As I understand it, there are a couple of related factors people argue about with regard to the risk of earthquakes in LA.

First, what is the long term rate of earthquakes? We know the geologic slip rates on the faults well enough to know the long-term rate of moment release in earthquakes, but the mix of 6's, 7's, and 8's is less well known. So there could be lots of 6.5's with a few 8's, or far fewer 6.5's and a couple more 8's per millenium, for example. A Northridge-sized each decade in LA is possible, but it may be an end member. Exact numbers would require a more specific magnitude description, a better specification of what is the LA area, and more knowledge than I have.

Second, are there large areas quiescent after large earthquakes? It seems that way in the Bay Area after 1906, and perhaps in LA after 1857. Big rate changes over the last century or two make estimates of the long-term risk, as I mention above, more difficult. If we're coming out of the stress shadow of the 1857 event, then the recent activity (Landers, Hector Mines, Northridge, San Fernando) should continue.

Another twist is a possible alternation of activity between LA and the Mojave Desert faults with a periodicity of at least hundreds of years.

I hope I have confused this topic sufficiently for now.

John