Researchers from the United States Geological Survey are looking at new evidence on Anegada to determine what caused an unusual flood from the sea that brought ashore large coral boulders around the 1400’s.
A team of researchers, led by Geologist, Dr. Brian Atwater, spent February 24 to March 17 on Anegada collecting evidence for analysis, as they seek to validate one of the three scenarios being tested.
The three scenarios being considered are two types of tsunamis generated by earthquakesin the Puerto Rico trench and an unusual super storm similar to the 2013 typhoon Haiyan in the Philippines.
Dr. Atwater said the new evidence include the development of a model using the three scenarios that will help the researchers understand the boulder movements, analyse homotrema and other microscopic evidence, examine conch shell mounds and Elkhorn coral to trace the over wash.
Homotrema are red organisms that encrust reef and are responsible for giving sand a pink hue when washed ashore. The researchers are analysing these organisms which are considered to be indicators of how far in land the waves travelled.
Dr. Atwater described Anegada as a guide to regional hazards as researchers can learn about its extreme wave history more readily and fully than most places. He said a big part of this year’s effort was to build up a collection of field observations on limestone boulders that can be used to estimate how fast the flow was and how deep the water was. This will be used for a comparison to different types of tsunamis and will also be applied to the storm scenario.
“We put a lot of effort into mapping and measuring limestone boulders,” Dr. Atwater explained, adding,“Approximately 300 individual limestone boulders were mapped with a handheld GPS and measured, to get an idea of how much force was required to move them and whether they had an orientation that would help us to figure out the flow direction.”
He said researcher Hira Lodhi will be working not just with the boulders but will make computer simulations of tsunamis that will give a time history of the flows, how strong the current was at that given place and then compare that with the size of the boulder.
The team also observed a series of conch shell mounds with flat tops on Anegada with the highest found being 12 feet.
“It turns out that the conch shell mounds are probably connected to the boulders and other evidence of over wash since the flat tops can be explained by over flow from an extreme event,” Dr. Atwater explained.
Field work began on Anegada in 2008 and previously the researchers focused on brain corals and used them as a fingerprint of an offshore source. This year, the team sought to identify other corals on the island which can be used as a possible sign of a tsunami generated from the north of the island.
Dr. Atwater said if it was a big event from the north that was destructive to the reef, those forest of Elkhorn coral should have been attached and in fact, very large clasps of Elkhorn coral were found in the Table Bay area in the north-east of Anegada.
The research on Anegada will continue and new evidence will be considered and tested before any conclusions can be drawn. The research team is comprised of Brian Atwater, Robert Halley, Haider Hasan, Bruce Jaffe, Din Mohammad Kakar, Hira Ashfaq Lodhi, Pedro Matos Llavona, Jessica Pilarczyk and Michaela Spiske.
Photo Caption: File Photo –Dr. Brian Atwater, Hira Ashfaq Lodhi, Din Muhammad Kakar and Robert Halley presenting findings at DDM’s Conference Room. Photo Credit: Department of Disaster Management