Centered on Mexico's Laguna Salada Fault in Baja California, the earthquake was triggered by the same processes that drive temblors on the San Andreas Fault, which runs all the way from Southern California to north of San Francisco. (See a California map.)

"On the scale of the Earth, it's all one fault zone: the plate boundary between the Pacific plate and the North American plate," said Mark Benthien, communications director for the Southern California Earthquake Center in Los Angeles. (Find out more about plate tectonics.)

As the Pacific plate grinds northward against the western edge of the North American plate, most of the motion occurs along