The San Andreas Fault, California's Nightmare is Waiting
The San Andreas Fault is the most famous fault in the world. Its notoriety comes partly from the disastrous 1906 San Francisco earthquake, but rather more importantly because it passes through California, a highly-populated state that is frequently in the news. And with many research institutions dedicated to studying such an accessible fault, the SAF has become a household name. But what is a fault? And what is the SAF?
A fault is a planar crack in a rock along which slippage has taken place. Most faults are small - even microscopic - and are not important. Some faults are many miles long.
Faults can be classified according to which of the three directions of space the rocks on either side move. When the motion is predominantly vertical, they are called dip-slip faults. Dip-slip faults with dips less than 45 degrees are called thrust faults.
If the motion is mostly horizontal and parallel to the fault plane, the fault is called a strike-slip (or transform) fault. The SAF is a right lateral transform fault. This means that if two people face each other across the fault and it moves, each person will see the other person move to the right. If the rocks move horizontally apart or together, they are called divergent or convergent, respectively. Convergent faults raise pressure ridges and mountain ranges. Divergent faults create gaps or sags. When plate boundaries are convergent there is always a subduction zone. When divergent, they usually open valleys on land and oceanic ridges like the Mid Atlantic Ridge.
At plate boundaries, the fault plane is seldom vertical, i.e. a dip of 90 degrees. Indeed, almost none of the SAF's fault plane is vertical.
The San Andreas Fault is a place where two tectonic plates touch, the North American and Pacific Plates. The plates are rigid (or almost rigid) slabs of rock that comprise the crust and upper mantle of the Earth. The SAF is about 700 miles long as the crow flies and about 800 miles long when its curves are measured. It is roughly ten miles deep and reaches from the Salton Sea in Imperial County to Cape Mendocino in Humboldt County.
The plates are continually moving but where the touch each other, they get stuck. As the rest of the plates moves, the stuck parts deform like compressing a spring so they build up stress in the rocks along the fault. When the rock breaks or slips, the suddenly plates move, causing an earthquake. The entire process is called elastic rebound. As they break and scrape by one another, they produce seismic waves that travel through the ground and shake the surface. We know this shaking as earthquakes. While we think of plates as rigid, they can stretch a little, like pizza crust. That is why we can have an earthquake on the SAF in northern California but not on the SAF in southern California.
There are many major faults in California that are parallel to the SAF, like the Hayward Fault.
This results in a curious effect: while the Pacific Plate is moving northwest relative to the North American
Plate at an average rate of about 2.5 inches per year (about as fast as your fingernails grow), the
average slip rate along the
SAF is less, about 1.5 inches per year. The difference is being taken up by slip along the other parallel
This can be demonstrated by a deck of cards.
People often think of the SAF as a line in the ground, and that by hopping across it, they can go from one plate
to the other. This is not true. Although the most recent break in the ground that defines the trace of the
SAF is a narrow line, the actual plate boundary should be viewed as a fault zone. The width of a fault zone can
be less than a mile wide (Olema Trough) or many miles across (Salton Trough).
California is famous for its earthquakes on the SAF (1906 San Francisco Earthquake, 1857 Fort Tejon earthquake),
but the state's quakes are not particularly large. The largest earthquakes in the world occur near subduction zones,
where one plate is sliding under the other (Alaska, Chile, Indonesia, etc.) Only in northwestern California
north of Cape Mendocino is there a subduction zone, the Cascadia megathrust, and it is not directly related to the
SAF. Here the small Juan de Fuca plate is subducting under the North American Plate. Given enough time, the
Cascadia megathrust will produce an earthquake much larger than any quake on the SAF. It has happened before.
The SAF is about 28 million years old. It 'began' when the East Pacific Rise, the boundary between the Farallon and
Pacific Plates, subducted under the North American Plate near what is now Los Angeles. Spreading northwest and southeast
, The SAF gradually grew, and it still growing. The grinding plates and earthquakes are gradually warping
and reshaping Califonia. In a few million years, California will look very different than it does today. Consider this:
California is about 1000 miles long.
1000 miles x 5280 ft/mile x 12 inches/ft = 63,360,000 inches
At the measured slip rate of about 2.5 inches per year, it will take
63,360,000 inches/ 2.5 inches/year = 25.3 million years
for the state to be sheared and twisted beyond all recognition. With similar arithmetic, we can see that 25 million years ago,
California shared not the faintest resemblance with the modern paradise we know today. Meanwhile, the Farallon plate continues
to scrape along the bottom of the North American plate, probably destined to sink into the mantle, melt and be recycled.
All states and all continents have faults but California has more than most. What makes the SAF so important is that it is a
plate boundary and that it is on land. Most plate boundaries are in the ocean and so they are hard to study.