Some additions to your information..Northridge only broke about 15 or 20 km of this shortening belt, on a fault dipping the opposite way from Sierra Madre. So, there could be plently of elastic strain stored on the Sierra Madre, enough for a much larger quake than you predict. But, trenching shows 1000s of years between quakes. But, being a reverse fault, all the slip need not get to the surface, if the hanging-wall above the fault is folding. It is possible that the segement where they get slow slip rates is actually just as fast as adjoining segments, but at depth. I don't know this fault, more than a couple of talks and maybe a paper or 2. You would expect that the GPS data would show contraction across the Sierra Madre fault, which is the big monster for that area (at least as far as thrust faults), but the analysis of Argus et al 2005 shows the action is south of the Sierra Madre, near and north of downtown LA, on the Puente Hills fault and faults beneath it. I have to finish a proposal on one of the faults beneath it by tomorrow night...luckily my co-PIs are doing a lot of this because I can of ran out of motivation. You don't get funded unless your proposals are excellent...and even some of those do not get funded. Too many of us, or too little money, or both.
Chris