ShakeAlert: How Earthquake Early Warning Works on the U.S. West Coast

ShakeAlert is the earthquake early warning system the USGS and its partners have been building for California, Oregon and Washington. It cannot predict earthquakes. It detects one as it starts and warns people and machines before the damaging shaking reaches them.

Why the West Coast

The Federal Emergency Management Agency estimates that 77% of the nation's earthquake risk — an average $4.1 billion a year in losses — is concentrated in those three states. Growing cities and dependence on power, water, telecommunication and transportation systems raise the stakes.

How it works

• The quake: a fault slips, and the rupture spreads along it like a crack across a windshield, sending seismic waves outward like ripples from a stone.
• The race: those waves travel through the shallow Earth at 0.5 to 3 miles a second. An alert moves through communication networks almost instantly — so shaking can take seconds or even minutes to reach you after an alert has.
• The system: sensors detect the first waves, computers estimate the quake's location, magnitude and rupture length and map the expected shaking, and alerts go out to people and devices.

Shaking spreading northwest from the M7.8 ShakeOut scenario earthquake, 10, 50 and 90 seconds after rupture began at the white star. USGS (Perry and others, 2008).

How much warning depends on distance from the epicenter and the system's speed. Close to the epicenter there is a zone where no alert can outrun the waves; a dense network of sensors near faults shrinks it. For big scenario quakes — M8 on the San Andreas or M9 on the Cascadia Subduction Zone — major cities could get several minutes.

What a few seconds buys

The networks behind it

• The California Integrated Seismic Network: more than 400 high-quality ground-motion sensors, run by the USGS with the State of California, Caltech and UC Berkeley.
• The Pacific Northwest Seismic Network: the USGS with the University of Washington and University of Oregon.
• Both are part of the Advanced National Seismic System. When complete, alerts will go out through Wireless Emergency Alerts, IPAWS, phone apps and social media.

Test users see a map of the epicenter and the approaching waves, a countdown to arrival and the expected intensity, while a voice counts down.

Intensity, not magnitude

Magnitude is one number for the energy a quake releases, set by the size of the rupture. Intensity is how hard the ground shakes where you are — it depends on magnitude, distance and local geology, varies from place to place, and is what does the damage. So ShakeAlert warns in intensity.

Progress

• In development since 2006; alerts to test users in California from January 2012.
• It has detected thousands of earthquakes. It alerted within 4 seconds of the start of the M5.1 La Habra quake (March 28, 2014), and gave test users in Berkeley 5 seconds of warning before shaking from the M6.0 South Napa quake (August 24, 2014).
• In February 2016 it moved from demonstration to "production prototype," letting selected users build pilot systems. Public alerts still need more development, testing and user education.

Before or after the Big One?

Most countries built warning systems after a disaster. Japan spent $600 million after the 1995 Kobe earthquake killed 6,400 people; in the 2011 M9.0 Tohoku quake no trains derailed, and 90% of Japanese surveyed said the system was worth it. China (after Wenchuan, 2008, 87,587 dead), Taiwan (Chi Chi, 1999, 2,415), Turkey (Izmit, 1999, 17,127) and Mexico (Mexico City, 1985, 10,153) did the same.

Sources

Based on Erin R. Burkett, Douglas D. Given and Lucile M. Jones, "ShakeAlert—An earthquake early warning system for the United States west coast," U.S. Geological Survey Fact Sheet 2014–3083, version 1.2 (2017); a work of the United States government in the public domain. The scenario maps are taken from the fact sheet's PDF. Its opening diagram, made with an Orange County Register artist, its warning-time map, modified from a figure in Nature (Allen, 2013), and its screenshot of the ShakeAlert user display are not reproduced.