When the December 2004 Sumatra earthquake struck, seismometers determined in a matter of seconds that it was big. But the instruments originally estimated its magnitude to be about 8.0, indicating essentially no risk of a major ocean-wide tsunami. Not until five hours after the quake struck did researchers revise the estimate to 9.0 or greater, which meant a high risk for a major tsunami. In the meantime, tsunami waves, moving at jet-speeds, had already crossed the Indian Ocean and violently crashed ashore in Thailand, India, Sri Lanka and elsewhere.

Indeed, time is of the essence when a giant earthquake strikes, especially underwater. Now, a team of researchers says that they have found a new way — using GPS — to more quickly determine if the quake is large enough to produce an ocean-wide tsunami.

The new technique relies on the global GPS network, in which stations are located all over the world, says Geoff Blewitt, a geophysicist at the University of Nevada in Reno. In the June 13 online Geophysical Research Letters, Seth Stein, Blewitt and colleagues reported that GPS stations can record even millimeter-sized permanent ground shifts caused by earthquakes hundreds to thousands of kilometers away from the epicenter.

The GPS stations “are very sensitive to the size of an earthquake and will pick up on the size of the earthquake within a matter of minutes,” says Stein, a seismologist at Northwestern University in Evanston, Ill. — something seismometers cannot do above magnitude 8 or so. That’s because seismometers only measure relatively “short-period” seismic waves in real time, which do not get bigger once an earthquake gets above a certain magnitude threshold, he says. Above a magnitude 8, the seismic waves become “ultra-long-period waves,” which take a matter of hours to be measured.

The problem for tsunami warnings, Blewitt says, is that a magnitude-8 earthquake would not set off an ocean-wide tsunami, but a magnitude-8.5 or larger earthquake in the right location would. Thus, as happened for the Sumatra earthquake, he says, the seismometers’ warning could come too late.

To test how GPS could help with tsunami warnings, the researchers analyzed data from 38 GPS stations up to 7,500 kilometers from the Sumatra quake’s epicenter, and found permanent ground movement within a few minutes of the quake’s strike. If these GPS stations had been transmitting data in real time, and a communication structure had been in place to provide adequate warnings, it is possible that some of the tragic loss of life could have been prevented, Stein says.

Any plan to make GPS stations part of a tsunami warning network, says Fred Pollitz, a researcher at the U.S. Geological Survey in Menlo Park, Calif., would require real-time communications and a better density of stations. While some areas of the world, such as the subduction zone off Japan and the Cascadia subduction zone along the U.S. Pacific Northwest and Canada, are already covered with a network of GPS stations, other areas, such as the Chilean subduction zone, have very few stations, he says. Thus, more stations would be needed to create a reliable global tsunami warning system that does not produce false alarms.

Additionally, Pollitz says, researchers need to better work out the algorithms to fully calculate tsunami risk based on an earthquake’s size and the ground movement measured by GPS. Despite such logistical issues, however, Stein’s team “convinced me that tsunami warning using GPS is feasible,” says Vasily Titov, a researcher at the Pacific Marine Environmental Laboratory in Seattle, Wash. “This is very exciting work.”

Megan Sever

Links:
"Reviewing Sumatra," Geotimes, December 2005
"Quake off Java triggers deadly tsunami," Geotimes online, Web Extra, July 17, 2006
Seth Stein's Web site
University of Nevada Geodetic Laboratory
Pacific Marine Environmental Laboratory

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