On this map, the two darkest red traces, are the traces of the San Andreas fault. If you were to walk along either strand, toward the other, you would have to make a big step to the right, right at Brawley.
Because the San Andreas fault is a right-lateral strike slip fault, the western portion of this map is moving to the northwest and the eastern portion of this map is moving to the southeast, relative to each other. This means that the crust at this stepover is tending to have a big hole ripped in it. Since we cannot rip big holes in the crust, the crust instead stretches to fill in the gap. In the upper crust it does this by forming normal faults. The predicted orientation of these normal faults is about 120 degrees away from the trace of the San Andreas fault. Interestingly, the orientation of the swarm at Brawley is more nearly perpendicular to the San Andreas trend, as is the trend of topographic features in the area, suggesting this is a long-lived fault orientation. Update (8/27/12) The centroid moment tensor solution http://earthquake.usgs.gov/earthquakes/eqarchives/gcmt/neic_c000c7i2_gcmt.php shows that the largest of these quakes moved with slightly oblique strike-slip motion on either a right-lateral fault paralleling the San Andreas trend or a left-lateral fault perpendicular to it.
In this area, the Salton Sea owes its existence to these normal faults and to the stretching of the crust. The stretching occurring at this stepover is producing a depression, the Salton Sink, which filled with water when a 1905 flood overwhelmed a newly-constructed irrigation canal and diverted water from the Colorado River into the depression. The diverted waters rapidly eroded the soft soils in the area, creating new river channels. The flooding of the Salton Sink went on for two years before finally being stopped, leaving the large puddle of water we now call the Salton Sea.
Earthquake swarms are distinctive in that they have several to many quakes of similar magnitude along a fault system, as opposed to the more common pattern of a single large mainshock followed by many smaller aftershocks. There are many reasons that have been suggested, including effects of fluid or magmatic pressures. There are no active volcanoes at Brawley, so we can rule out volcanic mechanisms, but it is possible that some sort of hydrothermal or other fluid flow played a role.
