Buying Seconds With Warnings

You cannot predict the exact moment an earthquake strikes, but you can outrun the shaking. An earthquake sends out two waves: a fast, mild P-wave and a slower, destructive S-wave. Sensors near the epicenter catch the P-wave first and race a warning ahead of the S-wave at the speed of light.

That head start is only seconds to a minute, but it is enough to stop trains, halt surgeries, shut gas valves, and tell people to take cover. Japan's nationwide system can flash alerts to phones before the strong shaking arrives, and Mexico City has used warnings to give residents crucial seconds to reach the street.