Anyone interested in the geologic evolution of California is advised to read Assembling California by John McPhee, published in 1993, which describes how the State was pieced together as a puzzle of terranes. Indeed, all of the U.S. from the Great Basin westward was added to North America in this fashion, arriving as blocks ripped from other Continents or as volcanic island arcs that formed above shifting subduction zones.
While most Americans are familiar with the San Andreas Fault (the border between the North American and Pacific Plates), which runs northwest from southeast California and exits the State northwest of San Francisco, there are many other faults that parallel the Pacific Coast and the Sierra Nevada, Coastal and Southern California Ranges; others straddle the Transverse Range or radiate from the volcanic centers of Northern California.
This morning's 6.0 magnitude earthquake (6.7 miles deep) was centered just north of San Francisco Bay (6 miles southwest of Napa); it thus resulted from pressure release along a "minor" fault, well east of the San Andreas corridor. All of Earth's major tectonic plates are in constant motion, grinding against one another at a rate that we humans cannot perceive; it is when that energy is released in the form of an earthquake that we are forced to acknowledge the ongoing geologic evolution of our planet. Today's quake demonstrates that tectonic pressure is transferred to lesser terranes as well, even though, in the course of our brief human life span, they appear to be part of the "stable mainland."
