Should East Coast residents worry about tsunami from Canary Islands?

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?

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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."

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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.
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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.