June 28th, 2006 10:34 EST
University Scientists Using Global Positioning System Software
University scientists using Global Positioning System software, developed by NASA, have shown GPS can determine, within minutes,
whether an earthquake is big enough to generate an ocean-wide
tsunami. This NASA-funded technology can be used to provide faster
A team led by Geoffrey Blewitt of the Nevada Bureau of Mines and
Geology and Seismological Laboratory, University of Nevada, Reno,
demonstrated that a large quake's true size can be determined within
15 minutes using GPS data. This is much faster than current methods.
"Tsunami warning is a race against time," said co-author Seth Stein,
Department of Geological Sciences, Northwestern University, Evanston,
Ill. "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," Blewitt said. "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, works by measuring the time
radio signals from GPS satellites arrive at ground stations located
within a few thousand kilometers of a quake. From these data,
scientists can calculate how far the stations moved because of the
quake. They can derive an earthquake model and the quake's true size,
called its 'moment magnitude.' This magnitude is directly related to
a quake's potential for generating tsunamis.
As illustrated by the magnitude 9.2-9.3 Sumatra quake of December
2004, scientific methods have difficulty quickly determining moment
magnitude for very large quakes. That quake was first estimated at
8.0 using seismological techniques designed for rapid analysis.
Because these techniques derive estimates from the first recorded
seismic waves, they tend to underestimate quakes larger than about
8.5. That is the approximate size needed to generate major ocean-wide
tsunamis. The initial estimate was the primary reason warning centers
in the Pacific significantly underestimated the earthquake's tsunami
The potential of GPS to contribute to tsunami warning became apparent
after the Sumatra earthquake. GPS measurements showed that quake
moved the ground permanently more than 0.4 inches as far away as
India; about 1,200 miles away from the epicenter. "With signals like
that, an earthquake this huge can't hide," Blewitt said. "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 NASA'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 0.3 inches. Only data that were available within 15 minutes of
the earthquake were used. Results indicated 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
"Modeling earthquakes with GPS requires a robust, real-time ability to
predict where GPS satellites are in space with exacting precision,
which our software does," said Frank Webb, a NASA geologist at the
agency's Jet Propulsion Laboratory, Pasadena, Calif. "This technique
improves rapid estimates of the true size of great earthquakes and
advances real-time tsunami modeling capabilities."
Results of the study are published in Geophysical Research Letters.
Media contacts: Jill Boudreaux, University of Nevada, Reno,
775-784-4611; Megan Fellman, Northwestern University, 847-491-3115;
and Harvey Leifert, American Geophysical Union, Washington,
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