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.