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.

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.

The Hayward Fault remains the most overdue of the local faults for a major earthquake.