E.E.W. allows seismologists to estimate, in real time, the potential amount of shaking an earthquake will cause. The principles are relatively simple. Seismometers can scan the ground for the “P waves,” or compression waves, which precede the slower and more powerful “S waves,” or shear waves, that are responsible for severe earthquake damage, like lightning before thunder. Using the P waves, a local data-processing center can calculate the likely reach and magnitude of a quake moments after it begins. The resulting alert, dispatched at the speed of light, usually beats the S waves, which ripple through rock at about two miles per second.

To call the warning “early” is generous. It usually arrives between a few seconds and less than a minute ahead of the quake—advance notice that, in duration, is somewhere between a sneeze and a red light. The Achilles’ heel of the system is what its designers call the “blind zone.” Those who might benefit the most from a warning—the people at the epicenter—won’t get one, because the S waves arrive too soon. Still, if you’re far enough outside that bull’s-eye, a few moments can be meaningful. In the past few decades, more than half of earthquake injuries in the U.S. have been caused by people or things falling—two occurrences easily avoided if you have time to take cover beneath a sturdy piece of furniture. For those who are landing planes, assembling electronics, operating cranes, or drilling into molars, even the smallest warning would be welcome. You might have just enough time to lock the wheels on your wheelchair, or to remove your scalpel from your patient’s chest. The effectiveness of the warning depends on how much can be done in a handful of seconds. The goal of E.E.W., therefore, isn’t just to sound the alarm. It’s to help transform knowledge into action as quickly as possible.

Vaguely related: The Unexpected Return of Duck and Cover.