According to the U.S. Geological Survey [USGS], four earthquakes occurred in Contra Costa County near the cities of Danville and Alamo in less than 24 hours this weekend, ranging in potency from a 2.7 to a 3.3 on the Richter Scale.
On Sunday, three quakes struck near Danville within a few hours of each other: a 3.0 shaker at 10:35 A.M., followed by a 2.7 just a few minutes later and then another 3.0 around 3:25 P.M.
Then on Monday morning a 3.3 quake jostled the area around Alamo at about 4:55 A.M, waking people as far away as Morgan Hill in Santa Clara County.
No one has reported any notable damage or injury, although the Monday quake did slightly slow down morning commutes as BART protocol demands that trains run slower after an earthquake in case of rail damage.
According to a USGS map, the affected areas are nearest the Northern Calaveras Fault. UC Berkeley seismologists concluded in 2015 that the Northern Calaveras is closely tied to the Hayward Fault, the Hayward Fault being “essentially a branch of the Calaveras Fault that runs east of San Jose,” and they found that one can stir up the other, although there’s no reason to believe that’s happened in this case.
The UK-based Express newspaper referred to the quakes as “a swarm,” although this term is pretty loose in this context. Earthquake swarms can mean hundreds of tiny earthquakes in a short span—the area around Reno, Nevada recorded more than 1,000 quakes in just two months in 2008, for example.
USGS seismologist Anne Marie Baltay told the Express that while a quick burst of small earthquakes is unusual relative to, say, a typical day in the East Bay, a similar series of clusters has occurred seven times in the past four decades.
In certain cases, smaller earthquakes may be foreshocks presaging a larger quake to come, although the USGS site cautions not to worry too much about such things:
Worldwide the probability that an earthquake will be followed within 3 days by a large earthquake nearby is somewhere just over six percent. In California, that probability is about six percent.
This means that there is about a 94 percent chance that any earthquake will NOT be a foreshock. In California, about half of the biggest earthquakes were preceded by foreshocks; the other half were not.
At this time, we cannot tell whether or not an earthquake is a foreshock until something larger happens after it.
While there’s no particular reason to be concerned about these quakes, they are another timely reminder that another large earthquake in the Bay Area is inevitable and will most likely happen within the next three decades, so there’s no time like the present for residents to start preparing.