I think there is a lot of controversy on the so-called 22 year period for Parkfield. Some of the earlier quakes may not have been at Parkfield. I have gotten the impression that the characteristic earthquake theory, at least for quakes that size, is fairly widely rejected. On the other hand, there are small repeating earthquakes at Parkfield and elsewhere...same little patch of fault; don't know if they are periodic. The 2004 Parkfield quake was opposite the 66 quake...ruptured the opposite direction, and highest slips were where the lowest 66 slips were.
I am skeptical of the same patch of the same fault being able to break until stress has built up again...I question whether faults (like Reelfoot in New Madrid) can store multiple earthquakes. On the other hand, it has been 4 years at Parkfield and the fault is moving fast (35 mm/yr?) at depth, so I won't rule out the fault being capable again.
From posts here, John V has a different take on this...I'd like to see what he thinks about Parkfield in particular.
Chris