Pre-Columbian Megathrust Earthquake Could Have Caused the Puerto Rico Tsunami Deposit

According to research presented at the 2023 Annual Meeting of the Seismological Society of America (SSA), tsunami deposits discovered in a coastal mangrove pond in Northwest Puerto Rico may have originated from a megathrust earthquake along the Puerto Rico Trench that took place between 1470 and 1530.

“These Puerto Rican tsunami deposits, along with similar age deposits at other islands in the Caribbean, suggest that the tsunami would have been triggered by a massive magnitude 8.7 or larger earthquake,” said Bruce Jaffe of the U.S. Geological Survey.

Given that there are only 500 years of written records of tsunamis in the Caribbean and a lack of evidence for earthquakes of magnitude 8.0 or more on the Puerto Rico Trench, the deposits could be crucial for assessing seismic risk in the area.

Previous research has uncovered Caribbean tsunami deposits along the northern segment of the Lesser Antilles arc in Anegada, St-Thomas (U.S. Virgin Islands), Anguilla, and Scrub Island (British Virgin Islands).

These deposits, which are thought to be between 1200 and 1500 years old, may have been caused by a tsunami brought on by an earthquake with a magnitude of 8.0 to 9.0 and a rupture of at least 300 kilometers, according to the experts.

If the Puerto Rico pond deposits came from the same event, “it could be a large length of fault that ruptured 400 kilometers, roughly,” said Jaffe.

Earthquake modeling for the Lesser Antilles arc deposits suggested a magnitude 8.7 earthquake. Models that include the newly discovered Puerto Rico deposits indicate that the tsunami-triggering earthquake “would have to be quite a bit larger,” he added.

The tsunami itself may have impacted both the Caribbean and Atlantic coasts of Puerto Rico.

Sediment swept up in the high-speed flow of a tsunami remains suspended in the water, because the turbulent eddies are strong enough to mix the sediment from the sea bed all the way up through the water column. It creates a very distinctive grading of sediment in the core.

Bruce Jaffe

Jaffe and colleagues scouted dozens of locations in Puerto Rico “looking for the right setting for a possible tsunami deposit to form and for it to be preserved,” he said.

They discovered a potential contender in a coastal mangrove pond in East Bajura near Isabela in Northwest Puerto Rico that was close to the sea and bordered by old dunes that could shield the pond from waves.

Researchers discovered a thin, half-meter-deep layer of sand covering the whole pond in sediment cores obtained from it. A distinctive gradation of sediment particles and an eroded basal layer are only two of the deposit’s many signs of a tsunami event.

Sediment swept up in the high-speed flow of a tsunami remains suspended in the water, “because the turbulent eddies are strong enough to mix the sediment from the sea bed all the way up through the water column,” explained Jaffe, who has analyzed deposits in the wake of six major tsunamis worldwide. “It creates a very distinctive grading of sediment in the core.”

To further narrow the timing and source of the tsunami, the researchers are analyzing huge cores retrieved from the pond using CT scans and X-ray fluorescence, among other technologies. They are looking for minute features like the direction of the grassroots and the mineral makeup of sediment grains.

One of the researchers, Matthew Baez, a graduate student working with Jaffe and Alberto López-Venegas of the University of Puerto Rico, Mayagüez is looking for similar ponds along the north coast of Puerto Rico that could corroborate the findings from the East Bajura pond. And Jaffe said there are plans to take a deeper core from the current pond to look for evidence of earlier tsunamis.

Study co-author López-Venegas said there has been a lack of paleo-tsunami research on major earthquakes affecting Puerto Rico’s northern coast. The 2nd May 1787 earthquake that most likely occurred offshore the northern coast of Puerto Rico, likely centered on the Main Ridge within Puerto Rico Trench, is one of the most significant seismic events to affect that part of the island.

“The bottom line is, we still have a lot of work to do as we do not have a good grasp of which events have occurred along the Puerto Rico Trench, and much more paleo-tsunami work is required to understand better what has occurred in the past and what may happen in the future,” López-Venegas said.

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