Monday, July 6, 2015

How could big Jersey wave be called a tsunami?

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Reports this week of a tsunami hitting Long Beach Island conjure up images for many of us of some kind of massive "tidal wave."

In 2004, for example, an Indian Ocean earthquake produced waves nearly 100 feet high that killed more than 200,000 people in Indonesia.

On June 13, near Barnegat Light, two people needed medical treatment after a six-foot wave swept three people off a jetty. (See "Tsunami in New Jersey linked to derecho?")

How exactly could that be called tsunami?

Basically, a tsunami is a kind of wave, not a size of wave, explains Philip Liu, a Cornell University professor of civil and environmental engineering.

A tsunami could be as short as two feet, said Liu, who led a team of scientists that studied the Indian Ocean disaster and has been studying ocean waves for three decades.

Unlike typical shoreline breakers, driven by winds and tides, that happen repeatedly, a tsunami is an unusual pulse-like wave generated by geophysical events such as underwater earthquakes, landslides or volcanoes.

Like an earthquake, which can level a city or just shake your chair, a huge range of magnitudes is possible.

In the New Jersey case, the cause might have a been a straight-line wind storm called a derecho, which suddenly created a kind of storm surge. "It is also possible that the slumping at the continental shelf east of New Jersey played a role," according to the National Oceanic and Atmospheric Admiinstration's Tsunami Warning Center.

"Slumping" suggests more of a collapsing than a landslide, Liu said.

Because weather was involved, this type of tsunami can be called a "meteotetsunami," he said.

A tsunami, by the way, is different from a "rogue wave," which a separate term for sudden large waves that don't seem to have a geophysical cause, he said. They might be caused by several waves randomly joining together, for example.

Contact staff writer Peter Mucha at 215-854-4342 or pmucha@phillynews.com.

Philly.com
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