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Tsunami warnings could reach vulnerable coastlines within minutes, thanks to an early-warning system that gauges how long an earthquake rumbles.

Most tsunami-warning systems work by measuring an undersea earthquake's magnitude, because those above magnitude 7.5 are considered highly likely to generate a tsunami. However, it takes at least 30 minutes to measure this accurately.

Previous studies have shown that quakes that shake for a long time are more likely to produce a tsunami. Now Anthony Lomax, a consultant seismologist based in Mouans-Sartoux, France, and Alberto Michelini of the National Institute of Geophysics and Volcanology in Rome, Italy, have developed a way to spot this signature quickly.

The pair studied the seismic waves from 76 underwater earthquakes. Sure enough, rumbles that produced high-frequency waves for more than 50 seconds had a high probability of generating a damaging tsunami wave. Using this information, they developed an algorithm to filter out quake duration from seismic data. If adopted in an early-warning system, "it could provide a warning within 10 to 15 minutes", says Lomax. The work will appear in Geophysical Research Letters.

"Used alongside other methods it could be promising," says Emile Okal, a seismologist at Northwestern University in Evanston, Illinois.

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