From Christian Science Monitor:
... As to what caused this week's swarm, researchers say it has to do with the region's distinctive geology.
"Compared to the rest of California, it's thinner crust, and hotter, and [has] more fluids," says Lucy Jones, a seismologist at the US Geological Survey in Pasadena, Calif. She describes the imperial Valley as "one big bowl of sediment."
"Those would likely be reasons that contribute to this," Ms. Jones says. "But we don't yet have the details of the physics."
The earth's thin crust in the region, and the proximity of hot magma beneath it, relates to the Imperial Valley's location at the top end of a fault in which tectonic plates are moving apart from one another to form the Gulf of California. The Brawley area, 100 miles east of San Diego, is "like the very end of a ridge that's trying to spread open. It's actually probably giving up on spreading open," Jones says....
