Understanding Earthquake Early Warning Systems

In 2013, Mark S. Ghilarducci, the director of the California Governor’s Office of Emergency Services (Cal-OES), announced a year-long series of stakeholder meetings that created a blueprint for developing an implementation plan for the California Earthquake Early Warning System (CEEWS). Cal-OES is committed to support of public-private partnerships to develop statewide earthquake warning capability.

Regional partnerships—including the Coachella Valley Regional Earthquake Warning System (CREWS); the Imperial County Regional Earthquake Warning System (ICREWS); and Seismic Warning Systems, Inc., in Scotts Valley, California—are implementing and testing warning systems in Riverside and Imperial Counties.

P Waves, S Waves, and Earthquake Warning

Earthquake advance warning systems detect the nondestructive primary waves (P waves) that travel quickly through the earth’s crust, in advance of the destructive secondary waves (S waves). The delay between the arrival of P waves and S waves controls the amount of advance warning that can be given. The interval increases the farther a location is from the epicenter of the earthquake. This interval generally ranges between 60 and 90 seconds for deep, distant, large earthquakes. An interval of one second for every two kilometers (km) from the quake’s origin is the rule of thumb.

The effectiveness of advance warning depends on the accurate detection of P waves and the rejection of false-positive ground vibrations caused by local activity. Table 1 compares the characteristics of typical P waves and S waves.

What to Do in a Few Seconds

Even a few seconds’ warning can provide enough time to take protective measures.

Table 1: P Wave and S Wave Characteristics

See also Modeling Communications Coverage” and “Partnerships Provide Earthquake Warning in California.”