Magnitude 4.5 quake jolts East Bay – The Mercury News

PLEASANT HILL – A magnitude 4.5 earthquake jolted the East Bay late Monday night, according to the U.S. Geological Survey, striking amid a small swarm of quakes in the area.

The USGS said the temblor hit just at 10:33 p.m. less than a mile south-southeast of Pleasant Hill at a depth of roughly 8½ miles. It was initially measured at 4.7 but downgraded to 4.5.

There were no immediate reports of any injuries.

BART service was impacted 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.

Pleasant Hill, like much of the Bay Area, is seismically active. The Concord Fault lies a mile northeast of the city and the Calaveras and Hayward faults sit about 7.5 miles and 11 miles to the southwest, respectively.

A USGS seismologist told the San Francisco Chronicle that the quake had taken place on an unmapped fault between the Concord and Calaveras faults, saying that such unmapped breaks were “not uncommon.”

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, USGS 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.”

Staff writer Lisa Krieger contributed to this report.

Check back for updates.