SAN LEANDRO (KPIX 5) — Scientists still can’t predict earthquakes, but they can provide a little bit of a warning. ShakeAlert is all about seconds, even fractions of seconds. Monday’s Bay Area quake is great example of that.

For anyone who went racing to their phone to see the epicenter of Monday’s quake, the first U.S. Geological Survey estimate was right in Chris Andrews’ front yard on Easy Street in Castro Valley.

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“Easy Street,” laughed Chris Andrews of Castro Valley. “Things, maybe a little less easy for us yesterday.”

“The shakes that we felt might have been potentially more amplified … I mean, the whole house was absolutely shaking.”

As always, the exact location and the magnitude get refined after a little while.

“The magnitude was actually adjusted to a 3.9,” said Robert de Groot of the U.S. Geological Survey. “It was originally a magnitude 4.”

De Groot works on the ShakeAlert system and he knows the first question you might have is why no text alert.

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“When we hit 4.5 and higher, that’s when alerts start being delivered to things like cell phones,” he explained.

Making that determination has to happen in a matter of moments. Monday, the system took under four seconds to size up the quake.

“To make that determination about ‘well, what’s the magnitude of the earthquake, and knowing what the estimated magnitude is, where is it going to shake and how hard is it going to shake,’” said de Groot.

In a large enough quake, like the USGS simulation of a 6.8 on the Hayward Fault, an immediate text could give people miles away several seconds of warning ahead of intense shaking. But the initial estimate will be just that, a best guess made in a hurry. That’s how some people living in Santa Clara County were alerted to a Truckee quake back in May.

“Had we waited a 10th of a second longer for the Truckee earthquake, the earthquake size would’ve come down considerably,” de Groot says. “But again, every fraction of a second matters when it comes to doing this sort of thing.”

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