The “earthquake drought” currently occurring along a notorious section of the San Andreas Fault could be due to the loss of an ancient lake that once existed in the region, scientists said.
Researchers led by Ph.D. student Ryley Hill of the University of California, San Diego, have presented a new idea as to why there has not been a major rupture along the fault for over 300 years, when historical records suggest that earthquakes of magnitude 7 or greater typically occur on average every 150 years.
The southern San Andreas Fault is capable of causing earthquakes of magnitude 7 and above, potentially putting the people living in the region at risk. Since there has not been a major earthquake, scientists are wondering when the next “big” quake might occur.
In a presentation at the 2020 annual meeting of the Geological Society of America, Hill and colleagues suggested that an old lake that once lay over this southern section may have been involved in fractures. They say that the weight of the water in Lake Cahuilla, which has been dried-out for nearly 1,000 years, may have added to the fault line and increased the frequency of major earthquakes, albeit to a very small degree.
The absence of a lake above the fault could explain why it has been so long since a major earthquake hit this section, the researchers say.
In their research, the team looked at historical records of earthquakes in the region going back 1,000 years. They then combined this information with models showing the water pressure in the rocks under the lake. Their results showed that the weight of the lake water, combined with the way it would have seeped into the ground below, may have increased the load on the rocks and made them weaker. At the time when the lake was at its deepest – between 1000 and 1500 AD – the pressure would have been greatest, which would have increased the probability of the fault breaking.
“It is not that [water]lubricates the fault,” Hill said in a statement. “Imagine your hands are stuck together and pressing inward. If you try to slide them side by side, they don’t want to slip off so easily. But if you imagine water between them, there’s a pressure that pushes [your hands]outwards – that basically reduces the strain on [your hands], and they slip really easily.
The weight, coupled with the water in the rocks, causes the pressure to build up to a critical point faster than if the water were not there.
Hill said her results did not mean that the fault was not fracture prone – the forces moving the tectonic plates that form San Andreas’ boundary are far greater than the pressure of the lost lake.
Sylvain Barbot, Assistant Professor of Earth Sciences at the University of Southern California, who was not involved in the study, told Tekk.tv that the research “draws attention to a critical segment of the San Andreas Fault that is expected to erupt into a major earthquake at any moment”.
Two earthquake shocks that occurred along this section of the fault, around the Salton Sea, drew attention to the potential hazard posed by the fault. “If the fault begins near the Salton Sea, the seismic waves from this imminent earthquake would be channeled towards the Los Angeles Basin and shake the city for a long time,” Barbot said in an e-mail.
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 was needed to show that a “drought” was occurring on the fault. “The problem with most paleoseismic records is that they are too short to have a clear idea of what a long-term recurrence is, what fracture patterns might be present, and so understand that the internal workings of a system generally lack the very important evidence of what actually happened over time,” he said in an email to Tekk.tv. “Unless you know this very well, the possibilities become endless.
He also said that the connection between the lake level and the time of earthquakes is “muddy at best”, and that few studies provide arguments for or against it. He said that if the assumptions made in the latest model are correct, then it could happen. However, he said he did not believe that the lack of water would be sufficient to “lock up” a fast moving plate boundary fault for hundreds of years.
“It is at least as likely [in my opinion]that this is the normal variability of a defect system that we simply don’t know that well,” Goldfinger said.