Just before 6:30 p.m. on Monday, a 3.9-magnitude earthquake struck in the East Bay was felt all over the Bay Area.
The quake was centered in Ashland, an incorporated area of San Lorenzo, outside San Leandro — just about directly on top of the Hayward Fault, as ABC 7's Drew Tuma tweeted. The initial magnitude was reported as 4.2M, but it was downgraded to 3.9M, as ABC 7 reports.
M4.0 earthquake looks to be on top of Hayward Fault pic.twitter.com/JecPFcDRCd— Drew Tuma (@DrewTumaABC7) June 29, 2021
Reports on social media and the USGS ShakeAlert Map suggest that shaking was felt in San Francisco, as far north as Vallejo and Fairfield, and as far south as San Jose.
This quake was not large enough to trigger to new ShakeAlert system, which would theoretically have sent warnings to people's cellphones ahead of the shaking.
Good evening Bay Area! Did you feel the M4.0 quake about 2 mi N of San Lorenzo? The #ShakeAlert system detected the quake. The event wasn't large enough for #ShakeAlert-powered alerts to be distributed to phones but at this magnitude automated actions could be triggered.@Cal_OES pic.twitter.com/XEltZXw7OK— USGS ShakeAlert (@USGS_ShakeAlert) June 29, 2021
The Hayward Fault remains the most overdue of the local faults for a major earthquake.