About 40 years ago a Japanese seismologist by the name of Kiyoo Mogi proposed the idea that before a large earthquake occurs, there are a series of earthquakes of increasing magnitude that occur as a "run-up" to the main event. 

These events surround the eventual fault that breaks in the main earthquake in a kind of "donut" pattern, with the main earthquake then occurring in the "hole" of the "donut".  Since that time, seismologists call this type of precursory pattern a "Mogi Donut".

The concern with the Mexicali earthquake is the section of the San Andreas fault that last broke about 500 years ago, the piece that runs from Bombay Beach near the Salton Sea, up to San Gorgonio and Cajon Pass.  The 1992 Landers earthquake broke several faults in the Mojave Desert at the northern end of this part of the San Andreas.  The Mexicali earthquake has now ruptured faults in the region at the southern end of this section of the San Andreas.  The question we have is whether these two events, the June 28,1992 M7.3 Landers earthquake, and the April 4, 2010 M7.2 2010 Mexicali earthquake, represent a donut pattern forming.

The new data- and theory could predict a threatening major Californian earthquacke

Another possibility is that yesterday's event seems to be on the Laguna Salada fault, the southern extension of the Elsinore fault that runs northeast of San Diego.  This would seem to directly threaten the region between San Diego and Los Angeles.

They are tracking these events with forecast technology on www.openhazards.com