Dozens of small earthquakes have shaken the San Francisco Bay Area over the last month, rattling nerves and raising fears that more seismic events may be on the horizon.
The clusters of earthquakes — the latest of which struck Monday — have all been underneath the East Bay suburb of San Ramon, which is close to the Calaveras fault.
There were at least 19 earthquakes of magnitude 2 or greater on Monday alone, with the largest, a magnitude 3.6, striking at 9:07 a.m.
That quake was strong enough to unsettle customers in the checkout line at a Safeway in Dublin, leaving shoppers feeling shaking for a few seconds — just enough to look at each other in bewilderment and alarm before the rumbling stopped.
Before that, there were six earthquakes in the 5 a.m. hour, giving many across the region an unpleasant wake-up call.
“Wee bit nervous,” one person wrote on Threads, adding that she planned to stock up on water and earthquake supplies.
“San Ramon is basically a massage chair today … but like, the stressful kind,” another person wrote on the social media site. “Dear Earth: we get it, you’re active. You can stop now.”
The seismic clusters began Nov. 9, when 13 earthquakes were detected — the largest being a magnitude 3.8. Two more came the next day, as well as one apiece Nov. 15, Nov. 17, Nov. 18 and Nov. 20.
As if Bay Area residents weren’t already on edge, a magnitude 2.9 earthquake ruptured at 2:55 p.m. Monday with an epicenter in Oakland’s Montclair neighborhood. That quake was close to the Hayward fault.
San Ramon, one of the largest cities in Contra Costa County, and the surrounding Tri-Valley area, is no stranger to earthquake swarms, according to Annemarie Baltay, a U.S. Geological Survey seismologist.
Seismic swarms have occurred in 2002, 2003 and 2015 in the San Ramon area; and in 2018 around nearby Danville — with similar characteristics and magnitudes, Baltay said. However, none of those swarms were precursors to a large destructive quake on the Calaveras fault, which has not occurred in the Tri-Valley area in the modern historical record.
Earthquake swarms in the past in this area have lasted for as few as two days, but the longest one was 42 days.
“It may be unsettling for folks who live there,” Baltay said.
But there is generally little predictive value in earthquake swarms.
Scientists say there’s a 5% chance that any particular earthquake will be followed by an even larger one nearby over the next week.
For instance, the magnitude 7.8 earthquake that struck the San Andreas fault through Central and Southern California in 1857 was preceded an hour earlier by a far-more-modest magnitude 5.6 quake in Monterey County, and a magnitude 6.1 earthquake an hour before that.
And in the months and years ahead of the deadly magnitude 6.9 Loma Prieta earthquake in 1989, there were modest earthquakes — in the magnitude 5 range — that in hindsight were foreshocks to the temblor that caused a partial collapse of the Bay Bridge and the collapse of a section of Interstate 880 in Oakland.
Most earthquake swarms do not result in major temblors.
One swarm that did attract significant attention occurred in 2016, with small earthquakes at the southern end of the San Andreas fault. Those events did increase the likelihood of a more major quake in Southern California, though that ultimately did not happen.
Scientists suspect that the Calaveras fault is capable of causing a magnitude 6.7 or larger earthquake. The fault runs from around the unincorporated community of Alamo in Contra Costa County southeast along Interstate 680 through areas around Danville, Dublin and Pleasanton.
After meeting up with the Hayward fault in Santa Clara County, the Calaveras fault continues southeast, running past Gilroy, before meeting up with the San Andreas fault in San Benito County.
The most recent large temblor on the Calaveras fault occurred in 1984, the magnitude 6.2 Morgan Hill earthquake, and resulted in at least 21 injuries and millions of dollars in damage. Prior to that, in 1911, an earthquake — with an estimated magnitude of 6.5 — hit a nearby section of the fault.
A modest magnitude 5.1 earthquake on the Calaveras fault, east of San José, occurred in 2022.
Scientists believe it’s possible that an earthquake on the San Andreas fault that begins in San Benito County — in the area around Hollister — could continue rupturing northwest up the Calaveras fault, and then potentially continue up the Calaveras or the Hayward fault. Whether such a scenario is particularly likely, however, is still being studied.
The USGS estimates that there is a 72% probability that an earthquake of magnitude 6.7 or greater will occur in the Bay Area by 2043.
One of the faults scientists are most concerned about is the Hayward fault, which runs through Richmond, Berkeley, Oakland, Fremont and San José. A hypothetical magnitude 7 quake on the Hayward fault could result in at least 800 deaths, 18,000 injuries, and leave as many as 2,500 people needing to be rescued from collapsed buildings and 22,000 from stuck elevators, scientists estimate.
The last major quake on the Hayward fault occurred in 1868, which was estimated to be between magnitude 6.8 and 7.0. It was one of the most destructive in California history.
Experts say small quakes are a good reminder to get prepared for larger ones.
People can take steps such as securing objects that can topple and cause injuries when shaken — such as strapping bookcases and securing TVs to walls. Property owners can also look into whether their single-family home or their apartment needs to be retrofitted.
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