PLEASANT HILL – Just before bedtime for some and well after it for others, one of the biggest East Bay earthquakes in years jolted residents, an event that experts said Tuesday should serve to be a wake-up call for everyone.
The magnitude 4.5 quake hit late Monday amid a small swarm of quakes in the area, U.S. Geological Survey officials said Tuesday morning. The temblor hit at 10:33 p.m. and was centered less than a mile south-southeast of Pleasant Hill at a depth of roughly 8½ miles, the USGS said.
“If anything, it’s a grim reminder to be ready for the next big earthquake to come that will affect the Bay Area more than this one did,” USGS geophysicist Brian Kilgore said Tuesday. “It’s a great wake-up call that it’s time to get your emergency kit ready.”
It also occurred on what the USGS called a fault line that is not mapped on their survey, to the north end of the Calaveras Fault and to the south of the Concord Fault.
“We’ve not seen a 4.5 or anything that big there, but we’ve had a whole bunch of magnitude 3s in the immediate facility, and a couple of 4s in that area, too, through the years,” Kilgore said. “We don’t have any information on anything larger than that so this particular one, while certainly widely felt, is not that unusual as far as having activity there.”
The USGS initially measured the quake at 4.7 but later downgraded it to 4.5. It was reported to be felt as far east as Sacramento, as far north of Sacramento and as far south as Santa Cruz.
The Concord Fault lies a mile northeast of Pleasant Hill, and the Calaveras and Hayward faults sit about 7½ miles and 11 miles to the southwest, respectively.
“Obviously, there’s a fault in there somewhere,” Kilgore said. “It’s not at the surface, so it’s not mapped and named. But the Calaveras (Fault) comes to an end, and the Concord Fault has shifted a bit east, and earthquakes between them happen all the time.”
There were no reports Tuesday morning of any injuries or damages to businesses.
BART service was affected by the quake, with trains running at reduced speeds for track inspections. Passengers were told to expect delays up to 20 minutes.
The earthquake came just days before the 30th anniversary of the Loma Prieta quake, a magnitude 6.9 that killed more than 60 people. It also served as a reminder that USGS scientists have predicted a 63 percent chance of a magnitude 6.7 or larger quake striking the Bay Area in the next 30 years.
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Monday night’s shaker was preceded by several smaller quakes, including a magnitude 2.5 at 10:23 p.m., according to the USGS. Caltech reported more than a dozen smaller quakes in the Pleasant Hill and Pacheco areas in the hour after the magnitude 4.5.
In a tweet, seismologist Lucy Jones warned that there was a chance that a larger quake could follow.
“Because any quake can be a foreshock,” she said, “there’s a slight increase in the chance of a bigger quake for the next few days, at the same location near the Calaveras Fault.”
The M4.5 quake at 10:33 pm was in the East Bay at 14 km depth. Because any quake can be a foreshock, there’s a slight increase in the chance of a bigger quake for the next few days, at the same location near the Calaveras fault
— Dr. Lucy Jones (@DrLucyJones) October 15, 2019
The USGS had a figure to share on the slight increase.
“According to our forecast, over the next [week] there is a 2% chance of one or more aftershocks that are larger than magnitude 4.5,” a statement posted to the earthquake’s event page said in part, adding that smaller earthquakes of magnitude 3 or higher — large enough to be felt near the epicenter — were likely but should “drop off over time.”
Early Tuesday afternoon, a 4.7 magnitude quake shook San Benito County, centered south of Hollister. The temblor was felt in the Bay Area, although seismologist Jones said that it was too far from the East Bay quake’s epicenter for the two to be connected.
Staff writer Lisa Krieger contributed to this report.
Check back for updates.
