When a rare earthquake shook the northeastern United States yesterday, residents reacted with a mix of excitement, fear, panic, humor and then lots of questions.
Among the concerns was the possibility of a tsunami. Even though yesterday's quake, for a variety of reasons, posed no threat of creating devastating waves, the question remains -- could a seismic event some day spark a tsunami on the East Coast, much like the ones that have devastated Japan, Indonesia and other parts of the world in recent years?
Not likely. But it's also not impossible, according to experts. For now, risk-assessment is a challenging problem. With so little historical evidence to work with, scientists still don't have a good picture of exactly how plates and faults are structured beneath the East Coast. It's also unclear how frequently earthquakes are expected to strike there.
"We are working on it, but it's very difficult to get a sense of how often things happen," said Uri ten Brink, a research geophysicist with the U.S. Geological Survey in Woods Hole, Mass. "In general, I would say that if you're talking about people's lifespans, people shouldn't be worried. But when we do work for the Nuclear Regulatory Commission that is concerned about probabilities on a much larger time-span, like 10,000 years, that we may have to worry about."
For an earthquake to cause a tsunami, the event must meet several requirements. First, the quake has to happen under water. Second, it needs to be big enough to cause the sea floor to shift up or down a considerable distance. The result is a bump in the ocean's surface, which then propagates around the globe, dumping large volumes of water onto seashores.
Normally, it takes a magnitude of eight or higher before offshore earthquakes start sparking tsunami warnings, said John Vidale, a geophysicist and seismologist at the University of Washington, Seattle. The magnitude-9 quake that hit Japan in March, for example, may have pushed one side of the fault as much as 40 meters (130 feet) or more above the other side.
Yesterday's Virginia-based quake, on the other hand, weighed in at just 5.8. That, plus the fact that it happened some 100 miles inland, gave it a zero percent chance of producing waves. And because there are no major faults or quickly moving tectonic plates off the East Coast, the chances of a huge earthquake ever striking there remain quite low, Vidale added, especially compared to places like Japan, Indonesia or even the Pacific Northwest, where magnitude-9 or higher quakes are known to strike an average of every 500 years.
But that doesn't mean that the East Coast is risk free. Theoretically, meteorites that strike the ocean can cause massive waves. A more likely wildcard scenario is that smaller quakes could cause underwater landslides, in turn producing tsunamis.
One major source of concern for that possibility lies in the Canary Islands, Vidale said. Experts also have their eyes on the Puerto Rico trench in the Caribbean and the Gibralter Plate Boundary, off the coast of Africa and southern Europe, said Lewis Kozlosky, a physical scientist with the National Weather Service Tsunami program in Silver Spring, Md. Both areas are seismically active and have a high likelihood of producing tsunamis, given big enough earthquakes.
And although a full historical picture is missing, tsunamis have been known to hit the East Coast before. The most destructive example in modern history happened off the shore of Portugal in 1755. The resulting tsunami killed more than 60,000 people in Lisbon, ten Brink said. It also caused damage in Brazil, the Caribbean and Newfoundland, among other places. For unknown reasons, the coast of the United States was unharmed by that event, possibly because the topography of the seafloor deflected the wave.
Much later, in 1929, a 7.2 earthquake struck off the coast of Newfoundland, causing a massive landslide on the continental slope there. The result was a tsunami that reached more than 40 feet high in some places and ended up killing 27 people in the region's fishing villages.
No one can predict what the future holds in store for the East Coast, and for the most part, the risk of being washed away by a tsunami is quite low for residents of New York, Washington D.C. and other coastal areas. But risks are different from hazards, ten Brink pointed out. So many people live in such high densities on the East Coast that if a tsunami did strike, consequences would be disastrous.
For now, it can't hurt to learn the signs of a tsunami and what to do if one is approaching, Kozlosky said, just in case. Watch for water retreating from shore, for example. And pay attention if you hear a roaring sound coming from the ocean.
"If you feel strong shaking, where it's hard to stand up or it lasts for 20 seconds or more, that's a big sign to go to high ground immediately," he said. "If you see natural warnings signs, don't wait for any official warnings."