California is overdue for a major earthquake on the southern San Andreas fault, which could be due to the absence of a large lake that may have triggered past quakes.
The San Andreas fault runs through California for 1200 kilometres, forming the boundary between the Pacific and North American tectonic plates. The southern section of the fault, east of Los Angeles, has been abnormally quiet, with no large earthquakes for nearly 300 years despite an average gap between quakes of 180 years over the past millennium. Movement of the plates has also built up a large amount of stress within the fault.
鈥淚t is 10 months pregnant,鈥� says at San Diego State University in California. He says a large earthquake on the fault poses the biggest seismic hazard in the state and could cause thousands of deaths and in damage.
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A for this delayed quake is the disappearance of Lake Cahuilla over the fault. For millennia, this lake formed when the Colorado river shifted course and dried up when it shifted back. But the lake last dried out near the end of the 18th century and the floods that might have refilled it are now prevented by dams on the river.
Before the lake vanished, its added weight, as well as water seeping from it, could have increased the pressure of fluid within the fault. Hill says this could have reduced friction between the fault surfaces, like the air between an air hockey table and a puck. However, until now it was unclear whether this would have been enough to trigger a major earthquake on the southern San Andreas.
To find out more, Hill and his colleagues studied layers of sediment from a site near the edge of the lake site, attempting to identify periods when the lake was full. They discovered that six of the seven major earthquakes in the area over the past 1100 years occurred close to the time when the lake was filled. The seventh quake, which happened around 1000 years ago, might have also occurred when the lake was filled, but its precise date was too ambiguous to be sure, says Hill.
The researchers also modelled how the added weight of water in the lake would affect pressure within the fault, finding that rapid filling on its own could add enough pressure to trigger an earthquake. Conversely, the absence of the lake over the past few centuries has had a 鈥渟tabilising鈥� effect on the fault, says Hill. 鈥淭hat air hockey table is getting turned off.鈥�
While that may have deferred the inevitable earthquake on the fault, the delay could have enabled more stress to accumulate, says Hill. That could make the eventual earthquake bigger when it eventually comes, he says.
More accumulated stress won鈥檛 necessarily lead to a bigger earthquake, says at the US Geological Survey. There could be two smaller earthquakes, rather than a single large one, for instance.
That said, Brothers finds the link between the filled lake and quakes convincing, even if more work is needed to nail down the precise dates. 鈥淚f the lake never comes back, we might be entering a new tectonic regime on the fault,鈥� he says.
