Puerto Rico tsunami deposit could have come from pre-Columbian
megathrust earthquake

Date:
April 20, 2023
Source:
Seismological Society of America
Summary:
Tsunami deposits identified in a coastal mangrove pond in Northwest
Puerto Rico could have come from a megathrust earthquake at the
Puerto Rico Trench that occurred between 1470 and 1530, according
to new research.


Facebook Twitter Pinterest LinkedIN Email
FULL STORY ========================================================================== Tsunami deposits identified in a coastal mangrove pond in Northwest Puerto
Rico could have come from a megathrust earthquake at the Puerto Rico
Trench that occurred between 1470 and 1530, according to research reported
at the Seismological Society of America (SSA)'s 2023 Annual Meeting.


========================================================================== 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.

The deposits could be very important for understanding seismic risk in
the region, since written records of tsunamis in the Caribbean extend back
only 500 years, and the evidence for magnitude 8.0 or larger earthquakes
on the Puerto Rico Trench is sparse.

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).

The researchers estimate that these deposits, ranging in age from 1200
to 1500 years before the present, could be the result of a tsunami
generated by a magnitude 8.0 to 9.0 earthquake that ruptured at least
300 kilometers.

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.

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. In a coastal mangrove pond at East Bajura
near Isabela in Northwest Puerto Rico, they found a likely candidate
close to the shore and flanked by ancient dunes that could protect the
pond from swells.

In sediment cores taken from the pond, the researchers found a thin
sand sheet about a half a meter deep that blanketed the whole pond. The
deposit bears several signatures of a tsunami event, including an eroded
basal layer and a unique gradation of sediment particles.

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." The researchers are examining large cores taken from the
pond with CT scans and X-ray fluorescence, among other tools, looking
for fine details such as the direction of grass roots and the mineral composition of sediment grains to further constrain the timing and the
source of the tsunami.

One of the researchers, Matthew Baez, a graduate student working with
Jaffe and Alberto Lo'pez-Venegas of the University of Puerto Rico,
Mayagu"ez 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 Lo'pez-Venegas said there has been a lack of paleo-tsunami research on major earthquakes affecting Puerto Rico's northern coast. The
2 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," Lo'pez-Venegas said.

* RELATED_TOPICS
o Earth_&_Climate
# Tsunamis # Natural_Disasters # Earthquakes # Ecology
o Fossils_&_Ruins
# Early_Climate # Fossils # Ancient_DNA # Paleontology
* RELATED_TERMS
o 1928_Okeechobee_Hurricane o Hurricane_Hugo o Bermuda_Triangle
o Mangrove o Paralititan o 2004_Indian_Ocean_earthquake o
Homo_rudolfensis o 2005_Kashmir_earthquake

========================================================================== Story Source: Materials provided by
Seismological_Society_of_America. Note: Content may be edited for style
and length.


==========================================================================


Link to news story: https://www.sciencedaily.com/releases/2023/04/230420171646.htm

--- up 1 year, 7 weeks, 3 days, 10 hours, 50 minutes
* Origin: -=> Castle Rock BBS <=- Now Husky HPT Powered! (1:317/3)