San Andreas Fault 'Earthquake Drought' May Be Due to a Long-Lost Ancient Lake

The "earthquake drought" currently taking place along a notorious section of the San Andreas Fault may be down to the loss of an ancient lake that once existed in the region, scientists have said.

Researchers led by Ph.D. student Ryley Hill, from the University of California San Diego, have put forward a new idea for why there has been no major rupture along the fault for over 300 years, when historical records suggest earthquakes of magnitude 7 or above normally take place once every 150 years on average.

The southern San Andreas Fault is capable of producing earthquakes of magnitude 7 and above, potentially placing those living in the region at risk. The lack of a large earthquake has left scientists wondering when the next "big one" may hit.

In a presentation at the Geological Society of America's 2020 Annual Meeting, Hill and colleagues suggested an ancient lake that once sat above this southern section may have been involved in ruptures. They say the weight of the water in Lake Cahuilla, which has been drying out for almost 1,000 years, may have placed additional strain on the fault, increasing the frequency of large earthquakes—albeit to a very small degree.

The lack of a lake above the fault may explain why it has been such a long time since a large earthquake hit this section, the researchers say.

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Map showing the area of the San Andreas Fault that the researchers studied. Rebecca Dzombak

In their research, the team looked at historical records of earthquakes in the region stretching back 1,000 years. They then combined this information with models showing water pressures in rocks under the lake. Their findings showed the weight of the lake's water combined with how it would have seeped into the ground beneath may have increased stress placed on the rocks, making them weaker. At the point in time where the lake was deepest—between 1000 and 1500 A.D.—the pressure would have been greatest, making the fault more likely to rupture.

"It's not that [water] lubricates the fault," Hill said in a statement. "Imagine your hands stuck together, pressing in. If you try to slip them side by side, they don't want to slip very easily. But if you imagine water between them, there's a pressure that pushes [your hands] out—that's basically reducing the stress [on your hands], and they slip really easily."

The weight, coupled with the water in the rocks, causes stress on the fault to build to a critical point quicker than if the water was not there.

Hill said their findings do not mean the fault is not at risk of rupture—the forces moving the tectonic plates that San Andreas forms the boundary of are far greater than the pressure from the lost lake.

Sylvain Barbot, Assistant Professor of Earth Sciences at the University of Southern California, who was not involved in the study, told Newsweek the research "brings attention to a critical segment of the San Andreas fault that is expected to snap into a great earthquake at any moment."

Two earthquake swarms that took place along this section of the fault, around the Salton Sea, brought attention to the potential danger the fault poses. "If the rupture starts near the Salton Sea, the seismic waves of this impending earthquake would be channeled towards the Los Angeles basin and shake the city for a long time," Barbot said in an email.

Chris Goldfinger, a director of the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University who was also not involved in the study, said more data is needed to show a "drought" is taking place on the fault. "The problem with most paleoseismic records is they are too short to have a clear idea of what the long term recurrence is, what patterns of rupture might be there, and so understand the inner working of a system commonly lacks the very important evidence of what actually happened over time," he told Newsweek in an email. "When you don't know that very well, the possibilities become infinite."

He also said the link between lake levels and the timings of earthquakes is "squishy at best," with limited research supporting arguments for and against. He said if the assumptions made in the latest model are correct, then it could happen. However, he said he does not think the lack of water would be enough to "lock up" a fast-moving plate boundary fault for hundreds of years.

"At least equally likely [in my opinion], is that this is normal variability of a fault system we just don't know all that well," Goldfinger said.

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Stock image showing the Salton Sea with mountains in the background. Researchers say a lost lake may be behind the "earthquake drought" along one section of the San Andreas Fault. iStock