Puerto Rico Tsunami Sediment May Have Come From Pre-Columbian Bombing Earthquake – ScienceDaily

A tsunami deposit identified in a coastal mangrove pool in northwestern Puerto Rico could have come from a massive earthquake in the Puerto Rico Trench that occurred between 1470 and 1530, according to research reported at the 2023 Annual Meeting of the American Seismological Society (SSA).

The Puerto Rican tsunami deposit, along with similar age deposits on other islands in the Caribbean, said Bruce Jaffe of the USGS, suggests the tsunami could have been caused by a massive magnitude 8.7 earthquake or a larger earthquake.

The sediments may be very important for understanding earthquake risk in the region, since written records of tsunamis in the Caribbean stretch back only 500 years, and evidence of earthquakes of magnitude 8.0 or greater in the Puerto Rico Trench is sparse.

Previous research revealed Caribbean tsunami deposits along the northern part of the Lesser Antilles arc in Anegada, St. Thomas (US Virgin Islands), Anguilla, and Scrap Island (British Virgin Islands).

The researchers estimate that these deposits, which are between 1,200 and 1,500 years old before present, could be the result of a tsunami triggered by a magnitude 8.0 to 9.0 earthquake that ripped at least 300 kilometers away.

If the pond sediments in Puerto Rico had come from the same event, “it could have been a huge crack ruptured — about 400 kilometers across,” Jaffe said.

Seismic models in the Lesser Antilles arc deposits indicate a magnitude 8.7 earthquake. He added that models that include newly discovered Puerto Rican sediments suggest that the earthquake that triggered the tsunami “must be a little bit bigger.”

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

Jaffe and his colleagues explored dozens of sites in Puerto Rico “in search of the right place for potential tsunami deposit formation and preservation,” he said. In a coastal mangrove lake in eastern Pajora near Isabela in northwestern Puerto Rico, they found a potential filter close to shore surrounded by ancient sand dunes that could protect the pond from swells.

In the sediment samples taken from the pond, the researchers found a thin sandy plate about half a meter deep that covered the entire pond. The deposition bears many signatures of a tsunami event, including an eroded basal layer and a unique gradation of sediment particles.

Sediment swept up in the high-velocity flow of a tsunami remains suspended in the water, “because the turbulent eddies are strong enough to mix sediment from [sea]”It creates a very distinct taxonomy for the sediments in the core,” explained Jaffe, who analyzed the sediments in the aftermath of six major tsunamis around the world.

Researchers examine large cores taken from the pond using CT scans and X-rays, among other tools, looking for fine details such as the orientation of grass roots and the mineral composition of sediment grains to pinpoint the timing and source of the tsunami.

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 basins along Puerto Rico’s northern coast that could confirm findings from the eastern Pajora pond. Jaffe said there are plans to take a core deeper than the current pond to look for evidence of previous tsunamis.

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

“The bottom line is that we still have a lot of work to do because we don’t have a good understanding of the events that occurred along the Puerto Rico Trench, and more work is needed in the ancient tsunami field to better understand what happened in the past and what happened,” López-Venegas said. It might happen in the future.”

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