Japan, China, Mexico, Turkey and other countries have built earthquake early warning systems, which can warn people and give them a moment to protect themselves from a potentially deadly quake.

But the United States, oddly, doesn’t have such a system — a prototype, ShakeAlert, has been tested in California, but Congress has yet to provide funding to expand it nationwide. Earthquake experts say that for less than $40 million, we could build a reliable U.S. system that would send out alerts on Twitter and Facebook to warn people of impending quakes.

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“We are sure there will be such a system after a big earthquake. We would like to do it before a big earthquake,” Doug Given, head of the U.S. Geological Survey’s Southern California Earthquake Monitoring project in Pasadena, Calif., told Gannett News Service.

The systems work by watching for seismic P-waves, which have short, fast wavelengths and do little damage, but which are followed usually several seconds later by the longer-wavelength S-waves that knock down buildings and cause landslides.

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Japan’s nationwide online earthquake alert system, launched in 2007, uses more than 1,000 seismographs scattered throughout the country, which is one of the most earthquake-prone on the planet.

As this Time magazine article describes, in March 2011 such a warning from the system buzzed on the cell phone of Kensuke Wanatabe, a college professor in the Japanese city of Sendai.  That gave him and his students just enough time to crawl under their desks and avoid injury from falling objects as the building began shaking violently from a 9.0 quake.

“It was terrifying,” Watanabe told Time. “But the mobile warning really helped.”

Photo: A satellite image shows the San Andreas fault. Credit: NASA