Earthquake early warning systems can give people crucial seconds to move to safety—but only if they send the message in time. Now, scientists working on such systems have discovered that the bigger the tremor, the longer it takes to issue an alert—giving people little time to prepare for the big one, but lots of time to brace for a ho-hum event.
All earthquakes start with P waves, which are fast moving and cause little damage. S waves come next, moving more slowly but causing more destruction. Early warning systems measure ground movement during the fast P waves to predict how much shaking the S waves will cause, and then send out an alert.