The UC Berkeley Seismological Laboratory issued an alert about the recent earthquake in California’s Napa Valley 10 seconds before it struck. That may not seem like much time — unless you’re a child of the 1950s and 1960s who was trained in school to duck and cover the second you saw a large bright nuclear flash.

Earthquake early warning systems can deliver alerts of impending seismic activity a few seconds to as long as four minutes before the tremors begin. The systems don’t predict earthquakes, but a quake’s energy waves move slowly enough to create an opportunity for a warning. The length of warning depends on the distance from the earthquake’s center.

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