A preliminary magnitude 3.8 earthquake shook Concord Thursday.
The quake struck at 11:18 a.m. along the Ygnacio Valley section of the Concord fault, according to U.S. Geological Survey. A magnitude 2.2 aftershock in the same area struck six minutes later.
No damage was immediately reported.
UPDATE: This article was updated to include information about the 11:26 a.m. aftershock.