Since I already have this thread started this post is being added here.

There was just what appears to be a powerful aftershock reported for that recent very powerful Oaxaca, Mexico earthquake.

2012/04/02 17:36:44 16.477 -98.287 12 6.3
2012/03/20 18:02:49 16.66N 98.18W 20 7.4

That one looks a lot closer to the surface than the origional. But news reports are stating that no major injuries have been reported so far.

My theories and those of probably some other researchers propose that powerful aftershocks can often occur at multiples of roughly 7 days after the original earthquake. That aftershock was 13 days after the original.