Wow a scientific question I actually know the answer to. Chris actually answered it first but to just clarify, EACH segment of any fault is only capable of a certain magnitude. For the SAF in the Aptos part, its prob. only 7.1 What happened in 1906 was that the aptos segment ruptured in conjunction with the Pennisula segment in conjuntion with the segment north of the Golden gate. Combined, they registered an 8+. Individually they were prob. no more than a 6.5 to 7.0. In the future SAF socal segment, it is suggested that when it zippers through from say Salton Sea up to Wrightwood, EACH segment will be having a quake of around 6.5 to 7.0 but collectively the magnitude would reach 8+. Because of this, the actaul ground shaking in LA will NOT be greater than what was experienced by SFV residents in Northridge and Sylmar (71), HOWEVER a great number of people will experience that intense shaking than if it was a just a single 6+ event with one epicenter.

It is thought that in 1906, the Santa Cruz segment, the one you live closest too, may have been a bit sluggish during the event, i.e. its sister segments to the north may have slipped causing quakes in the 6.7 range while teh Santa Cruz segment may have been just at a 6. Therefore not all the energy went and thus the rather quick turn around time from 06 to 89 for the next one. The Pennisula segment was the next segmenet to release not as much, hence why it is now a potential candidate for a 6.5+ quake in next 30 years and the segment north of that released the most.

Hope this makes sense!