A smattering of very small earthquakes hit the Bay Area Monday night. Though mild in size, the pair of quakes came just minutes apart from each other, which left some folks in the region shaken.
According to the U.S. Geological Survey (USGS), the first one hit north of San Ramon at a quarter of 10 p.m. That quake measured a modest 2.7, so small that only 14 people reported it.
(Ordinarily small quakes receive hundreds of responses on USGS’s “felt report.”)
The shaking didn’t extend very far, felt only around Concord, Oakland, and San Leandro. A minor shake-up like that barely brooks comment in the Bay Area—except that another small shaker came hot on its heels.
At 10:21 p.m., a 2.5 quake occurred north of Morgan Hill. This time USGS recorded no responses from residents at all, meaning that this quake went almost completely overlooked by all but seismographs.
Both Morgan Hill and San Ramon sit nearest the 76-mile long Calaveras Fault.
According to USGS, the largest quake ever recorded on the Calaveras was a 6.5 way back in 1911. More recently, a 6.3 quake in Morgan Hill hit in 1984, followed by a 5.6 in 2007.
Seismologists calculate only about a 26 percent chance of a large quake (6.7 or greater) on this line over the next 30 or so years.
However, in 2015, researchers at UC Berkeley reported that the Calaveras Fault is physical connected to the nearby (and much more dangerous) Hayward Fault. Since the length of a fault determines the worst quake it can produce, this may make it more dangerous than previously anticipated.
USGS declared only three Bay Area earthquakes large enough to be “significant” in 2018: A 3.5 in Oakland in May, a 3.8 near San Jose in April, and a 4.4 in Berkeley in January.
So far, 2019 has seen only one significant Bay Area quake in February, a 3.7 near Yountville.