In the arid expanse of what is today Southern California, a vast lake once waxed and waned. Lake Cahuilla spanned thousands of square kilometers before drying away to nothing and then reappearing in a decades-long cycle that repeated over millennia. When the lake was full, it served as a resting place for migratory waterfowl, and its ecosystem provided plentiful food and fresh water to communities of Indigenous Peoples.
Now, a new study published in Nature has suggested that Lake Cahuilla had an impact not only on the surface but also underground. The filling of the lake may have caused major earthquakes along the southern reach of the San Andreas Fault, as the weight of the water increased pressure on the rocks below. Today, with the lake gone and unlikely to return, those earthquakes have settled down—though the tectonic strain that caused them has continued to accumulate and will one day lead to another large earthquake in the region.
A Massive Lake Appears
Lake Cahuilla formed as the Colorado River periodically changed course, spilling into the low-lying Salton Trough instead of the Gulf of California. The trough typically would fill to a depth of around 100 meters (330 feet) before the Colorado returned to its normal course, and the lake would gradually evaporate.
“Lake Cahuilla extended from Palm Springs all the way down into Mexico,” said Thomas Rockwell, a geologist at San Diego State University and a study coauthor. “It was a large, large lake.”
Radiocarbon dates from sediments and charcoal found within Lake Cahuilla’s onetime shoreline indicated that the lake appeared and vanished six times over 1,100 years, with the last lake reaching its zenith around 1733.
That date and the dates of the five lake highstands before it match up nearly perfectly with the dates of major earthquakes in the region, Rockwell and his coauthors found. “Earthquakes can punch through layers of sediments, leaving a visible scar that scientists can use to date approximately when an earthquake happened,” explained Ryley Hill, a Ph.D. candidate at San Diego State University and the Scripps Institution of Oceanography at the University of California, San Diego and lead author of the study.
Previous work pairing radiocarbon dating of Lake Cahuilla with paleoseismic data hinted at a connection between the lake’s high points and prehistoric earthquakes. But with an increased number of data points and a more precise series of dates, the researchers were able to establish the relationship with greater certainty.
“The idea is not new, but the data is really amazing,” said David Sandwell, a professor of geophysics at Scripps who was not involved with the research. “This matchup is so good that it’s probably very likely the two are related.”
The connection probably comes down to what’s known as induced seismicity, which describes earthquakes triggered by forces outside of the normal stress generated as Earth’s tectonic plates interact with each other. It’s been known to happen as large reservoirs are filled with water and explains why hydraulic fracturing, or fracking, is often accompanied by swarms of earthquakes.
When it was full, Lake Cahuilla, which sat above a portion of the southern San Andreas Fault, exerted pressure on the fault equal to nearly 10 atmospheres, the researchers said. That’s tiny compared with the pressure already inside the crust, but it could be enough to contribute to earthquakes.
The weight of the lake’s water increased the fluid pressure along the fault, pushing the sides away from one another and making the fault more prone to slipping. It’s a phenomenon similar to how airflow on an air hockey table helps the puck glide along more easily by lifting its edges off the surface, Hill explained.
The slight depression in Earth’s crust created by the lake played a role, too, making earthquakes more likely at the edges of the depression.
“Earthquakes are more frequent when there’s water” involved, Rockwell said.
The Desert Returns
In the area today, the Colorado River is held back by Hoover Dam on the border of Arizona and Nevada, making any reappearance of Lake Cahuilla improbable at best. In its place is the Salton Sea, a shallow, briny lake about one sixth the size of Lake Cahuilla created when an irrigation canal burst in 1905. The Salton Sea isn’t big enough to noticeably affect the odds of an earthquake happening on the San Andreas Fault, Rockwell said. It is also shrinking and, with agricultural fields and highways lining its rim, unlikely to ever be allowed to grow significantly in size.
The disappearance of Lake Cahuilla makes it more difficult to predict when an earthquake might happen along the San Andreas Fault, said Debi Kilb, a project scientist at Scripps who wasn’t involved with the research.
With the lake gone, the main force contributing to earthquakes along the fault today is the slowly building tectonic stress between the North American and Pacific plates.
Where once earthquakes happened in the region about every 180 years, now “the recurrence time has been reset,” she said.
—Nathaniel Scharping (@nathanielscharp), Science Writer
5 July 2023: This article has been updated to correct Ryley Hill’s affiliation.