MILPITAS — A magnitude 3.4 earthquake rattled the South Bay on Sunday night with an epicenter at the Calaveras Reservoir, where crews are finishing a replacement dam for one that had engineers concerned, according to the United States Geological Survey.
It created enough of a scare that the San Francisco Public Utilities Commission conducted visual inspections of both dams Sunday night and did not detect anything to cause alarm.
“No issues have been found,” commission spokeswoman Deilia Jackson Darby wrote in an email to this news organization, adding that secondary inspections would be performed by Tuesday at the latest.
The temblor struck at 7:10 p.m., about nine kilometers east of Milpitas, smack dab at the Calaveras Reservoir. It had a depth of about 7.2 kilometers, according to the USGS.
On the northern tip of the reservoir lies the old 1920s-era Calaveras Dam, which state regulators ordered lowered in the past when they determined a 7.25-magnitude quake on the Calaveras Fault, which runs north and south through the reservoir, could cause the sides of the old dam to slump. That weakening, they determined, could cause a 30-foot-high wall of water to cascade down on Fremont.
However, Sunday’s 3.4 temblor was much weaker and officials do not believe it could have damaged any of the facilities, SFPUC spokeswoman Betsy Rhodes said.
“Based upon its magnitude and its location, they review the impact radius to determine whether or not any SFPUC appurtenances are within the potentially affected area and might be affected by earth movement,” she wrote in an email Sunday. “The upper limits of the impact radius are close enough to the dams for our operators to do a visual inspection (Sunday) evening to confirm that all is fine out of an abundance of caution.”
The agency said crews perform visual inspections on a daily basis. Rhodes added that reservoir levels are already low due to the ongoing construction on the new dam which will be able to withstand a magnitude 7 quake on that fault.
In 2001, the state Division of Safety of Dams ordered the San Francisco Public Utilities Commission to reduce the water level by 60 percent because of the earthquake concerns.
The new dam project is expected to be completed next year. The replacement dam will have a height of 220 feet and is designed to handle a maximum level quake on the Calaveras Fault. The reservoir will then be able to hold 31 billion gallons of water, providing for the PUC’s 2.6 million customers in the Bay Area.
Staff writer Robert Salonga contributed to this report.
Did you feel it, North San José? #SJD4 #SanJose #Earthquake pic.twitter.com/dE91gnIVsb
— Councilor Lan Diep (@SanJoseD4) September 3, 2018