By Elizabeth Svoboda
02:00 AM Dec. 19, 2005 PT
GPS satellite receivers are already navigational must-haves for hikers and drivers. Now scientists are hatching plans to press them into service as tsunami predictors.
International organizations like the Pacific Tsunami Warning Center, or PTWC, in Hawaii currently depend on coastal seismic stations to record deep-sea earthquakes that could cause giant waves. But according to Jeff Freymueller, a geophysicist at the University of Alaska Fairbanks, data from GPS receivers could provide quicker, more accurate estimates of the magnitude of a tsunami-causing quake, buying time for evacuation. Freymueller presented his findings at this week's American Geophysical Union conference in San Francisco.
Unlike seismometers, GPS receivers can measure the movement of the ground in real time. Because quake magnitude is a direct function of how much the earth shifts, Freymueller has demonstrated that the receivers can obtain precise measurements of a massive quake's severity in as little as 20 minutes.
"Seismometers measure the velocity of the ground, and you have to collect a number of cycles of the important wave in order to get that measurement," he said. "GPS receivers measure the static displacement of the earth, and after the first few minutes of a quake, that doesn't change much."
Freymueller envisions a new tsunami-warning strategy that would use seismic and GPS data in tandem to calculate a wave-causing quake's strength soon after its onset. This would enable more-accurate computer simulations of the coming wave, allowing more-targeted evacuation strategies. Planting the receivers every hundred miles in tsunami-prone areas, he added, could be done in a matter of months, and each receiver would cost less than $10,000.
"Early warnings from GPS could save thousands of lives," he said. "In last year's Indian Ocean tsunami, there were potentially one to two hours for evacuation, had an accurate warning system been in place. Every minute counts."
Seismic measurements of very large quakes like the one that caused last year's Indian Ocean tsunami take several hours to fine-tune, because the moving vibrations must be recorded at a variety of stations in different locations. When the quake that caused the giant Southeast Asian wave first hit, scientists at the PTWC estimated its magnitude at 8.0, but revised their estimate to 8.5 an hour later. After a few more hours passed, a team at Harvard University pegged the quake at 8.9. The final reading, 9.2, was not agreed upon until months afterward.
Yehuda Bock, a geologist at the Scripps Institution of Oceanography, has also investigated the possibilities of using GPS receivers in tsunami-warning systems. His results are similar to Freymueller's, indicating the receivers can gauge the ground movements created by tsunami-causing quakes with unprecedented precision and speed.
"With GPS, the displacements are measured second by second," said Bock, who also presented at the American Geophysical Union conference. "Within 70 seconds you have a good idea of the final deformation." In addition to predicting tsunamis, he thinks GPS modules could be used to monitor the activity of volcanoes and landslides in real time.
Like Freymueller and Bock, Peter MacDoran, a GPS expert who works for George Washington University's Space and Advanced Communications Research Institute, wants to make GPS receivers part of disaster-prediction networks. But he foresees using them in a different way: to track the movement of tsunami-associated pressure waves in the Earth's atmosphere.
"Quakes that cause tsunamis create deformation on the surface of the water, and that causes an atmospheric 'thump,'" MacDoran said. "A compression wave travels into the upper atmosphere, and that disturbance causes subtle changes in the way GPS signals travel." Digital processing of the changed signals coming from nearby receivers would indicate that a tsunami was imminent.
MacDoran has proposed setting up networks of GPS-connected personal computers to monitor these signals, especially in tsunami-prone areas like Southeast Asia, the United States' Atlantic coast and the Pacific Northwest.
He emphasized, however, that his goal is to complement seismic-based tsunami-detection strategies, not replace them. "The quake sensors we have work well. Seismic sensing is a highly developed art," he said. "It just doesn't give you all the information you need."