Another smallish earthquake rattled the Bay Area this morning, the second in a series that began last night at 6:09 PM.

According to the USGS, Thursday morning’s quake struck at 9:13 AM, and was initially reported as a magnitude 3.7 event. The USGS later revised that to a magnitude of 3.5.

It was centered 2 miles from Vallejo, and about 23 miles north-northeast of San Francisco City Hall, the USGS says. It has a depth of 5.7 miles.

As of 9:37 AM, BART was experiencing 10-minute delays as the system undergoes its usual post-quake checkup.

Last night’s quake, which struck the same area and had a similar depth, was a 3.5 magnitude.

Sue Simon, the assistant to the Vallejo city attorney, told Bay City News she felt both.

“It was a small jolt then a big jolt,” Simon said of this morning’s quake.

She said the jolts were about a half-second apart, and that she felt them from City Hall.
“I was told the large windows in the building flexed out a little,” Simon said.

The USGS has received responses from throughout the Bay Area about the quake, most of them from the cities of Vallejo, Benicia, Napa, American Canyon and Martinez.

Richard Allen, director of the Seismological Laboratory at the University of California at Berkeley, said he felt the “weak shaking” this morning that lasted a less than a second.

He said it occurred close to the continuation of the West Napa Fault, which runs east of Santa Rosa southeast to Vallejo and is parallel to the Rodgers Creek and Hayward faults.

Because both quakes were of magnitude 3.5, he considers them a cluster. If this morning’s quake had been of a lesser magnitude, it would be considered an aftershock, Allen said.

Clusters are not that unusual, Allen said, noting that there were three or four small quakes in Berkeley in October.

In a dissenting view, however, David Oppenheimer, a seismologist with the Geological Survey in Menlo Park tells the Chron that he believes both quakes originated on “a spur of the nearby Franklin Fault,” and said “today’s quake could be considered an aftershock of the first quake.”

Did you feel it? As always, the USGS would love to hear from you about your shaking (or lack thereof) here.

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the author

Eve Batey is the editor and publisher of the San Francisco Appeal. She used to be the San Francisco Chronicle's Deputy Managing Editor for Online, and started at the Chronicle as their blogging and interactive editor. Before that, she was a co-founding writer and the lead editor of SFist. She's been in the city since 1997, presently living in the Outer Sunset with her husband, cat, and dog. You can reach Eve at eve@sfappeal.com.

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  • Jeremy_ofSF

    I think we need a new term for earthquakes under 4.0. Maybe “quaklet” or “earthquaklet”?

  • Jeremy_ofSF

    I think we need a new term for earthquakes under 4.0. Maybe “quaklet” or “earthquaklet”?