I'll add to that a bit - the 2 segments are treated differently and have different schedules for average rupture - the parkfield-tejon pass - wrightwood segment has the 140 year average.
The southern section (cajon pass to salton sea) has a major quake about every 300 years - the latest studies I've seen put the souther section at having quakes in 1100 +/- 65 yrs, 1350 +/- 20 years and 1680 +/- 40 years. So its been about 321 years since it ruptured the southern section.

When I'm talking Big I mean 7-8 sized quake.

Then this brings up the fact that a large quake on one of the other faults in Southern California that is in a more densely populated area will cause more damage and be more devastating than the Big One on the san andreas.

Canie