How does an Early Earthquake Warning System work?

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For example, Japan’s EEW system. I always thought that earthquakes couldn’t be predicted, but they seem to be able to warn people before one happens at least a little beforehand. How do they do that?

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The system doesn’t warn people before the earthquake occurs. Instead the shaking of the earthquake travels through the ground at between 0.5 to 3 miles per second, while electronic signals can travel at nearly the speed of light. This means that an electronic system can provide early warning of incoming earthquake shaking before it actually arrives.

They’re designed to warn as soon as seismic waves are felt. In general, earthquakes have an initial wave of lower intensity. These systems detect those and warn of the incoming bigger wave.