I am posting on Brian's thread because the one below is getting too big/complicated.
Brian, I thought you were headed to east coast?
Caution: following is highly speculative.
Are the aftershocks in southern California spreading? Is there a vague arc-like "front" of aftershocks from Brawley west then SW? What is with the aftershocks along and parallel to the USA border, west of the main rupture? Are these quakes (mostly 3M3+) real and located correctly?

Does anyone want to figure out whether it is spreading? You could take the northern end of the main rupture on the USA-Mexico border, and make a bunch of half circle arcs spaced at, say, 10 km, and count M3+ (or 2.5+) quakes within each pair of 10 km-spaced arcs for each 6 hour period, and make a series of, say, 10 graphs with day on horizontal axis and number of M2.5 + quakes on vertical axis, and show all 10 graphs. You could then count the number of quakes within each pair of arcs and plot radius of arc vs. number of quakes (I'm confusing myself).

You may have to not have equal steps of the arc because the farther from the northern end of rupture, the larger the radius, so the larger the area between arcs. You could try an equal area aproach....

A geologic reason why a migration of quake might occur is if there is creep along some detachment at depth, or just plain old hot crust or upple mantle beneath Salton Trough.

As usual, there may be no pattern
Chris