It's the world's first early warning system for earthquakes. It was developed by Japan meteorological agency, and as the earthquake shook the country on Friday, the warning system sent a warning message that went on national TV, radio and mobile phones. People in Tokyo got the message before they felt the earthquake.

From the Wall Street Journal:

The agency's computer system analyzes the first, faster-traveling wave and quickly estimates how powerful the second wave will be; if it is likely to be very powerful, the system immediately issues a warning message.

The message comes on TV and radio, and many Japanese cell phones can receive and display the message. Some power and gas facilities as well as railway and industrial systems automatically halt their operations the moment they receive the warning message, the spokesman said.

Japan is the only country with this kind of warning system and it had never really been used in a big earthquake before. If you live near the epicenter of the quake, you wouldn't get the warning in time.

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