Earthquake Early Warning:
How Much Notice Do You Get?
Earthquakes can't be predicted before they start. But once one begins, there's a brief window — seconds, not minutes — where a warning can outrun the shaking. Here's how that's possible, and what it isn't.
Early Warning Isn't Prediction
No system can tell you an earthquake is coming before it starts. Earthquake early warning (EEW) works differently: it detects a quake that has already begun and races to warn people before the strongest shaking reaches them.
That's possible because of a simple physics fact — energy travels through the ground slower than a radio signal travels through the air.
Two Types of Seismic Waves, Two Different Speeds
P-Waves (Primary)
The fastest waves an earthquake produces, but weak and usually not damaging. Sensors detect these first — often felt as a small jolt or rumble before the real shaking starts.
S-Waves (Secondary)
Slower than P-waves, but stronger — these carry most of the shaking that causes damage. Early warning systems detect the P-wave, calculate the quake's size and location, and send an alert before the S-wave arrives.
How Much Warning You Actually Get
The warning window depends entirely on how far you are from the epicenter:
Near the epicenter
Little to no warning — the P-wave and S-wave arrive close together. People closest to where a quake starts almost always feel the shaking with no advance notice.
10–30 miles away
A few seconds of warning — enough to drop, cover, and hold on before strong shaking arrives.
50+ miles away
Tens of seconds of warning in some cases — enough time for automated systems to slow trains, open elevator doors, or pause surgeries, in addition to a phone alert.
ShakeAlert: The US System
In the United States, USGS operates ShakeAlert, an earthquake early warning system covering California, Oregon, and Washington, built on a network of hundreds of seismic sensors. It powers alerts on Android's built-in earthquake warning system and apps like MyShake. Outside the West Coast, there is currently no dedicated early-warning sensor network, since EEW requires a dense array of ground sensors near active fault lines.
ShakeAlert's Track Record
ShakeAlert has already demonstrated real warning time in practice. During the 2019 Ridgecrest earthquakes in California, the system correctly detected the events and issued alerts, though the network was still in a limited public rollout at the time. Since then, ShakeAlert-powered alerts have reached millions of phones via Android's built-in earthquake alerts and the MyShake app during numerous California quakes, typically giving several seconds of warning to people tens of miles from the epicenter — enough time to drop, cover, and hold on before the strongest shaking arrives.
Early Warning vs. Post-Quake Notification
It's worth being clear about the distinction, since both get called "earthquake alerts":
- Early warning (ShakeAlert-style) fires during a quake, before shaking reaches you — only available where dense sensor networks exist, and only useful at a distance from the epicenter.
- Post-quake notification (what most nationwide apps provide, including StormCast) fires after USGS records and publishes the event — not fast enough to warn you before shaking, but useful for knowing what just happened, how strong it was, and whether to expect aftershocks.
Get Earthquake Notifications Nationwide
StormCast monitors the USGS earthquake feed every 10 minutes for M2.5+ events anywhere in the US. When a quake happens near you, you get a push notification with magnitude, location, and distance from the epicenter — whether or not you're in a ShakeAlert coverage area.
