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 / Updated  / Source: Live Science

How unusual slow earthquakes can spawn powerful tsunamis is a long-standing mystery that researchers may have finally solved. Called "tsunami earthquakes," these slow quakes are capable of creating huge waves that can cause serious damage to coastal cities.

Tsunami earthquakes are not like typical earthquakes. They happen slowly and don't generate the same kind of violent shaking as typical earthquakes — the tell-tale sign that it's time to evacuate.

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Scientists first discovered tsunami earthquakes 35 years ago and they happen so rarely there has been little opportunity to study them since. Now, a new study suggests that tsunami earthquakes happen when two sections of Earth's crust, called tectonic plates, get hung up on extinct volcanoes on the ocean floor, called seamounts. The seamounts act like tread on a tire and make tectonic plates stick.

The research team realized these extinct volcanoes sometimes get squashed in subduction zones. A subduction zone is where one tectonic plate is sliding under the other plate.

The actual rupture of tsunami earthquakes is slow compared with regular earthquakes, which happen at 335 to 670 miles per hour. In regular earthquakes that rupturing can happen two or three times faster. The slow rupture allows time for huge waves to swell.

Tsunamis, like the one that stuck Aceh, Indonesia, can cause serious flooding and submerge entire villages.United States Navy

Bell and the team estimate two tsunamis that struck New Zealand in 1947 might have reached 43 feet. Since the New Zealand tsunami earthquake, scientists think there have been nine other tsunami earthquakes. In 2006, an earthquake off the coast of Indonesia with the same magnitude created a wave 23 feet tall that drowned 637 people.

"These tsunami earthquakes create very little ground shaking, but they shake the ground gently for a long time," lead researcher Rebecca Bell from Imperial College London, told Live Science. "The best warning for residents living close by is that if they feel a very prolonged earthquake, even if the shaking is gentle, they should evacuate to high ground. New tsunami warning signs in New Zealand now use the tag line 'Long, strong, gone'."

- Kelly Dickerson, Live Science

This is a condensed version of a story that appeared in Live Science. Read the entire story here. Follow Kelly Dickerson on Twitter. Follow us @livescience, Facebook & Google+.

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