How is the San Andreas Fault made and what is the Big One?
What is the San Andreas Fault?
There is a high earthquake-prone zone in California that is over 1,200 km long. It is the San Andreas Fault, a large fracture of the lithosphere that runs through California, which causes an average of about 10,000 earthquakes each year. This fault is of the strike-slip type, that is, it moves with horizontal movements with respect to the earth’s surface. Its motion is decidedly evident when looking at a photograph of it from above, in which slippages of the ground that have occurred over the years are very clear.
The San Andreas Fault seen from above.
Two large lithospheric plates move along the San Andreas Fault and slide close to each other: the Pacific plate and the North American plate. Active at least since the Jurassic period, the fault slides the western portion of California northward at a rate of about 3.5 cm per year.
Map of the San Andreas Fault and the movement of the Pacific and North American plates.
These continuous movements have led to the formation of a series of cracks in the earth’s crust and, of course, numerous earthquakes over the centuries. In 1906, a violent earthquake triggered by the fault movement hit San Francisco, causing the destruction of the whole city.
The Big One
To date, the San Andreas Fault represents a threat in all respects. In fact, as the fault moves, energy is built up that must be released. For too long there have been no earthquakes in the southern part of the fault. It is therefore expected that some sections of the fault that have high seismic risk potential, currently inactive, may move in the near future, releasing an enormous amount of energy. This event that scares seismologists is called the Big One and is expected to occur as a massive earthquake.
