The San Andreas Fault Is About to Crack

May 20, 2016

The director of the Southern California Earthquake Center, Thomas Jordan, made an announcement recently that would have sent a chill down the spine of every Californian: that the San Andreas fault appears to be in a critical state and as such, could generate a large earthquake imminently.

Of course, the reiteration of the seismic hazard to Californians will be nothing surprising, but what is new is the warning that the southern portion of the fault “looks like it’s locked, loaded and ready to go.”

Why is this eminent seismologist making these alarming statements? Well, the fact is that there has not been a major release of stresses in the southern portion of the San Andreas fault system since 1857. In simple terms, the San Andreas is one of many fault systems roughly marking the border between the Pacific and North American tectonic plates.

Both plates are moving in an approximately northerly direction but the Pacific plate is moving faster than its North American counterpart, meaning that stresses between the plates are constantly building up.

In 1906, some of these stresses were catastrophically released in the San Francisco Bay area in a 7.8 magnitude event and again, in northern California, during the 6.9 magnitude 1989 Loma Prieta earthquake.

Events of these magnitudes, however, have not occurred along the San Andreas fault in the south of the state—the 1994 Northridge event was associated with a nearby, but separate, fault system—leading to the suggestion that one is imminent and, given the amount of stress that might actually have accumulated, when it arrives it will be the “Big One.”

How Big Is ‘Big’?

So just how big could this potential earthquake be and is it possible that the destruction demonstrated in the film San Andreas could actually come to fruition?

In short, Californians will be (reasonably) pleased with the answers to these questions. In the film, the San Andreas fault produces an earthquake with a magnitude of 9.0. While not unheard of globally, earthquakes of this size are generally confined to regions of the earth where subduction—where one tectonic plate is being forced below another—is happening, for example in Chile and Japan. The tectonic situation in California is different. Here, two plates are sliding past each other.

As such, recent predictions limit the possible maximum earthquake magnitude along the San Andreas fault system to 8.0, although with a 7 percent probability estimate that such an event could occur in Southern California in the next 30 years; over the same period, there is a 75 percent chance of a magnitude 7.0 event.

While magnitudes of 7.0, 8.0 and 9.0 might sound negligibly different, the energy that such events would unleash varies significantly, with a magnitude 9.0 event releasing 32 times more energy than a magnitude 8.0 and 1,000 times more energy than a magnitude 7.0.

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