In the seismic annals of California, Monday’s 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 in one important way, 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, and hundreds of sensors embedded in the ground immediately sent an alert to seismologists at Caltech in Pasadena. They had 30 seconds warning before the quake was felt there.

“It was right,” said Kate Hutton, a seismologist with Caltech. “I sat really still to see if I could feel it and it worked.”

The system has been in place for more than a year. But Monday’s quake offered a rare opportunity to actually see – and feel -- if it worked.

The sensor 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 sensor gave an early warning of several magnitude 5 quakes last year in Imperial County, but the temblors hit too far away for them to felt in Los Angeles.

The Anza quake was different.

Even though it measured magnitude 4.7, its location on solid granite made the shaking stronger and more widespread. People reported to the USGS 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.

Hutton and other declared the test a success, with some caveats.

The system initially overestimated the quake’s magnitude, saying it was a 5.2. But U.S. Geological Survey 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, so the warning system accurately captured that.

The early earthquake 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 from moderate to strong earthquakes.