local economy," says Brian McAdoo, a disaster researcher at Duke University who was not involved in the work.

Anegada's coral boulders, which measure meters in size, have long baffled scientists. "I've been to many islands in the Caribbean, and I've never seen such large boulders washed so far inland," says study author Hali Kilbourne, a marine scientist at the University of Maryland Center for Environmental Science.

Some researchers have hypothesized that hurricanes might be responsible. But modeling work has shown that even Category 5 hurricanes wouldn't cause flooding violent enough to uproot coral boulders and carry them so far from their native environment. That leaves just one other possibility: an even more destructive tsunami.

The ages of the boulders should indicate when such an event might have occurred. But previous work relying on radiocarbon dating had yielded estimates that spanned several centuries. Kilbourne and her colleagues used a different technique based on radioactive uranium. It naturally occurs in seawater and is incorporated into living coral, where it gradually decays into thorium. The relative proportions of those two elements indicate when the coral was last immersed in seawater. By analyzing uranium and thorium levels in nine of Anegada's coral boulders, the researchers estimated that at least some were hurled inland between 1381 and 1391 C.E. "That clearly was when something big happened to wash all of these things ashore," Kilbourne says.

The Caribbean Sea experiences plenty of earthquakes from its shifting tectonic plates. But a subduction zone, where a plate plunges into the mantle, is generally necessary for generating the vertical motion that triggers tsunamis. Kilbourne thinks the suspected tsunami likely came from a magnitude 8 or greater earthquake on the Puerto Rico Trench, where the North American tectonic plate dives under the Caribbean plate just 100 kilometers or so north of Anegada.

It isn't entirely surprising that the boulders are the only surviving evidence for the medieval quake, says Louise Cordrie, a geoscientist at CINECA, a nonprofit research consortium in Italy. Precolonial records in the Caribbean are virtually nonexistent. Moreover, the Puerto Rico Trench isn't very active. Because the two tectonic plates there are converging by only about 2 centimeters per year, stress is slow to build up. That pace would lead to a major earthquake only about every 1000 years or so, Cordrie says.

Nevertheless, a tsunami originating from the Puerto Rico Trench poses a distinct danger to multiple Caribbean islands. Cordrie and her colleagues previously showed that tsunami waves triggered by a large earthquake on the trench would wash up on Anegada within just a few tens of minutes. For an island that's barely 8 meters above sea level at its highest point, that's a terrifying thought. "There's not much time," Cordrie says.

Kilbourne and her colleagues hope to corroborate their new dates with evidence from outside the Caribbean. One possibility: historical records from across the Atlantic Ocean, in the British Isles, Kilbourne says. The waves that rolled ashore there from the Caribbean tsunami might have been small, she says, but perhaps someone noted something. "It's worth looking."