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Tsunami "shadows" may provide for valuable warnings

For the first time, scientists with the National Oceanic and Atmospheric Administration have demonstrated that tsunamis in the open ocean can change sea surface texture in a way that can be measured by radars on satellites.

The finding may one day help save lives by improving detection and forecasting of the direction and intensity of tsunamis at the ocean’s surface, NOAA officials say.

"We've found that roughness of the surface water provides a good measure of the true strength of the tsunami along its entire leading edge,” said Oleg Godin of NOAA's Earth System Research Laboratory and the Cooperative Institute for Research in Environmental Sciences, who was lead author on the study.

Large tsunamis crossing the open ocean stir up and darken the surface waters along the leading edge of the wave, according to the study. The rougher water forms a long, shadow-like strip parallel to the wave. The more obvious the shadow-like strip is, the stronger the tsunami is.

The new research challenges the traditional belief that tsunamis are too subtle in the open ocean to be seen at the surface. The findings confirm a theory, developed by Godin and published in 2002-05, that tsunamis in the deep ocean can be detected remotely through changes in surface roughness.

In 1994, a tsunami shadow was captured by video from shore moments before the wave struck Hawaii. That observation and earlier written documentation of a shadow that accompanied a deadly tsunami on April 1, 1946, inspired Godin to develop his theory.

He tested the theory during the deadly December 26, 2004, Indian Ocean tsunami, the result of the Sumatra-Andaman earthquake.Satellite data revealed clear evidence of an increased surface roughness along the leading edge of the tsunami as it passed across the Indian Ocean between two and six degrees south latitude.

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