Catching Waves: Ocean-surface changes may mark tsunamis

A new theoretical model that describes a tsunami’s interaction with winds may explain enigmatic observations associated with some of the high-speed ocean waves and could lead to a technique for spotting approaching tsunamis long before they hit shore.

On several occasions, people have observed dark, kilometer-wide bands on the ocean surface as tsunamis approached or passed by—a phenomenon that researchers call a tsunami shadow. In 1946, a pilot flying north of Hawaii reported a dark band on the ocean surface that outraced his aircraft. That shadow was associated with a tsunami spawned by a temblor off the coast of Alaska.

On Oct. 4, 1994, observers at several coastal sites in Hawaii spotted such shadows upon the arrival of tsunamis that had been triggered by an undersea earthquake near Japan. One of these shadows, which was captured on videotape, stretched across the horizon as it raced toward shore.

Tsunamis can be generated not only by underwater earthquakes but also by submarine landslides or even large meteorites splashing into the sea. The waves travel more than 700 kilometers per hour where the ocean is 4 km deep or deeper, says Oleg A. Godin of the National Oceanic and Atmospheric Administration in Boulder, Colo. Even at those high speeds, however, a tsunami can take up to half a day to traverse an ocean basin.

Godin’s mathematical models of the shadows, which he chronicles in an upcoming Journal of Geophysical Research (Oceans), suggest that even though speeding tsunamis are only centimeters tall on the open sea, they significantly disrupt winds just above the ocean’s surface. That disturbance in the winds, in turn, affects the ocean surface, roughing it up in some places and smoothing it in others. Tsunamis with enough power to be destructive once they reach shallow waters could change open-ocean roughness by 10 to 15 percent, says Godin. When these differences in water-surface roughness are clearly bounded, the result may appear to be a shadow, he says.

If this explanation of tsunami shadows is correct, these phenomena could identify—and possibly suggest the size of—approaching tsunamis. Sensors on satellites, aircraft, or radar that can gaze over the horizon and even through clouds could look for such surface anomalies.

Such monitoring schemes could be a valuable supplement to the few deep-ocean sensors deployed to detect tsunamis, says Daniel A. Walker, a retired geophysicist who serves as adviser to the Oahu Civil Defense Agency in Honolulu. A well-placed satellite could watch the entire Pacific for tsunami-induced changes in ocean roughness and possibly alert scientists more quickly than the current network of buoys does, he notes.

Any technique that could reduce the number of false alarms related to tsunamis would be welcome, says Walker. He estimates the average cost of unnecessary evacuations in Hawaii at about $60 million per event.

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