The San Andreas Fault System “The Big One”

Introduction The California landscape is cut by many faults capable of making earthquakes, but one of these, the San Andreas Fault, more important than all the others. It is the principal boundary between the Pacific Plate and the North American Plate, and its displacement rate is the highest of all California faults – more than one inch per year. Two of the greatest earthquakes in the history of California, the 1857 Fort Tejon Earthquake and the 1906 San Francisco Earthquake, resulted from slippage along this fault.

Tectonic Evolution of the San Andreas Fault Zone The San Andreas Fault that we see today however, is merely a snapshot in time, as the fault has migrated eastward. To understand this, let’s start about 30 million years ago. Three tectonic plates are involved, the North American, Pacific and Farallon Plates. Dividing the Farallon and Pacific oceanic plates was the East Pacific Rise (divergent boundary), and the Farallon was being subducted beneath the North American continental plate.

By 20 million years ago, the westward leading edge of the North American Plate began to overrun over the East Pacific Rise. As this continued, the once convergent boundary instead became a transform boundary. M – Mendocino Triple Junction R – Rivera Triple Junction

The proto-San Andreas Fault probably ran near or slightly off the coast of California. For perhaps million years, the plate boundary, although gradually lengthening, remained at or near the edge of the continent. About 5 million years ago, the long sliver of coast that we now know as Baja California was split off from the mainland of Mexico by an extension of the East Pacific Rise and started drifting northwestward with the Pacific Plate.

The San Andreas Fault at this time “stepped inland” to roughly its position today. The spreading center that now runs up the Gulf of California to the Salton Sea is broken into numerous short segments by long transform faults. The “stepping” of the San Andreas Fault also caused the fault to take on a rather distinctive “big bend” in the vicinity of the Los Angeles Basin, causing the uplift of the Transverse Ranges.

In cross-section, this change over from a convergent, subduction boundary to a transform environment is shown above. Divergent Boundary Transform Boundary

Another way of looking at this change over time diagrammatically. The proto-San Andreas Fault begins as a fault paralleling the coastline. As the East Pacific Rise extended inland, the San Andreas likewise stepped inland.

Depending on which way a strike-slip fault like the San Andreas “bends”, it may be a “restraining bend”, as it is in southern California, or a “releasing bend” as it is more commonly in areas more to the north. Restraining Bend Restraining bends will often cause the generation of hills or mountains due to compression. The mountains surrounding the Los Angeles Basin in the LANDSAT image to the right were generated along a restraining bend on the San Andreas fault.

Releasing Bend A releasing bend will generally create a “pull apart” or sag pond due to extensional forces acting in the area. Lost Lake, a spring fed sag pond generated by a releasing bend along the southern San Andreas Fault near Cajon Pass.