Researchers find why San Andreas fault hasn’t caused a big earthquake in L.A. - yet

The southern San Andreas fault in California is in a seismic drought, going more than 300 years without a major earthquake. New research 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.

One of the largest faults in the world, the 800-mile-long San Andreas marks the meeting of the North American and Pacific plates in western California. The fault has three sections, but the southern section from the Salton Sea to Parkfield, Calif., has been historically the quietest - and that’s not a positive. The pent-up energy, when released, could be catastrophic to nearby populated cities.

“This fault poses the largest seismic hazard in all of California,” said Ryley Hill, lead author and PhD candidate at San Diego State University. “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.”

Earthquakes generally form when two tectonic plates are essentially stuck at their edges because of friction, building stress. When the stress becomes greater than the frictional forces, the two blocks can suddenly slip past one another and release the energy in waves and cause shaking.

The U.S. Geological Survey estimates a high probability that an earthquake at a magnitude of 6.7 or greater could occur in the next 30 years in the Los Angeles area.

The new study, published Wednesday in Nature, investigated earthquake activity along the southern San Andreas fault over the past 1,000 years. 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 high water levels of the nearby ancient Lake Cahuilla.

“If previous earthquakes on it occurred every 180 years, plus or minus 40 years, why is it that we sit on 300 years without an earthquake?” asked Hill. “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 inform our understanding of what might happen in the future.”

The team created a computer model that simulated how a full lake affected the fault. They found high water levels in Lake Cahuilla prompted activity along the fault in two aspects. First, the weight of the lake water caused the crust beneath it to bend, unlocking the plates so they don’t touch as much. The lake water also seeped into the cracks and pores in Earth’s crust beneath it, increasing the fluid pressure inside the fault and further unclasping the plates.

Think of the scenario like playing a game of air hockey, said Hill. Without air, the puck does not slide easily across the table, much like when frictional forces hold two stuck tectonic plates together. When air (or water from the lake) is added, it helps relieve tension and makes it easier for the two to slip past one another.

“The correlation between the stressing rate from lake loading … and the observed earthquake likelihood is astounding,” Götz Bokelmann, a geophysicist at the University of Vienna who was not involved in the study, wrote in an email. “If this holds up, then this will be quite important.”

Bokelmann said that there will probably be some debate following this study because earlier studies in the region have come to different conclusions, but he finds this mechanism “quite plausible.” The new study counters previous research that found the effects from Lake Cahuilla weren’t as significant as found here and that looked at secondary faults within the Salton Sea as additional stress contributors.

“Our study shows that the lake by itself was sufficient enough to trigger events on the southern San Andreas fault - and large events,” said Hill. “This is pretty applicable to pretty much anywhere where hydrologic loading, either natural or anthropogenic, exists.”

Hill points out other areas where added stress from water played a part in triggering an earthquake. China’s Wenchuan earthquake in 2008, 7.9-magnitude, was linked to the impoundment of the Zipingpu Dam. Water level changes have also been associated with historical earthquakes at the Dead Sea.

But could changes to the present-day Salton Sea help trigger an earthquake? Maybe.

The good news, Hill said, is that the odds of Salton Sea refilling back to the size of ancient Lake Cahuilla “is effectively impossible, thankfully.” Lake Cahuilla was about 32 times as big as the present-day Salton Sea and was also fed from the Colorado River. Today, because of drought and overallocation, the Colorado River is unable to deliver as much water to the Salton Sea, he said.

The bad news, however, is that Hill 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. This is problematic because of plans to restore and potentially refill 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.”