Tsunami forecasting: A new use for GPS?

A new tsunami-detection technique that uses Global Positioning System data could potentially save many lives by helping alert people in time to escape. University researchers have shown that the technique, which relies on NASA-funded GPS software, can determine within 15 minutes whether an earthquake is strong enough to generate a tsunami like the one that devastated many parts of Asia in December 2004.

Now scientists can estimate an earthquake’s potential for causing a tsunami using deep ocean buoy readings and seismometer data. But those methods have weaknesses. Adding GPS data to the mix may provide an earlier and clearer indication of a potential tsunami.

The new approach, called GPS displacement, works by measuring the time it takes for radio signals from GPS satellites to arrive at ground stations close to the earthquake. Using that information, scientists can calculate how far the earthquake pushed the ground station. They enter that distance into a computer model that calculates the earthquake’s magnitude, which can then indicate whether a tsunami might occur.

GPS-based detection offers the most advantages when an earthquake has a magnitude of at least 8.5, NASA scientists say. With the current detection methods, scientists struggle to rapidly assess the size of large earthquakes.

Sizeable events, such as the 2004 earthquake near Sumatra, Indonesia, stir ground motions that overwhelm nearby seismometers. For example, seismological techniques first gauged the 9.2-9.3 Sumatra earthquake at 8.0. NASA researchers say that initial prediction was the main reason warning centers in the Pacific region underestimated the earthquake’s tsunami potential.

Data from deep ocean buoys measure size more accurately but not until some time later when the tsunami reaches a buoy’s location.

GPS data can offer an early, vivid depiction of what is to come because most earth surface movement takes place within 15 minutes of an earthquake, said Geoffrey Blewitt, a professor of space geodesy at the University of Nevada who led the research team. The GPS’ value is that it bridges the gap between seismic and buoy data, he said.

Scientists are unsure, however, whether GPS data could help predict confined tsunamis, such as the recent one off the coast of Java, Indonesia. NASA scientists who assisted in Blewitt’s study said GPS could work in such circumstances if the GPS global network were more robust.

Chile’s coast is an example of an area that lacks adequate GPS coverage even though it is near an active earthquake zone. On the other hand, GPS coverage for scientific research already exists in the U.S. Pacific Northwest, and the federal government has funded the installation of more GPS stations in Washington, Oregon and Alaska. But most GPS stations would need upgrades to continuously transmit data, said Willy Bertiger, a NASA Jet Propulsion Laboratory GPS systems engineer and a member of the NASA team that wrote the GPS software used in the tsunami research.

GPS detection requires 15 minutes of data after the earthquake, but no GPS ground stations exist in Java and many other vulnerable areas. The GPS software could have spotted the recent Java tsunami, which took about an hour to form after the earthquake, if the global GPS network had been more concentrated, Blewitt said.

More regional networks of real-time GPS transmitters and receivers are necessary along coastlines that are prone to earthquakes for this technique to be effective, Bertiger said. “JPL has constructed a [global] infrastructure for real-time GPS data return,” Bertiger said, adding that scientists could expand the infrastructure along areas where two tectonic plates meet.

“We believe that with a denser GPS station coverage than is currently available and given sufficient computing power, an early warning might have been able to reach [Java’s] Pangandaran beach closely prior to the tsunami arrival,” said JPL geophysicist Sharon Kedar, one of Bertiger’s colleagues.

Saving lives, however, requires more than just good forecasting. “We couldn’t have done anything in 2004 even if we had the system that we wanted,” Blewitt said. Alerting people requires an effective communication system that immediately warns public officials, first responders and people in danger. Lawmakers are working with U.S. and international officials to improve community preparedness.

Adding GPS-based detection to the tsunami observation system would not cost much, researchers say. The computer models can run on a desktop computer.

Installing several dozen $10,000 GPS receivers in each major area where two tectonic plates meet might be expensive, but other agencies that use GPS, such as the Federal Aviation Administration, could share the costs, Blewitt said.

So why didn’t researchers think of using GPS for tsunami detection before now?

Seismology is a decades-old discipline, but a global network of GPS stations has been available for only a few years, Blewitt said. “When the instruments are not there, you tend to not think of ways in which you can use them,” he said. “In retrospect, I guess it should have been more obvious.”