By Lisa Grossman, Science News
WARNING STOP. INCOMING TSUNAMI STOP. Giant waves might one day send scientists such an underwater telegram via telecommunication cables on the ocean floor.
Ocean water interacting with Earth’s magnetic field could create strong enough signals in the underwater cables to alert scientists that a tsunami is on the way, a paper in the February Earth, Planets and Space argues.
“This is a very good supplementary, augmentative system to the existing tsunami warning,” says Manoj Nair, a geophysicist at NOAA’s National Geophysical Data Center in Boulder, Colo., and a coauthor of the study. “It can be information in places where we don’t have any information.”
Though tsunamis move tremendous amounts of water at hundreds of kilometers per hour, their passage barely makes a ripple on the surface of the open ocean. But moving huge volumes of saltwater can make another kind of wave below: electromagnetic waves.
Because it is so salty — and therefore full of freely moving sodium and chloride ions — ocean water is a good conductor of electricity. As water passes through Earth’s magnetic field, the movement of positive and negative ions generates a weak electric field.
That electric field can induce a voltage, the force that drives electric current, in the kilometers-long telecommunication cables that crisscross the ocean floor. So a tsunami moving large amounts of water quickly can generate a pulse in the voltage. If the voltage is large enough to be detected above background noise, it could serve as a warning from an imminent wave.
This effect had been hinted at as early as 1971 and is now routinely used to monitor long-term ocean water flow across the Florida Strait as a measure of climate change. In 1995, Bell Labs researchers suggested that water movement triggered by the 1992 Cape Mendocino earthquake could be detected through submarine cables, as well. But detailed studies of how much voltage a tsunami would produce were lacking.
“Only recently we had the capability of doing such sophisticated numeric and computer simulations,” Nair says.
Nair and his colleagues built a 3-D computer model of the 2004 Indian Ocean tsunami, which was triggered by a magnitude 9.3 earthquake off the coast of Indonesia and killed more than 180,000 people. The researchers calculated that three cables at the bottom of the Indian Ocean could have seen voltages of about 500 millivolts, well above the estimated 100 millivolts of background noise from the ionosphere and other sources. Future research will investigate ways to further subtract out background noise, Nair says.
The system wouldn’t replace current tsunami early warning systems, which use direct measurements from pressure sensors on the ocean floor. Those systems are still superior — they are more reliable and give more information about the origin and direction of a tsunami, Nair says. But using the preexisting cables could cheaply supplement existing observations.
“We argue that we have cables already, and setting up a voltage difference measurement system is pretty easy and inexpensive,” he says. “This can really augment the tsunami measuring system we have already in existence.”
The paper doesn’t address the logistics of monitoring such a system, however.
The idea sounds feasible, says Mark Everett of Texas A&M University in College Station. “I think the results are sufficiently encouraging that a move could be made to develop an experimental system for the Indian Ocean.”
Smaller but still-devastating waves could slip under the noise threshold unnoticed, cautions Alan Chave of Woods Hole Oceanographic Institution in Massachusetts. “Being able to detect the tsunami of the century is not really what’s needed,” he says. “You’d need to be able to detect things that are more common that would place people at risk.”
Chave adds: Regardless of the detection method, the usefulness of any tsunami early warning system is limited by authorities’ ability to quickly warn the public.