A peaceful stretch of ocean floor runs along the edge of North America, where the land slowly disappears and falls into the deep Atlantic. This area probably doesn’t get nearly as much attention as it should. It’s known as the continental slope by scientists. A lot of people have never heard of it. What worries researchers like Uri ten Brink, a geologist with the U.S. Geological Survey, is that not enough people know about these underwater canyons. He has spent years mapping them, one sonar image at a time, looking for something that no one wants to find.
Based on what he and his coworkers have found, the risk seems real. A landslide under the water along this continental slope, caused by an offshore earthquake with a magnitude of 4.5 or higher, could send waves over 26 feet high. Underwater bays and canyons could push those waves further inland, making them bigger in ways that are hard to plan for and hard to predict. It sounds like something out of a disaster movie until you find out it has already happened, just close enough to serve as a warning.
A 7.2-magnitude earthquake happened south of Newfoundland in 1929. It caused a huge underwater landslide. The resulting wave killed 28 people on the island. As it sped up into narrow coastal inlets, it made walls of water up to 43 feet high. The effects were pretty minor along most of the U.S. East Coast—no deaths and smaller waves. That’s both comforting and wrong, though. Submarine landslide tsunamis usually hit hard along the coasts they are close to before losing power as they move farther away. It was not in New York or Boston, but in Newfoundland, which is close to Canada. The next one might not be.
For more than five years, Ten Brink’s team has been carefully laying the groundwork. They have been collecting sediment cores from underwater canyons, studying them for signs of past slides, and trying to guess how often these landslides really happen. It’s boring and takes a long time. Sediment core samples don’t come with any exciting field missions or news stories. But this kind of baseline research is needed to figure out how dangerous something is, and for most of the East Coast, this information hasn’t been available until recently. “There are more questions than answers at this point,” ten Brink has said. Coming from someone who has worked on the problem for years is an honest statement in and of itself.
Not only the continental slope is a worry. The Puerto Rico Trench is in the northeast Caribbean. It has a subduction zone, which is a type of geological feature that is usually found in the Pacific Ring of Fire. The 2004 Indian Ocean tsunami was very bad, so the U.S. government paid for studies to see if the Puerto Rico Trench was also dangerous. So far, what we know about the fault there suggests that it probably won’t cause a wave big enough to hit the East Coast directly and destroy it. But the Caribbean is a different story. There was an earthquake in this area in 1918 that caused a tsunami that killed 116 people in Puerto Rico. Since the 16th century, there have been 27 recorded tsunamis in the Caribbean. Each one adds to a pattern that is harder to ignore than it might seem on a calm afternoon in coastal Virginia.
Researchers also keep an eye on a major fault off the coast of Portugal, the volcano in the Canary Islands that is sometimes mentioned in scary news stories, and a slow-moving fault near Cuba that hasn’t been studied much because of political problems between the two countries. Some of these ideas, like the one about the Canary Islands, have been largely shot down by scientists. Ten Brink himself doesn’t think that threat is real from the East Coast. Some things are still not well understood because no one has looked closely enough.
What’s harder to take back is the main point that Hurricane Sandy made clear in 2012. The U.S. East Coast has some of the world’s most densely populated and economically important cities, but it doesn’t have any infrastructure that would help with a tsunami. Even though Sandy wasn’t a tsunami, it still caused a lot of damage. Take a moment to think about that. We are still learning more about what lies below the Atlantic. The talk about being ready hasn’t really begun yet.
