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The deposits, found in a mangrove pond on the northwest corner of Puerto Rico by researchers from the USGS Pacific Coastal and Marine Science Center and the University of Puerto Rico, are believed to date back to between 1470 and 1530. They suggest that the tsunami was triggered by a massive megathrust earthquake that occurred at the Puerto Rico Trench. Similar age deposits at other Caribbean islands up to 400 km to the east, as well as prior seismic modeling, suggest that the earthquake was of a magnitude 8.7 or larger. An earthquake of that magnitude in the Caribbean would generate tsunamis that cross the Atlantic Ocean.

“These Puerto Rican tsunami deposits provide valuable information for assessing the seismic and tsunami risks of the region,” said Bruce Jaffe, USGS Research Oceanographer, who led the research. “This work contributes to a better understanding of the frequency and magnitude of earthquakes and tsunamis in the area.”

These findings are significant because written records of tsunamis in the Caribbean extend back only 500 years, and evidence of earthquakes of magnitude 8.0 or larger on the Puerto Rico Trench is sparse. Understanding the likelihood of such events is crucial for developing effective disaster management strategies. 

“When buildings are engineered to withstand earthquakes in areas where tsunamis occur, it is often the tsunami, rather than the earthquake, that results in greater loss of life and property,” said Jaffe. “The most striking example is the 2004 Indian Ocean tsunami, which killed nearly a quarter million people. A small fraction of these deaths was attributed to earthquake-related damage. The 2011 Great Tōhoku Earthquake in Japan was similar: the tsunami caused widespread destruction and the loss of nearly 20,000 lives, while earthquake damage to the buildings was much reduced thanks to their design.” 

Jaffe and colleagues presented their findings at the Seismological Society of America's 2023 Annual Meeting. 

Read the related press release from Seismological Society of America.

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