Earthquake early warning systems don’t predict earthquakes. Instead, they detect ground motion as soon as an earthquake begins and quickly send alerts that a tremor is on its way, giving people crucial seconds to prepare.
In 1985, Caltech seismologist Tom Heaton published the first paper on the concept of earthquake early warning systems, networks of ground-based sensors that send alerts to users when the earth begins to tremble.
Such systems, including ShakeAlert on the West Coast of the United States, operate on the principle that while seismic waves travel at just a few miles per second, electronic alerts from the region of the epicenter can be sent almost instantly. Here's how it works:
During an earthquake, several types of seismic waves radiate out from the quake's epicenter. First, weaker but faster-moving P-waves trigger sensors that, in turn, transmit signals to data processing centers.
Algorithms quickly estimate the earthquake's location, magnitude, and intensity: Where is it? How big is it? Who is going to feel it?
The system then sends an alert before slower, but more destructive S-waves and surface waves arrive.
Although people who are near the epicenter will have little, if any advance warning, those farther away may have critical seconds to brace for shaking. Paired with automated responses that can slow trains or shut off gas lines, early warning systems may help prevent some of the injuries and damage typically associated with major quakes.
An earthquake just happened. Why didn't I receive an alert on my smartphone?
You might be too close to the epicenter. Early-warning alerts are typically delivered three to five seconds after an earthquake starts. That's the time it takes for seismic waves to travel to the closest stations, and for computers to analyze the data. If you are less than 10 miles from the epicenter, it is unlikely you will get a warning.
The shaking might not have been strong enough. It is important to remember that most people experience weak shaking during an earthquake. This is because large earthquakes are rare, and because individuals are often too far from the epicenter to experience significant shaking. Currently, apps, such as ShakeAlertLA, are designed to send alerts only for stronger shaking. This could change as scientists and public officials continue to tune the system's parameters.