Dried-up lake may explain why California is ‘overdue’ major earthquake on San Andreas fault

California is overdue for a significant earthquake on the southern San Andreas fault, which may very well be because of the absence of a giant lake which will have triggered previous quakes.

The San Andreas fault runs via California for 1200 kilometres, forming the boundary between the Pacific and North American tectonic plates. The southern part of the fault, east of Los Angeles, has been abnormally quiet, with no massive earthquakes for almost 300 years regardless of a median hole between quakes of 180 years over the previous millennium. Motion of the plates has additionally constructed up a considerable amount of stress throughout the fault.

“It’s 10 months pregnant,” says Ryley Hill at San Diego State College in California. He says a big earthquake on the fault poses the most important seismic hazard within the state and will trigger hundreds of deaths and a whole lot of billions of {dollars} in injury.

A doable clarification for this delayed quake is the disappearance of Lake Cahuilla over the fault. For millennia, this lake fashioned when the Colorado river shifted course and dried up when it shifted again. However the lake final dried out close to the tip of the 18th century and the floods which may have refilled it are actually prevented by dams on the river.

Earlier than the lake vanished, its added weight, in addition to water seeping from it, might have elevated the strain of fluid throughout the fault. Hill says this might have diminished friction between the fault surfaces, just like the air between an air hockey desk and a puck. Nevertheless, till now it was unclear whether or not this is able to have been sufficient to set off a significant earthquake on the southern San Andreas.

To seek out out extra, Hill and his colleagues studied layers of sediment from a website close to the sting of the lake website, trying to establish intervals when the lake was full. They found that six of the seven main earthquakes within the space over the previous 1100 years occurred near the time when the lake was crammed. The seventh quake, which occurred round 1000 years in the past, may need additionally occurred when the lake was crammed, however its exact date was too ambiguous to make sure, says Hill.

The researchers additionally modelled how the added weight of water within the lake would have an effect on strain throughout the fault, discovering that fast filling by itself might add sufficient strain to set off an earthquake. Conversely, the absence of the lake over the previous few centuries has had a “stabilising” impact on the fault, says Hill. “That air hockey desk is getting turned off.”

Whereas which will have deferred the inevitable earthquake on the fault, the delay might have enabled extra stress to build up, says Hill. That would make the eventual earthquake greater when it will definitely comes, he says.

Extra amassed stress received’t essentially result in a much bigger earthquake, says Danny Brothers on the US Geological Survey. There may very well be two smaller earthquakes, reasonably than a single massive one, for example.

That mentioned, Brothers finds the hyperlink between the crammed lake and quakes convincing, even when extra work is required to nail down the exact dates. “If the lake by no means comes again, we is likely to be getting into a brand new tectonic regime on the fault,” he says.