California Earthquakes Along San Andreas Fault May Be Triggered By Ancient Lakes

A 3.2-magnitude earthquake hit near Kernville, California, on Sept. 28, according to the U.S. Geological Survey. (USGS)

Researchers have discovered that ancient rising lakes may trigger California earthquakes along the San Andreas fault line.

The new study, published Wednesday in Nature, investigated earthquake activity along the southern San Andreas fault and found that six significant earthquakes on the fault line occurred when the prehistoric Lake Cahuilla was rising.

Collecting field data from rocks near the fault, Hill and his colleagues found earthquakes occurred about every 180 years, give or take 40 years, and coincided with the high water levels of the nearby ancient Lake Cahuilla.

Data gathered by Ryley Hill, coauthor of the study and graduate student at the University of California San Diego, shows the lack of seismic activity may be due to the drying of the nearby Salton Sea and provides clues on future potential earthquake triggers, including projects aimed to refill the body of water.

This is a typical scene in the aftermath of the great San Francisco earthquake on April 18, 1906. This view is southwest of the corner of Geary and Mason Streets, San Francisco, Ca. (AP Photo/U.S. Geological Survey)

“We have found the Southern San Andreas Fault, which poses the largest seismic hazard in all of California, was likely triggered by the filling of an ancient lake called Lake Cahuilla. The lack of this lake since ~1725 might also help explain in part why we have not had a large earthquake on this section of the fault for 300 years,” Hill told Newsweek.

The San Andreas Fault is the border section between two massive tectonic plates under the surface of the Earth and stretches for more than 800 miles through California, past San Francisco down to San Diego.

The southern end of the fault lies next to what is now known as the Salton Sea, a remnant of the ancient lake. Lake Cahuilla used to fill periodically over the past thousand years but is now causing a seismic drought, going more than 300 years without a major earthquake.

The San Andreas Fault has been causing concern among experts as multiple segments appear to be significantly stressed. For this reason, they fear that a gigantic earthquake could be imminent.

“This fault poses the largest seismic hazard in all of California,” Hill explains. “The southern San Andreas fault is a locked section, and when this fault ruptures … it would cause significant damage to the Los Angeles metropolitan area.”

According to this new study, this earthquake could have been triggered by a rapid filling of the Salton Sea.

“This section of the San Andreas Fault has accumulated much tectonic stress. While it is doubtful that Lake Cahuilla will fill again, there is potential that the Salton Sea, the modern-day remnant of Lake Cahuilla, may be filled again,” Hill said. “There is evidence to suggest that it might not just be the total weight/size of the lake that can trigger an event but the rate at which the lake fills. So if we were to rapidly increase the Salton Sea filling, I would be apprehensive about potentially triggered earthquakes on the [southern fault].”

The U.S. Geological Survey has previously predicted that it is very likely that some areas across the San Andreas Fault will experience a magnitude 6.7 earthquake in the next 30 years.

“If previous earthquakes on it occurred every 180 years, plus or minus forty years, why is it that we sit on 300 years without an earthquake?” Hill questioned. “This made a lot of scientists scratch their heads for many years. Understanding the history of this fault and what may have caused ruptures in the past helps us understand what might happen in the future.”

Hill says the positives are that the odds of the Salton Sea refilling back to the size of ancient Lake Cahuilla “is impossible, thankfully.” But warns that he and his colleagues found that it’s not necessarily the volume that could add stress but the rate at which the lake could be filled.

Lake Cahuilla was about 32 times as big as the present-day Salton Sea and fed from the Colorado River. Hill adds that the Colorado River cannot deliver as much water to the Salton Sea because of drought and overallocation.

This is problematic because of plans to restore the Salton Sea, which has increasingly been affected by climate change and an area of toxic dust.

“If you suddenly rapidly increase the filling of the lake, that might actually stimulate seismicity,” said Hill. “That would be really bad for this area because we already know that so much stress has accumulated on this fault.”