A magnitude 3.6 earthquake struck at 12:19 pm today, a mile from the Contra Costa suburb of Danville. No immediate reports of damage or injury have been reported, but the Bay Area Rapid Transit system did halt the trains temporarily to check for any problems.
We'll update with any significant updates. In the meantime, this is probably why you experienced transit delays in the last hour.
It's not the first earthquake/earthquake-based delay of the day. BART also called a short halt early this morning do to previous seismic activity.