Rare tsunami may have hit U.S. East Coast in mid-June

By all accounts, there was a tsunami earlier this month, along the east coast of the United States. If you didn't notice it, you're not alone; it only became apparent when the National Oceanographic and Atmospheric Administration (NOAA) looked the observations made by coastal buoys put the data together with some first-hand human accounts.

Not exactly what you'd expect from a tsunami after seeing the massive destruction in Japan and around the Indian Ocean from other ones, right?

The phenomenon that occurred along the northeast coast two weeks ago hasn't been identified yet, but it was most likely what's known as a 'meteotsunami'. As you can probably guess from the prefix tacked on there, the term refers to tsunami-like waves caused by a weather system — in this case, a strong storm known as a derecho — that sped across the Midwest and headed off the Jersey Shore just before the odd waves were recorded.

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The West Coast/Alaska Tsunami Warning Center — the branch of NOAA that keeps an eye on such things — received an eyewitness report from a spear fisherman who observed the waves in action while fishing along the north-central coast of New Jersey. Brian Coen witnessed a strong, out-going tide around 3:30 pm on June 13; strong enough that it carried divers over the submerged breakwater which was usually 3 to 4 feet underwater and was ultimately exposed by the out-rushing water. The water continued flowing out for a couple of minutes before Brian spotted a 6-foot wave rushing back into the inlet, along with a sudden reversal of the strange current. The surge managed to knock people off rocks that were 5 to 6 feet above sea level, resulting in some injuries.

Similar tsunami-like wave action was detected by more than 30 tide gauges.

"From North Carolina up through Massachusetts, we can find that signal, even though it's very small, which tells us there was something going on," said Mike Angove, head of NOAA's tsunami program, according to the Associated Press. "We're trying to piece this back together."

Meteotsunamis require a few ingredients. To generate the phenomenon, you need a weather feature, like a front or squall line, moving at about the same speed as the waves in the water, and then some landform — typically a bay or inlet — to amplify the waves when they interact.

While meteotsunamis are not known for causing extensive damage like their earthquake-generated relatives, they do contribute to beach erosion and can cause damage to harbours and boats docked there. They also pose a threat to swimmers and unsuspecting beach visitors who might get caught by sudden currents or large waves.

Those living near the Great Lakes might be familiar with a similar occurrence — known as a seiche. These occur when water in an enclosed area, like a lake, is pushed by strong winds to form a standing wave, effectively piling up water at one end of the lake and lowering it, sometimes dramatically, at the other end. Lake Erie, with its long, oval shape and relatively shallow bottom experiences some of the most dramatic seiches on the Great Lakes, where water levels are sometimes as much as 16 feet higher on one end of the lake than the other.

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Seiches move a bit like a pendulum; because the water has nowhere to disperse (as it would over the ocean), it can continue sloshing back and forth for days after the weather system that stirred it up has passed. Hurricane Hazel famously caused a seiche along the shores of Lake Ontario in 1954, resulting in a storm surge-like effect that caused extensive flooding near Toronto, then when the wave reversed and water piled up on the southern shore of the lake, went on to cause flooding and damage there, as well.

(Images courtesy: NWS/NOAA, Great Lakes Commission)

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