Brian,
I delayed replying to your post for a couple of days, but had been wondering for a while whether the grim outlooks for Los Angeles that had been presented a few years ago at Southern California Earthquake Center Annual meetings were exaggerated. I'm more specifically talking about a combinations of the 3D ground motion modeling and the engineering geologist: on what scale will steel-frame buildings with brittle welds fail (collapse), and same question for ductile welds. I have only been to one of the last 3 of these meetings, so don't know the latest thinking. For example, are measured ground motions for the M5+ couple of quakes consistent with the 3D models?
It's just a thought on my part that a M8 or 7.8 quake on the San Andreas fault might not be quite as bad for the relatively-distant parts of Los Angeles as the models predict. My comment is just a gut reaction; I don't directly have expertise. On the other hand, a M7+ quake on Whittier fault or M7+ quake on thrust faults beneath the basin could be pretty ugly. Especially of the magnitude approaches 7.5 so that the ground shaking goes on for longer.
The issue with Los Angeles compared to other cities is the 5 km-deep sedimentary basin beneath much of the city with slow velocities for seismic waves.
Chris