Earthquake Early Warning Basics
Earthquake early warning systems like ShakeAlert® work because an alert can be transmitted almost instantaneously, whereas the shaking waves from the earthquake travel through the shallow layers of the Earth at speeds of one to a few kilometers per second (0.5 to 3 miles per second). This diagram shows how such a system would operate. When an earthquake occurs, both compressional (P) waves and transverse (S) waves radiate outward from the epicenter. The P wave, which travels fastest, trips sensors placed in the landscape, transmitting data to a ShakeAlert® processing center where the location, size, and estimated shaking of the earthquake are determined. If the earthquake fits the right profile a ShakeAlert® message is issued by the USGS. The message is picked up by ShakeAlert® partners which could be used to produce an alert to notify people to take a protective action such as Drop, Cover, and Hold On and/or trigger an automated action. USGS image created by Erin Burkett (USGS) and Jeff Goertzen (Orange County Register) and updated by Robert de Groot (USGS).