A Big Earthquake at San Andreas Is Overdue—What Will Happen When It Cracks?

A huge earthquake caused by the San Andreas Fault is long overdue and continues to cause concern among seismologists.

The fault is the border section between two massive tectonic plates under the surface of the Earth. It stretches for more than 800 miles through California, past San Francisco and nearly as far south as San Diego.

These plates are relatively static, meaning they can see large pressures build up over time. And when they move, they can produce big earthquakes.

The fault continues to cause concern as multiple segments appears to be significantly stressed. Director of the Southern California Earthquake Center, Thomas Jordan, has previously said that the fault appears "locked, loaded and ready to go," according to the Los Angeles Times.

Both of the fault's plates are moving in a northern direction but the Pacific plate is moving faster, according to an article for the website The Conversation by Matthew Blackett, Senior Lecturer in Physical Geography and Natural Hazards at Coventry University in England. This causes increasing amounts of pressure.

Stresses have been released before. Most famously, a magnitude 7.8 earthquake hit San Francisco Bay in 1906 and a 6.9 earthquake hit Loma Prieta on California's Central Coast in1989.

The current stresses have not been released in years meaning an earthquake may be right around the corner.

How likely is a big earthquake?

"There are many plausible scenarios for big earthquakes up and down the the San Andreas fault system, including ones that would be highly disruptive in California," Rick Aster, Professor of Geophysics and Department Head at the Colorado State University told Newsweek.

"Large earthquakes on the San Andreas fault system are a geological inevitability, but individual earthquake occurrence over years to millennia is chaotic. It's thus possible to forecast earthquakes probabilistically but not, unfortunately, to predict the size, time and location of individual earthquakes at this time."

According to the U.S. Geological Survey, it is highly likely that some areas across the fault will experience a magnitude 6.7 earthquake in the next 30 years.

In areas near the state boundary, it is nearly 100 percent likely to occur. In the San Francisco and Los Angeles regions, the USGS estimates that there is a 10 percent chance of a strong magnitude 6.7 earthquake in the next 30 years.

"Some of these probabilities along particular faults are quite high, but it cannot be said with much confidence if any particular fault will next slip tomorrow, in 30 years, or at some other interval," Aster said. "This is fundamentally because the interactions between the fault systems are quite complex and interdependent, and our knowledge of the key physical conditions inside the Earth that control their behavior is very limited."

What will happen when an earthquake occurs?

Visualizations of what a huge earthquake caused by the San Andreas fault may look like have been conveyed in popular culture, such as in the 2015 movie San Andreas. In the film, the fault causes a magnitude 9.0 earthquake and destroys most of San Francisco and Los Angeles. The earthquake subsequently causes an tsunami.

But predictions from the USGS remain that the fault will not cause an earthquake bigger than 8.0, Coventry University's Blackett wrote.

The USGS predicted in 2008 what kind of destruction a hypothetical 7.8 earthquake caused by the fault could result in. It was believed that this would be most serious in areas that straddle the fault and that $33 billion worth of damage would be done to buildings.

Gas mains would also be damaged, causing subsequent fires to blaze across affected areas.

California, however, is strict on earthquake readiness. Technologies and scientific investigation of the fault are also accelerating. This means warning systems could be in place when the fault does inevitably cause a large earthquake.