Sometime between 1381 and 1391, an earthquake exceeding magnitude 8.0 rocked the northeastern Caribbean and sent a tsunami barreling toward the island of Anegada.

, depositing coral boulders hundreds of meters inland. The corals died but their skeletons remain. More than six centuries later, scientists are learning that these skeletons hold clues about tsunami history. showed the flooding likely resulted from a tsunami generated during a large earthquake in the nearby Puerto Rico Trench.

Now, in an open-access paper , researchers narrow the tsunami time frame to the last decades of the 14th century. The researchers expect this finding to support ongoing .

“If you’re designing a school or a hospital near the coast, you want to know whether there’s a chance that a very big earthquake could occur, and you want to design that building to withstand it,” said corresponding author , a 91Ě˝»¨ affiliate professor of Earth and space sciences and research geologist with the United States Geological Survey.

Anegada is the northernmost of the British Virgin Islands, sitting just south of the , where the Caribbean and North American plates converge. Most of the islands are protected by a broad, shallow continental shelf. Waves lose energy as they roll across the expanse, decreasing the chances of a tsunami hitting Caribbean shores. Anegada is different — the seafloor slopes steeply toward the deep trench, making the island more hazard prone.

Written records from the northeastern Caribbean go back five centuries, but none provide evidence for a tsunami from the Puerto Rico Trench. Geology allowed the researchers to evaluate tsunami history on a longer timescale.

Researchers began surveying the region after a massive earthquake and tsunami struck the Indian Ocean in 2004, .

The disaster surprised everyone, including researchers, prompting officials in the U.S. to on the Atlantic seaboard. , one of the project leads and a research geophysicist at Woods Hole Coastal and Marine Science Center, asked Atwater to check for signs of similar activity on Anegada. Atwater spent years in Indonesia after the tsunami.

The evidence uncovered on Anegada drew various research teams to the island and produced a series of discoveries.

In the most recent study, led by , an associate research professor at the University of Maryland Center for Environmental Science, the researchers present a time frame for the medieval tsunami based on how old the coral was when it died.

They calculated age by measuring two radioactive elements — uranium and thorium — that decay at known rates. These measurements were made on samples from the inside of the coral skeletons, due to weathering and potential contamination. The researchers then added the number of annual growth bands between the dated sample and the exterior of the coral to estimate when the tsunami occurred.

“Corals have annual density bands, much like tree rings,” Kilbourne said. “We were able to count how many years passed between the top density bands and the sections we used for dating.”

Kilbourne can also gather valuable environmental data from the coral skeletons, which store information about temperature and salinity, and plans to continue studying the samples to better understand climate change over longer timescales.

For more information, contact Atwater at atwater@uw.edu or Kilbourne at kilbourn@umces.edu.

Additional co-authors include and at Paris Institute of Earth Physics; at Aix-Marseille University; at the University of Delaware; at National Taiwan University and at Colorado Mesa University and the United States Geological Survey. 

This research was funded by the U.S. National Science Foundation, the University of Paris-IPGP, the French National Research Agency, Academica Sinica, the Higher Education Sprout Project of the Taiwan Ministry of Education, the National Taiwan University Core Consortiums Project, the Taiwan National Science and Technology Council.

A new , to be published in the September issue of , may shake the research community’s confidence in what the sediment record can say about past earthquakes. The lead author is , a 91Ě˝»¨affiliate professor of Earth and space sciences and a geologist with the U.S. Geological Survey.

The report focuses on the Cascadia subduction zone — a giant active submarine fault that slants eastward beneath the Pacific coast of southern British Columbia, Washington, Oregon and northern California. Studies in the past three decades have provided increasingly specific estimates of Cascadia earthquake sizes and repeat times, which affect public safety through seismic provisions in building design and tsunami limits on evacuation maps.

At issue is not whether the Cascadia subduction zone produces enormous earthquakes, Atwater said. What the report asks instead is how much geologists can say, with confidence, about the history of those earthquakes going back thousands of years.

Sediment cores collected over the past two decades near the foot of the continental slope, about 100 miles off Washington’s coast and on seafloor sloping to nearly two miles deep, are among a broader set of cores that underpin influential estimates of Cascadia earthquake size and recurrence in 2012.

The new report questions the geologic basis for these estimates, not by collecting newer samples, but by looking at a larger suite of cores collected and first analyzed in the 1960s and 1970s. Authors visited the 91Ě˝»¨Libraries and dusted off oceanography doctoral dissertations submitted more than four decades ago.

The new study draws on sediment core log notes handwritten by co-author Bobb Carson (’71), and another 91Ě˝»¨alumnus, William Barnard (’73), during research cruises off the Washington coast. Both were graduate students with Dean McManus, a 91Ě˝»¨emeritus professor of oceanography. Other samples were collected by co-author , then a graduate student at Oregon State University and now a scientist at the University of California, Santa Cruz.

Those Nixon-era sediment cores were originally collected for research unrelated to earthquakes. The scientists were interested in tracing – beds of sand and mud laid down by bottom-hugging, sediment-driven currents that infrequently emerged from submarine canyons onto the deep ocean floor. Not until a 1990 by a Canadian government geologist would turbidites be reinterpreted as clues to Cascadia earthquake history.

The new study asks how well geologists have managed to read that evidence. Images of the marine sediment layers collected in 2012 by co-authors , a 91Ě˝»¨professor of oceanography, and current 91Ě˝»¨oceanography graduate student suggest that in some of the deep-sea canyons along the Washington coast, shaking would not send the sediment flowing as the current models predict.

The historical samples confirmed that this can happen.

“Few earthquakes managed to register at Bobb [Carson]’s core sites offshore of northern Washington,” Atwater said. “For seismology it would be great if earthquakes and turbidites were to correspond one for one. But Bobb’s geology makes clear that it’s not nearly that simple.”

Carson said he was surprised to be contacted about his 91Ě˝»¨doctoral work.

“Wouldn’t anyone be?” said Carson, who went on to become a professor and dean before retiring from Pennsylvania’s Lehigh University. “Usually you think the data will be used once in a paper, and people will refer to it for four or five years, but you don’t expect it to be resurrected after 45 years.”

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For more information, contact Atwater at 206-553-2927 or atwater@uw.edu.

This article was adapted from a USGS   See also a from UC Santa Cruz.