Geophysics

Tsunami and Its Shadow

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Science  11 Jun 2004:
Vol. 304, Issue 5677, pp. 1569
DOI: 10.1126/science.304.5677.1569a

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Tsunamis are long-wavelength and long-period surface waves generated by coastal earthquakes, landslides, or volcanoes. Tsunamis can travel great distances without losing much energy, e.g. from Chile to Japan or Alaska to Hawaii, yet they are undetectable in the open ocean because of their slow speed and negligible amplitudes. When a tsunami reaches shallow water near land, it slows down further, but grows in height, typically to 2 to 3 m, although devastating tsunamis as high as 30 m have been recorded.

A tsunami shadow is a darkened strip of water observed ahead of the wave on rare occasions. Godin determined that a tsunami creates perturbations in the wind velocity along a thin layer of air above the sea. The perturbations enhance the roughness of the sea surface and create a darker strip parallel to the wave front between the troughs and crests. With this model of the shadow, a tsunami can be detected in the deep ocean, far away from land, with airborne and satellite-based radars and radiometers. This should allow tsunami warnings in distant coastal areas to be more accurate and more effective. — LR

J. Geophys. Res. 109, C05002 (2004).

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