A report by David Fleshler for The Sun Sentinel.
Everyone in South Florida accepts the risk of hurricanes. But tsunamis?
A new study from the University of Miami found that South Florida faces a small but real risk of tsunamis generated by landslides on the undersea slopes west of the Bahamas. Such landslides have occurred in the distant past, presumably causing tsunamis then, and they are likely to occur again, according to the study published in the journal Scientific Reports.
In the worst-case scenario, the largest possible quantity of mud and sand would plunge to the ocean floor from Great Bahama Bank, displacing enough water to slam South Florida with a 15-foot tsunami. While a wave of that height may not sound impressive, it represents something far more powerful than an ordinary 15-foot wave, since a tsunami is a wall of water that carries a vast amount of water behind it.
But there’s no reason to flee to higher ground yet. The National Oceanic and Atmospheric Administration rates the tsunami risk for the east coast of Florida as low, compared to “very low” for the Gulf coast and “high to very high” for the Pacific coast. The authors of the study say there’s no reason to revise that assessment.
“We’re still low risk,” said Jara Schnyder, lead author of the study and a graduate student at UM’s Rosenstiel School of Marine and Atmospheric Science.
She said the frequency of such events could be measured in the hundreds of thousands of years.
“Even if we have an earthquake that might trigger this, it would have to trigger a landslide with a big enough mass and fast enough propagation to generate a tsunami. That’s why we call these low-probability but high- impact events.”
Working with her were other scientists from UM, as well as scientists from the University of Delaware, Université de Bordeaux in France and University of Bremen in Germany.
In the most likely scenario, an earthquake off the Cuban coast would generate the landslide, although landslides could also occur spontaneously.
Gregor Eberli, professor of marine geosciences at UM and senior author of the study, said it would take a powerful earthquake, one scoring 5.5 on the Richter scale or higher, to generate a big enough landslide to cause a tsunami, and those are rare.
The area at risk would depend on the location of the landslide. The study looked at Great Bahama Bank, one of the geologic platforms that supports the islands of the Bahamas. If the landslide took place there, the area at risk would range from Cuba to the Keys to Miami Beach. If the landslide took place at the Little Bahama Bank, the at-risk area would extend up the Florida coast to southern St. Lucie County, Eberli said.
And there are scenarios that fall short of the worst-case. The size of the tsunami would depend on the amount and velocity of the mud and sand sliding to the ocean floor. At best, it might be barely noticeable or generate severe rip currents.
Other scenarios call for significant tsunamis, but ones smaller than a house-smashing 15-footer. Facing far more devastation would be Cuba. The worst-case for that island would be a tsunami reaching a catastrophic 31feet.
Tsunamis are most common in the Pacific Ocean, where continental plate boundaries that rim the Pacific generate the string of volcanoes and earthquake zones known as the Ring of Fire. But while earthquake-generated tsunamis are the most common, there have been tsunamis in which undersea landslides, often caused by earthquakes, played the largest role in generating the giant waves.
A rare Atlantic tsunami took place in 1929, when an earthquake off Newfoundland created an undersea landslide that caused a tsunami that killed more than two dozen people. In the Pacific, a 1998 earthquake caused an undersea landslide that generated a 23-foot tsunami that destroyed coastal villages and killed more than 2,000 people.
Eberli said a tsunami hitting Florida would be unlikely to be that devastating, although he said it could be a cause of concern for the Turkey Point nuclear power plant.
“The tsunami risk is not great enough to make damage like what they have in Japan,” he said. “It could be quite devastating for coastal buildings. But it will not be like tens of thousands of people will drown. It’s not that kind of event.”