The earthquakes of California are caused by the movement of huge blocks of the earth's crust. Southern California straddles the boundary between the Pacific and North American plates. These large sections of the earth's crust (the North American plate extends east to Iceland while the Pacific plate extends west to Japan) are moving past each other. The Pacific plate is moving northwest, scraping horizontally past North America at a rate of about 50 millimeters (2 inches) per year.
About two-thirds of this 50 millimeters per year occurs on the San Andreas fault and some parallel faults—the San Jacinto, Elsinore, and Imperial faults (see map). These four faults are among the fastest moving, and therefore most dangerous, in Southern California. Over time, these four faults produce about half of the significant earthquakes of our region.
The rate of plate movement along the San Andreas fault, 33 millimeters (1.3 inches) each year, is about how fast your fingernails grow. As a result, Los Angeles City Hall is now 2.7 meters (9 feet) closer to San Francisco than when it was built in 1924. It would take a mere (geologically speaking) 2 million years for your nails to extend 100 kilometers (60 miles) from San Bernardino to Palmdale. It took many millions of years of movement on faults (earthquakes) to shape Southern California’s current landscape.
However, this is not the whole picture. Unlike Central and Northern California, much of the plate movement in Southern California is not parallel to the San Andreas fault. Between the southern end of the San Joaquin Valley and the San Bernardino mountains, in the so-called "big bend," the San Andreas fault runs in a more westerly direction.
Where the fault bends, plate motion is complex. The Pacific and North American plates push into each other, compressing the earth's crust into the mountains of Southern California and producing faults and earthquakes. While these 300 or so faults are generally much shorter and slower moving than the four faults mentioned previously, over half of the significant earthquakes in Southern California occur on these faults.
The greatest concentration of these faults is in and near the mountains that have formed around the big bend of the San Andreas fault (the San Bernardino, San Gabriel, and Santa Ynez mountains). These mountains, like most mountains in California, are there because earthquakes are pushing them up. Many of these faults can be seen at the earth's surface, though some are buried beneath the sediments of the Los Angeles basin and the inland valleys.
"BEACHFRONT PROPERTY IN ARIZONA"
The idea of California falling into the ocean has had an enduring appeal to those envious of life in the Golden State. Of course, the ocean is not a great hole into which California can fall, but it is itself land at a somewhat lower elevation with water above it. The motion of plates will not make California sink — western California is moving horizontally along the San Andreas fault and up around the Transverse Ranges.
As the Northridge earthquake confirmed, some faults are not known until they move in large and damaging earthquakes. What do we do about these unknown faults we can't see and don't know about yet? Do we still have to wait until the next earthquake reveals them?
Not necessarily. In 2001, scientists of the Southern California Earthquake Center completed the Southern California Integrated GPS Network (SCIGN), an advanced system of 250 Global Positioning System (GPS) receivers. With this network the positions of locations throughout Southern California can be precisely measured. This network is now apart of an even larger system, the Plate Boundary Observatory, which is measuring movement throughout the western United States.
By measuring these locations for several years, we can see how different sites are moving relative to each other — for instance, Palos Verdes is moving toward Pasadena at about 4 millimeters (5/32 inch) per year. If movement between two locations is greater than the movement on known faults, then we have a reasonable idea that there may be another fault in the area, perhaps buried by sediment. This can lead to focused research using other methods to identify the unknown fault.