A team led by Geoffrey Blewitt of the Nevada Bureau of Mines and Geology and the University of Nevada, Reno, demonstrated that a large quake's true size and potential to generate a major ocean-wide tsunami can be determined within 15 minutes using GPS data, much faster than is possible with current methods. They report their findings this month in Geophysical Research Letters, published by the American Geophysical Union.
"Tsunami warning is a race against time," says Seth Stein, of Northwestern University in Evanston, Illinois, a co-author of the paper. "Tsunamis travel at jet speed, so warning centers must accurately decide, within minutes, whether to issue alerts. This has to be done fast enough for the warning to be distributed to authorities in impacted areas so they can implement response plans. Together with seismometer and ocean buoy data, GPS adds another tool that can improve future tsunami danger assessments."
"We'll always need seismology as the first level of alert for large earthquakes, and we'll need ocean buoys to actually sense the tsunami waves," added Blewitt. "The advantage of including GPS in warning systems is that it quickly tells how much the ocean floor moved, and that information can directly set tsunami models into motion."
The new method, called GPS displacement, is based upon measuring precisely when radio signals from GPS satellites arrive at ground stations located within a few thousand kilometers [miles] of a quake. From these data, scientists can calculate how far the stations moved because of the quake, and then derive the quake's true size, called its "moment magnitude." This magnitude is directly related to a quake's tsunami-generation potential.
Current scientific methods cannot quickly determine moment magnitude for very large earthquakes, as illustrated by the magnitude 9.2-9.3 Sumatra quake of December 2004. That quake was first estimated at 8.0 using seismological techniques designed for rapid analysis. Because these techniques derive estimates from the first seismic waves to arrive, they tend to underestimate earthquakes larger than about 8.5, the minimal size that can generate major ocean-wide tsunamis. The initial estimate of 8.0 was the primary reason warning centers in the Pacific significantly underestimated the earthquake's tsunami potential.
The potential of GPS to contribute to tsunami warning became apparent after the Sumatra earthquake, when GPS measurements showed it moved the ground permanently more than one centimeter [0.4 inches] as far away as India, more than 2,000 kilometers [1,200 miles] away from the epicenter. "With signals like that, an earthquake this huge can't hide," said Blewitt. "We hypothesized that if GPS data could be analyzed rapidly and accurately, they would quickly indicate the earthquake's true size and tsunami potential."
To test the feasibility of their approach, the scientists used JPL's satellite positioning data processing software to analyze data from 38 GPS stations located at varying distances from the Sumatra quake's epicenter. The software pinpoints a station's precise location to within seven millimeters [0.3 inches]. Only data that were available within 15 minutes of the earthquake were used. Results indicated that most of the permanent ground displacements occurred within a few minutes of the arrival of the first seismic waves. Their analysis inferred an earthquake model and a moment magnitude of 9.0, very near the earthquake's final calculated size.
The GPS software technology described in this report was funded by NASA.
Harvey Leifert | American Geophysical Union
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