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 and characterized both the magnitude 6.4 foreshock and 7.1 mainshock within seconds — though at the time, public alerting was only piloted in Los Angeles County, and no public alert actually went out there since the predicted shaking fell below the app's alert threshold. Since California, Oregon, and Washington expanded to statewide public alerting starting in late 2019, 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.
How ShakeAlert Calculates a Warning in Real Time
ShakeAlert runs on more than 1,700 seismic sensors across California, Oregon, and Washington. When several stations detect the start of a P-wave within a short window, the system's algorithms estimate the earthquake's magnitude and location using only the first fraction of a second of data — then continuously refine that estimate as more sensors report in over the following seconds. It calculates, for every populated area within range, how many seconds until the S-wave (the damaging shaking) is expected to arrive, and only sends an alert to areas where the expected shaking exceeds a minimum intensity threshold. That entire detect-calculate-broadcast pipeline typically completes in under 2 seconds from the moment the quake begins.
Why Nationwide Coverage Doesn't Exist Yet
Earthquake early warning requires a genuinely dense network of ground-motion sensors near active faults — ShakeAlert alone represents a multi-decade, hundreds-of-millions-of-dollars federal investment concentrated on the West Coast, where the seismic hazard is highest and most well understood. Other earthquake-prone regions of the US, including the New Madrid Seismic Zone in the central Mississippi Valley and parts of the interior West, don't yet have comparable sensor density, which is why there's no equivalent early-warning system available there today. Expanding coverage nationally would require a similarly large, sustained investment in new sensor infrastructure.
Frequently Asked Questions
Does my phone already have earthquake early warning built in?
If you're on Android in a ShakeAlert coverage area (CA, OR, WA), Google's Android Earthquake Alerts system uses ShakeAlert data automatically, with no app required. iPhone users in those states can get similar alerts through California's MyShake app or the state's Wireless Emergency Alerts.
Can early warning ever give false alarms?
Yes, occasionally — usually from a rapid magnitude overestimate in the first fraction of a second of data. Systems like ShakeAlert are continuously tuned to minimize this, but it's an inherent tradeoff of prioritizing speed.
Is a few seconds of warning actually useful?
Yes — a few seconds is enough to drop, cover, and hold on, for automated systems to stop trains and elevators, and for surgeons and dentists to pause procedures. It's not enough time to evacuate a building, which is why "drop, cover, hold on" remains the standard advice rather than running outside.
Will early warning eventually cover the whole country?
It's technically possible but would require substantial new investment in seismic sensor networks in each region. There's no announced nationwide timeline as of this writing.
Get Earthquake Notifications Nationwide
When an earthquake happens near you, StormCast sends a push notification with magnitude, location, and distance from the epicenter — whether or not you're in a ShakeAlert coverage area.
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A note on this guide: This is general safety information, not medical or professional emergency advice, and specific criteria can vary by region. In an actual emergency, call 911 and follow instructions from local officials and the National Weather Service.