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San Andreas Fault Is ready for the Big One

Southern Californias section of the immense San Andreas Fault is building towards acatastrophic rupture. At the opening of the National Earthquake Conference in Long Beach, Thomas Jordan, director of the Southern California Earthquake Center (SCEC), said that the springs on the San Andreas system have been wound very, very tight. And the southern San Andreas fault, in particular, looks like it’s locked, loaded and ready to go.

Based on the movement of the tectonic plates in the region, earthquakes should be relieving roughly 4.9 meters (16 feet) of stress every century. As reported by the Los Angeles Times, the San Andreas fault hasnt been doing this at all, meaning that over a 100 years worth of stress is waiting to be unleashed.

The last time the southern section of the fault ruptured was in 1857, when a stretch of 360 kilometers (225 miles) fractured at the surface, causing a magnitude 7.9 earthquake that lasted for three entire minutes. Although there are constant, small tremors, major stretches of it have refused to budge. One section, near the Salton Sea, hasnt experienced a huge event since the late 17thcentury.

A magnitude 7.8 quake in 1906 killed 3,000 people in San Francisco, and was one of the most powerful of the 20thcentury. Although this released a lot of stress, this was in the northern section of the fault line, and the southern section hasnt experienced anything like this since.



A simulation of a M8.0 quake along the San Andreas fault. SCEC via YouTube

Luckily, San Andreas doesnt lie directly beneath Los Angeles; however, this city of 3.9 million people is only 48 kilometers (30 miles) away from it, and it will certainly be affected by the next Big One. Unfortunately, pinning down when the next cataclysmic earthquake will happen is, as always, proving difficult.

As far as we know, there are no precursors or warning signs to quakes; the best scientists can do is to say where they will happen, and if a large amount of time has passed since the last tremor, its likely that the next will be particularly powerful. The longer nothing happens, the worse it will be when it does.

In any case, its not just the San Andreas fault that seismologists are concerned about: Fault systems are complex and interconnected, and what happens to one fault affects the others attached to it.

For example, beneath California lies the San Jacinto fault. Although comparatively small, theres compelling evidence that in the past its rupture has triggered its larger companion to also jut forwards. These double-fault quakes are not necessarily more powerful than single ruptures, but they do show how small earthquakes can trigger larger ones.



Mapping the likelihood of ruptures along the complex fault network. SCEC via YouTube

If one kickstarted San Andreas into causing a magnitude 7.5 earthquake, it would unleash 1,000 Hiroshima atomic bombs worth of energy in the blink of an eye. Frighteningly, things could be even worse: The SCEC ran a supercomputer simulation in 2010 to simulate a magnitude 8.0 quake on the region, and it revealed that the entire Los Angeles basin would be hit extremely hard, with major casualties and hundreds of billions of dollars of damage a near certainty.

Jordan concluded that the best course of action is to batten down the hatches: If Californias infrastructure isnt reinforced and designed to resist a magnitude 8.0 tremor, then thousands of people will die, and the city may not be suitable to live in for many months afterwards.

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