LOS ANGELES - In the seismic annals of California, Monday's magnitude 4.7 earthquake was little more than a footnote. It gave Southern California a small morning jolt but caused no damage and was largely shrugged off by noon.
But the quake was highly significant because it marked an advance in California's burgeoning earthquake early-warning system.
The quake struck in the desert town of Anza, about 35 miles south of Palm Springs, and hundreds of sensors embedded in the ground immediately send an alert to seismologists at California Institute of Technology in Pasadena. They had 30 seconds' warning before the quake was felt there.
"I sat really still to see if I could feel it, and it worked," said Kate Hutton, a Caltech seismologist.
The system has been in place for more than a year. But Monday's quake offered a rare opportunity to see - and feel - if it worked.
The sensors have warned scientists of numerous quakes, but the vast majority were either too small to feel or too far away to be felt in the Los Angeles area. For example, the sensors gave an early warning of several magnitude 5 quakes last year in Imperial County, but the temblors hit too far away for them to be felt in L.A.
The Anza quake was different.
Though it measured only magnitude 4.7, its location on solid granite made the shaking stronger and more widespread. People reported to the U.S. Geological Survey that they felt it as far away as Arizona and Central California.
At Caltech, computer screens flashed with a 30-second countdown to when the shaking would hit Pasadena. Sure enough, it came on time.
The system initially overestimated the quake's magnitude, saying it was 5.2. But USGS seismologist Susan Hough was not overly concerned about the error. She noted that the main job of the system is to alert people to a coming quake, not to gets its magnitude precisely right. The Anza quake caused an unusually intense amount of shaking, Hough added.
The earthquake early warning system is a pilot project for what scientists hope will eventually be a statewide network using thousands of sensors to notify people about imminent shaking.
Backers say an early warning would give utilities time to shut down, trains a chance to slow so they don't derail and workers a chance to move away from hazardous materials or precarious positions. Warnings would be sent to the public through text messages, emails and other special alerts.
Similar systems are already operating in Japan, Mexico and Taiwan. Two years ago, Japan's program alerted some 50 million residents ahead of the devastating 9.0-magnitude earthquake.