The USGS is now reporting that there was in fact a 2.9 earthquake centered in Pleasant Hill at 2:52 this afternoon.
Who felt it?
The Earthquake was on the Concord-Green Valley Fault.
According to the USGS “this fault is mostly a right-lateral strike-slip fault with up to approximately 4 mm/yr (1/6 inch/year) of creep. The Concord Fault lies to the south of the Green Valley Fault. The total slip rate on the Concord Fault segment is 4 mm/yr (1/6 inch/year) and on the Green Valley Fault segment is about 5 mm/yr (1/5 inches/yr).
The Concord Fault maybe connected to the Greenville Fault through a complex set of faults under Mount Diablo, including the Mount Diablo Thrust. The Concord Fault may also be connected to the northern Calaveras Fault through a complex set of faults south of Walnut Creek. The Green Valley Fault is a direct continuation of the Concord Fault north of Carquinez Straight. The northern end of the Green Valley Fault is not well defined at the surface or from seismicity.
At present, there is little information on the date of the last surface-rupturing event on the Concord-Green Valley Fault. The current best estimate is that the last large earthquake on this fault occurred between 200 to 500 years ago.”
The 2003 Working Group for California Earthquake Probability assigned a 4% probability that the Concord-Green Valley Fault system would produce a magnitude 6.7 or larger earthquake in the next 30 years.