Well..if you want a scientific basis, you look at the faults that slip the fastest and have the longest time since the last earthquake. If we are talking about southern California and earthquakes close to M8, that would be the southern San Andreas. Since you can have 10 M7 quakes for each M8, you might look at faults that fail often in M7s...meaning they have a high slip rate and fail in smaller quakes. The San Jacinto fault system comes to mind. If you think about where activity has been lately....you might think about a thrust earthquake west of Northridge in Ventura basin, or one in the Mojave...maybe north of Landers or Hector mines. As for the really devasting earthquakes close to Los Angeles...who knows....
Chris