Work by, I think, Kerry Sieh's group shows that it is almost 300 years between ruptures of the SAF in Carizzo (sp?) plain, and that the slip there is generally 9 or 10 meters. That would say that there might be a low probability of it failing, and I think that this segment was assigned a low probablilty of failure in 30 years by the working group that assigned probabilities to segments of the San Andreas. I tend to think things are not as regular as they think, and would not be amazed if Carizzo plain failed sooner, especially if a big earthquake propagated towards it. (This segment was part of the 1857 earthquake). But, a repeat of the 1857 earthquake is the next 20 years is not as likely as you suggest....Chris