The United States Geological Survey (USGS) has ominously said that Southern California is overdue for a major earthquake along the Grapevine just next to Los Angeles. According to its new analysis, significant quakes there happen once a century, which means that the region is 60 years “overdue”.

The study, published in the Journal of Geophysical Research: Solid Earth, does have a key caveat, though. The once-a-century average comes from a careful analysis of 1,200 years’ worth of earthquake data recorded in the rock around and in San Andreas, but it’s important to note that it’s a very rough average.

In terms of quakes on this stretch of the fault, “individual intervals range from 22 to 186?years,” which means that an earthquake could take place anytime today or not for another few decades.

As for the kind of earthquake they predict, the geological history suggests that it’ll unfortunately be a 7.5M – something that will do a lot of damage to any city it’s nearby when it strikes. When it does, it’s near-certain that it will rupture the land alongside it for hundreds of kilometers, and land will be completely shifted in one direction or another by about 2.7 meters (around 9 feet).

“There’s no getting out of this,” USGS geologist and lead researcher, Kate Scharer, told The Los Angeles Times. “This [earthquake] would be broadly felt across the basin. It would impact our ability to be a world-class city.”

The San Andreas Fault is not a single fault line, but a network of them. Every now and then, an extension of it, or a peripheral fault, is discovered and heavily scrutinized.

^{The USGS map of the readiness of faults to rupture. The entire San Andreas Fault is about twice as likely to jolt forwards as others in the area. USGS}

Generally speaking, the San Andreas Fault as a whole is around 1,300 kilometers (800 miles) long, and it’s divided into a northern stretch and a shorter, southern segment. Both have moved separately of each other, but the entire fault can move in one sudden jolt.

The last time the southern section ruptured was in 1857, when a 360-kilometer-long (224-mile-long) section burst forwards at a shallow depth. This is the quake that the authors of the report are referring to.

This registered as a 7.9M event, and it lasted for three minutes, but not every fault in the area moved. One segment, near the Salton Sea, hasn’t moved since the 17th century, and is well overdue.

The northern section last had a major rupture back in 1906. Back then, a far less populated San Francisco took the brunt of the tremors and 3,000 people died.

This means that estimating when the next major quake will occur, and precisely where it will take place on the fault network, is difficult to tell. The best estimates, as always in this case, come from the USGS – they claim that strain rates on the fault indicate that there’s a 99 percent chance a shallow 6.7M quake will emerge from San Andreas in the next 30 years.

Los Angeles has the most to worry about, as there is a 33 percent chance that it will experience a 7.5M event in the same time period. This would definitely qualify as the prophesied “big one”, enough to wipe off between 1-2 percent of US GDP in an instant and leave millions of people homeless.

The takeaway message from all this is that every day that passes without a major quake, the more likely it will be that the next event will be even more powerful and damaging. We know the big one’s coming, but it’s very difficult to pin down exactly when and scientists are doing their best to find out.

^{Part of the San Andreas Fault, as seen from above. Ikluft/Wikimedia Commons; CC BY 4.0}

[H/T: LA Times]