Early Warning Systems

Japan's earthquake early warning system detects P-waves (which arrive first but cause less damage) and immediately broadcasts alerts before the destructive S-waves arrive. This can provide 10 to 60 seconds of warning — enough to stop trains, shut off gas lines, and take cover.

Mexico City has a similar system called SASMEX that uses sensors on the Pacific coast to detect earthquakes before the waves reach the capital, 300 km inland. The warning time can reach 60 seconds or more. California's ShakeAlert went live in 2019, and its MyShake app and wireless emergency alerts now push warnings straight to phones across the state.