Bay Area earthquake cluster that took off Tuesday is still going

Seismic activity in the San Francisco Bay Area from Dec. 21 to Dec. 28. 2017. Seismic activity in the San Francisco Bay Area from Dec. 21 to Dec. 28. 2017. Photo: USGS Photo: USGS Image 1 of / 108 Caption Close Bay Area earthquake cluster that took off Tuesday is still going 1 / 108 Back to Gallery

Almost 30 small quakes, including one 3.9 temblor that rattled homes and residents, have been recorded around the San Francisco Bay Area in recent days.

A pair of shallow shakers on the Calaveras Fault kicked off the quake storm on Tuesday: a magnitude 3.1 at 7:19 p.m. near San Martin and a magnitude 3.9 at 10:32 p.m. near Alum Rock.

While the quakes didn't cause any damage or injuries, they were widely felt.

Dr. David Schwartz, a geologist with the United States Geological Survey, says people felt the shakes because the quakes struck close to the earth's surface with the first being 4.5 miles deep and the second larger one being 6.5 miles deep.

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"Over 8,500 people reported the 3.9," he says. "When you approach a magnitude 4.0, it produces shaking over a large enough area that if it's heavily populated, people are going to say, 'Oh is that a big truck moving by?' And then realize that no, it's an earthquake."

Seven more temblors ranging in magnitude 2.5 to 3.3 were recorded near Hollister on Wednesday and early Thursday.

Related video: The biggest quakes ever recorded. Story continues below.

In the past seven days, Schwartz says the USGS has recorded 28 quakes ranging in magnitude from 1.1 to 3.9 between Santa Rosa and Hollister. A quake of magnitude 2.5 to 3 is the smallest generally felt by people.

"This is normal," Schwartz said. "That's the message. Anything that's not normal, we'll let the public know."

The last big quake to hit the Bay Area was a magnitude 6.9 near Loma Prieta in Santa Cruz in 1989 (see photos of the destruction in the gallery at top). In 2007, a 5.5 hit near Alum Rock.

Schwartz says the Bay Area will see another big quake at some point, but geologists can't say when that will be.

These recent small quakes "are not telling us anything we don't already know about the likelihood of something larger. The Bay Area has been very quiet and eventually one of these big faults or a subsidiary has to move and give us something larger."