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Since the prototype went live, users have hardly felt any shaking because the quakes are either too weak or too distant. Occasionally, they get notice of a jolt that they care about like the magnitude-4.2 centered near Newhall, a bedroom community about 25 miles north of downtown Los Angeles.

Though a mild tremble by seismic standards, it rattled nerves across a wide swath.

Cochran, who recently left the academic world to join the USGS in Pasadena, has been running the program in the background for about a month before the Newhall rumble. She has grown used to the constant pinging whenever the ground heaves. The Newhall quake caught her interest because the countdown was short _ a sign that it occurred close by.

So Cochran sat still and waited for the shaking. Had it been stronger, she would have ducked under her desk.

“It was the first time that I had gotten a warning and actually felt it,” she said.

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Online:

U.S. Geological Survey: http://earthquake.usgs.gov

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Follow Alicia Chang’s coverage at http://twitter.com/SciWriAlicia