The Big One at San Andreas Fault: A Huge Earthquake Will Hit California—It's Just a Matter of When

California's Antelope Valley freeway passes near folded layers of sediment above the San Andreas Fault near Palmdale, California June 22, 2006. Lucas Jackson/Reuters

Last Tuesday, the Northwestern part of Los Angeles was shaken by an earthquake. A relatively small quake of magnitude 3.6 with its epicenter in the Santa Monica mountains, the incident made itself felt with only "mild" to "moderate" tremors, though it was enough to provoke some alarmed tweets and freak-out reaction GIFs.

That same week, a massive 7.1 magnitude quake struck Mexico, causing widespread damage and loss of life including in the densely-populated capital, Mexico City.

The second event was a reminder that, while the California quake barely registered in most people's daily lives, the chances of a similarly violent incident striking the Southwestern state are strong.

"It's a different system," says Matthew Blackett, Senior Lecturer in Physical Geography and Natural Hazards at Britain's Coventry University, "But the system that is causing these quakes in Mexico is by and large similar to what's happening in California." In both locations, tectonic plates are sliding past one another.

In California, the Pacific plate and the North American plate are both moving northward, but the former is moving faster than the latter, leading to a buildup of tension. Some of this was released in a catastrophic 1906 quake that hit San Francisco. But there's still thought to be a lot of unresolved pressure along the San Andreas fault in the South of the state. In other words, a quake is probably coming, and it's going to be a "big one."

American government projections, says Blackett, place the likely magnitude of that quake somewhere between magnitudes 7 and 8.

"The San Andreas fault in southern California last had a major quake in 1857 (magnitude 7.9)," explains Robert Graves, a Research Geophysicist at the U.S. Geological Survey (USGS), in an email interview with Newsweek. "Studies that have dated previous major offsets along the fault trace show that there have been about 10 major quakes over the past 1,000-2,000 years," Graves continues. "The average time between these quakes is about 100-150 years."

That means we could be due another one soon. Graves cautions, however, that these things aren't easily predictable. "In some cases, the time separation between quakes is as short as 60 years, and in others it is around 300 years. This variability is one reason that makes forecasting when the next quake will occur quite difficult."

"But," he says, "it will happen sometime."

When it does, what will that mean? California has stringent and well-enforced building regulations designed to ward off the worst impacts of large earthquakes. But that doesn't mean a "big one" wouldn't still do serious damage.

Back in 2008, the USGS modeled the impact of a 7.8 magnitude quake in Southern California. It found that "the total financial impact of this earthquake is estimated to be… about $200 billion with approximately 1,800 fatalities."

So is such a quake on the way? The unfortunate answer is that nobody really knows. Graves tells Newsweek that the recent level of earthquake activity is "normal" and that "there is nothing in our current measurements... that would suggest a major rupture is imminent." But, he adds, "it may be that a major rupture will occur without any precursory signals. The bottom line is that precise earthquake prediction is not currently possible."

Looks like all Californians can do is practice their legendary chill. The "big one" is coming, and there's not too much more to do but wait.