Todd...depends on what you compare 5 mm/yr to. The San Andreas fault through Carrizzo Plain slips at 35 mm/yr. But, there are not many people or buildings in Carrizo Plain. The parts of the San Andreas fault that slip at 20 to 35 mm/yr are going to have earthquakes of a given size more often than will a fault system slipping at 5 mm/yr. But, the problems with the 5 mm/yr is it is occurring on faults that are directly under part of LA metro, and shortening=contraction indicates thrust faults as opposed to the strike-slip motion on the San Andreas fault. Thrust faults for a given thickness of strong crust are wider. The ground motion from a thrust earthquake is larger than for strike-slip motion (everything else being equal). So, how often you will have thrust earthquakes beneath or adjacent to L.A. depends on how large they are...you can have many M6.7 earthquakes for one M7.3...for example. For a USGS proposal I just submitted, I added the area of offshore parts of thrust faults that we have interpreted to the Compton thrust ramp of Shaw and Suppe, and ran it through the fault area-magnitude simple equation, and (I forget exactly) the 7.0 of Shaw and Suppe becomes a 7.2 or 7.3....because the fault area doubles.

Other questions for LA are how much of that shortening is on faults that are creeping and not capable of big earthquakes, and of faults that are capable of big earthquakes, how much of the fault area is going to put out a lot of energy...

Chris