Northridge was only 6.7? I thought it was more. :-P
It was 6.7, but it was shallow, which made it very destructive. I lived through the 6.8 here in Seattle five years ago. That one was deep, so it did much less damage.
Northridge was indeed "just" Magnitude 6.7
Keep in mind the energy release, and thus destructive power of a quake, goes up 1.4 times for each .1 magnitude, 3 times for each .3 magnitude, 5.5 times for each .5 magnitude, and 32 times for each full magnitude point.
Northridge was 32 times more powerful than this quake in Iran, for example.
The biggest credible quake you could get directly under LA is in the 7.2 to 7.4 range, or upwards of of 5+ times more powerful than Northridge.