New research led by scientists at San Diego State University (SDSU) in collaboration with UC San Diego’s Scripps Institution of Oceanography finds that as the Salton Sea in Southern California has dried up, it has stabilized the southern section of the San Andreas Fault, perhaps delaying the next “big one.”
This section of the fault poses the largest seismic hazard in all of California because of the damage a significant earthquake could cause to the populous Los Angeles metropolitan area.
The paper, published on June 7, 2023 in the journal Nature, shows that for the last 1,000 years major earthquakes emanating from the southern San Andreas Fault have coincided with periods when the basin that holds the Salton Sea filled with water to form the prehistoric Lake Cahuilla – a body of water with 40 times the volume of the present-day Salton Sea.
The researchers investigated this relationship with computer modeling and found that when the Salton Sea basin filled, Lake Cahuilla’s weight bent the surrounding crust and its water penetrated deep underground, each of which altered the forces acting on the fault in ways that could help trigger a massive rupture and severe shaking.
This research, funded by the Southern California Earthquake Center, National Science Foundation, NASA, and the U.S. Geological Survey, suggests that the tight relationship between big-time seismic activity and the filling of the Salton Sea basin may also help explain why the southern section of the San Andreas Fault is long overdue for its next major shakeup. However, delaying the “big one” may only give the fault more time to accumulate stress, potentially making the eventual earthquake more powerful.
The San Andreas Fault is a roughly 800-mile-long fracture in Earth’s crust where the Pacific and North American tectonic plates meet. The two plates are slowly sliding by one another horizontally at a rate of almost two inches a year on average. The Pacific plate is on the west side of the fault moving roughly northwest and the North American plate is on the east side sliding southeast.
Earthquakes occur because sections of the craggy upper crust along the fault get stuck due to friction and other forces that clamp the two plates together. When part of the fault locks up and can’t slide, the stress accumulates until it overwhelms the forces causing the deadlock, at which point the fault ruptures and each side suddenly grinds past the other to make up the distance they would have traveled in the time they spent stuck.
The northern section of California’s San Andreas Fault is notorious for causing the hugely destructive San Francisco earthquake of 1906 and the Loma Prieta quake of 1989. By contrast, the southern section, which runs from Bombay Beach to the San Bernardino Mountains, has been surprisingly quiet.
For the last 1,000 years or so, the southern San Andreas Fault went around 180 years between major earthquakes, but it’s now been more than 300 years. Yuri Fialko, the study’s co-author and professor of geophysics at Scripps, described the southern San Andreas as “10 months pregnant” and, more ominously, “locked and loaded” for a big shake that could cause an estimated 1,800 deaths and $200 billion in damage if it strikes at a magnitude of 7.8.
Fialko and his coauthors wanted to dig into the mystery of why the southern San Andreas had been dormant for so long. Foundational to this inquiry was a 2022 paper by study co-author Thomas Rockwell of SDSU that used hundreds of samples of prehistoric lake sediment to establish a more precise history of when the Salton Sea basin had filled and emptied.
This history of the ancient Lake Cahuilla, which was filled six times in the last 1,000 years by Colorado River flood waters, facilitated a more refined seismic history of the southern San Andreas Fault by providing geological context. Armed with this improved context the team was able to reinterpret samples that were previously used to establish a history of earthquakes along the southern San Andreas Fault.
Combining these two histories revealed what Fialko called an “amazing correlation” between major earthquakes and the periods when Lake Cahuilla was filled to the brim with water. “Once we saw that correlation,” Fialko said, “the question was what is going on and how is it working mechanically.”
To investigate, Fialko and his co-authors developed a numerical model that incorporated new layers of complexity that made it even more realistic. Specifically, the model was better able to account for the porousness of Earth’s crust and the ways that water percolating through these cracks and pores alters pressure along the fault.
Previously, the calculations required to account for these additional factors overwhelmed even supercomputers. Even with today’s technology, SDSU’s supercomputer took around five weeks to run the five iterations of the model the study required.
The results confirmed that the filling of ancient Lake Cahuilla added significant stress to the southern San Andreas Fault – enough to trigger the major earthquakes the team observed in the geological record.
Lake Cahuilla added stress to the southern San Andreas fault in two main ways.
The first is that the weight of all that water bent the crust beneath it like a plastic ruler. Because the lake sits to the western side of the fault, the flexing of the crust beneath the lake ended up tugging at the western edge of the fault in such a way that it reduced the pressure clamping the two tectonic plates together and that had prevented them from slipping.
The second way has to do with water seeping into the cracks and pores in Earth’s crust beneath the lake and the weight of the lake itself. This infiltrating water increases the fluid pressure inside the fault, further counteracting the clamping pressure keeping the fault’s tectonic plates locked in place.
The effect of increased fluid pressure in a fault is a bit like an air hockey table. With the air on, the puck glides easily, but when the air is off, friction makes it hard to slide the puck. So too with an increase in fluid pressure inside the fault, the water pushes out against the two sides of the fault, making it easier for them to overcome friction, slide by one another and trigger an earthquake. This is also how hydraulic fracturing, which injects pressurized water into the ground, can induce seismic activity, said Fialko.
“While the factors of the Salton Sea drying contribute somewhat to stabilizing the southern San Andreas fault, the tectonic stress driven by plate motion is considerably greater and continues to accumulate stress on the fault," said Ryley Hill, the study’s first author and a PhD candidate in the Geophysics Earthquake Science and Applied Geophysics Joint Doctoral Program between SDSU and Scripps.
Besides Hill, Rockwell, and Fialko, SDSU geologist Matthew Weingarten contributed to the study.
The study’s findings also add the specter of triggering a massive earthquake into what is already a complex debate over the prospect of refilling the Salton Sea. A panel rejected a proposal to refill the Salton Sea with water from the ocean in 2022.
“We know this part of the fault is locked and loaded,” said Fialko. “But the past is key to the future here. By unraveling this history of events, this gives us a better idea of the hazard the fault poses and how it may behave in coming decades.”
Fialko said the model developed to better understand the southern San Andreas could also be applied to other places on Earth where there are large and sudden changes in hydrologic loads, like reservoirs that are filled and emptied.
About Scripps Oceanography
Scripps Institution of Oceanography at the University of California San Diego is one of the world’s most important centers for global earth science research and education. In its second century of discovery, Scripps scientists work to understand and protect the planet, and investigate our oceans, Earth, and atmosphere to find solutions to our greatest environmental challenges. Scripps offers unparalleled education and training for the next generation of scientific and environmental leaders through its undergraduate, master’s and doctoral programs. The institution also operates a fleet of four oceanographic research vessels, and is home to Birch Aquarium at Scripps, the public exploration center that welcomes 500,000 visitors each year.
About UC San Diego
At the University of California San Diego, we embrace a culture of exploration and experimentation. Established in 1960, UC San Diego has been shaped by exceptional scholars who aren’t afraid to look deeper, challenge expectations and redefine conventional wisdom. As one of the top 15 research universities in the world, we are driving innovation and change to advance society, propel economic growth and make our world a better place. Learn more at ucsd.edu.