Like a shadow of death racing across the ocean surface, a tsunami churns the air and darkens the water along its path. Researchers now report that they can detect this activity with radar, improving the chances that coastal populations can escape these killer waves.
Even in the best of circumstances, tsunamis are difficult to track. They can speed along at more than 500 kilometers per hour in the deep ocean, yet they are barely perceptible on the surface. Currently, scientists detect tsunamis with sensitive instruments aboard buoys and satellites that record subtle changes in sea-surface altimeter--or elevation. But coverage is sporadic. Satellites might not be stationed over the parts of the ocean where a tsunami has formed, and buoys aren't positioned in many areas where tsunamis have struck. That's what happened on 26 December 2004, when the Sumatra-Andaman earthquake struck the Indonesian island of Sumatra, generating a tsunami that spread across the Indian Ocean Basin. Hundreds of thousands of people perished in the resulting coastal flooding, and most of them never received a warning about the impending disaster.
In 2004, two developments revealed a potential new way to track tsunamis. First, researchers led by physicist Oleg Godin at the National Oceanic and Atmospheric Administration (NOAA) in Boulder, Colorado, theorized that tsunamis could be spotted based on the turbulence the waves stirred up in the lower atmosphere. If so, "side-looking" radar instruments, which view the ocean surface at an angle, could monitor the speed and direction of this turbulence across many hundreds of kilometers.
The second development was the Sumatra-Andaman earthquake. When it hit, an orbiting NOAA satellite called Jason-1 just happened to be in position over the Indian Ocean. Although the satellite did not carry the kind of side-looking radar suggested in the theoretical study, Godin's team was able to extract much of the information they needed. Over the next several years, the NOAA team analyzed the radar data and compared them with previous years of satellite observations of the same area. The result, the researchers report this month in Natural Hazards and Earth System Sciences, is that they could clearly see a tsunami racing across the ocean toward the coastlines.
The observations "suggest the potential for 'seeing' large, destructive, oceanwide tsunamis well in advance of their hitting populated shorelines," says seismologist Daniel Walker, a tsunami adviser to the City and County of Honolulu, Hawaii. Walker, who was not affiliated with the study and whose previous work on tsunamis helped provide the basis for the new satellite-detection technique, says that if the properties of the tsunami shadow can be correlated with the size of the wave, then "a powerful new tool will be available to improve the reliability of tsunami warnings."
It's "a landmark paper, agrees seismologist Emile Okal of Northwestern University in Evanston, Illinois. But, he cautions, the technique "is still a long way from becoming operational." The problem is that it takes years to deploy new satellites, he says, so perhaps other technologies could accomplish the same task more quickly and cheaply. For example, ground-based, over-the-horizon radars, which bounce their signals off Earth's ionosphere to see far beyond their lines of sight, can detect the same sea-surface patterns as satellites, and they could be deployed much sooner, he says.
This article has been amended. The Jason-1 satellite does not carry side-looking radar, as originally reported.