Combining two data sources could bring better warning of earthquakes to millions of people in vulnerable areas of the western United States, researchers say.

Scientists at the annual meeting of the Seismological Society of America in San Diego said combining information from GPS monitors to data recorded by a vast network of seismometers already dotting the western states could produce more accurate early warning of powerful earthquakes.

The seismometers are currently the only source of information used to alert people in the moments before powerful temblors arrive, they said.

"We depend on seismic networks for early warning, but they have limitations, especially for large earthquakes," Yehuda Bock, a research geodesist at Scripps Institution of Oceanography in La Jolla, Calif., said.

While seismometers close to an earthquake can detect the energy it releases and send that information in real time to distant places, providing warning before the ground shaking reaches a population center, they have trouble accurately assessing magnitudes larger than 7.

GPS, on the other hand, is very good at assessing magnitude, because it accurately records how much the ground is moving.

"We're talking about real-time accuracy within centimeters," Richard Allen, director of the Berkeley Seismological Laboratory said.

Researchers sited Japan's March 2011 earthquake, which authorities estimated as a magnitude 8 earthquake within about 120 seconds,

"A magnitude 8 is a very large earthquake, but it's not expected to produce nearly as large a tsunami as you did have," Walter Szeliga, a geodesist and research professor at Central Washington University, told OurAmazingPlanet.

Combined seismic and GPS data would have revealed far more quickly that the quake was in fact a magnitude 9, about 30 times more powerful, he said.

Such a combined system could protect the western United State in a similar situation, researches said.

"When people experience an earthquake, they just experience the strong shaking at their location," Allen said. "But the earthquake probably started seconds or minutes earlier at some distant location; you use the instruments close to the epicenter to constrain its size, so people get a warning before they feel the shaking."