Coral skeletons left by major earthquake over 630 years ago serve as a warning for Caribbean region

Coral skeletons left by a tsunami over 630 years ago are a warning for the Caribbean region, according to new research.

An earthquake exceeding magnitude 8.0 rocked the north eastern Caribbean sometime between 1381 and 1391, sending a tsunami toward the island of Anegada, say scientists.

Flooding scattered debris across the island, in what is now the British Virgin Islands (BVI), depositing coral boulders hundreds of yards inland.

The corals died, but their skeletons remain.

More than six centuries later, scientists say that the skeletons hold vital clues about the history of tsunamis.

Computer models showed the flooding likely resulted from a tsunami generated during a large earthquake in the nearby Puerto Rico Trench.

The new study, published in the journal Geophysical Research Letters, narrows the tsunami time frame to the last decades of the 14th Century.

The researchers say their findings will support ongoing efforts to prepare for future Caribbean tsunamis.

Corresponding author Professor Brian Atwater said: “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.”

Prof. Atwater, of the University of Washington and a geologist with the United States Geological Survey, added: “Anegada is the northernmost of the British Virgin Islands, sitting just south of the Puerto Rico Trench, 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 region 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.

Scientists began surveying the region after a massive earthquake and tsunami struck the Indian Ocean in 2004, killing more than 250,000 people.

The disaster surprised everyone, including researchers, prompting US officials to take a closer look at coastal hazards on the Atlantic seaboard.

Prof. Uri ten Brink, one of the project leads and a research geophysicist at Woods Hole Coastal and Marine Science Centre, asked Prof. Atwater to check for signs of similar activity on Anegada.

Prof. 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 Professor Hali Kilbourne, researchers presented 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.

The measurements were made on samples from the inside of the coral skeletons, due to weathering and potential contamination.

The research team then added the number of annual growth bands between the dated sample and the exterior of the coral to estimate when the tsunami occurred.

Prof Kilbourne, of the University of Maryland Centre for Environmental Science, said: “Corals have annual density bands, much like tree rings.

She added: “We were able to count how many years passed between the top density bands and the sections we used for dating.”

Prof. Kilbourne plans to continue studying the samples to better understand climate change over longer timescales.