Researchers have discovered why a massive earthquake has not yet destroyed Los Angeles

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The southern San Andreas fault in California is in a seismic drought, which has lasted more than 300 years without an earthquake. New research shows that the lack of seismic activity may be due to the drying up of the nearby Salton Sea and provides clues to potential future earthquake triggers, including projects aimed at refilling the body of water.

One of the world’s largest faults, the 800-mile San Andreas Fault marks the meeting of the North American and Pacific plates in western California. The fault has three sections, but the portion south from the Salton Sea to Parkfield, California, has historically been the calmest—and that’s not a positive. The pent-up energy, when released, can be disastrous for nearby populated cities.

said Riley Hill, lead author and Ph.D. Candidate at San Diego State University. “The San Andreas South Fault is a closed section, and when this fault bursts… it will cause significant damage to the Los Angeles metropolitan area.”

Earthquakes generally form when two tectonic plates are essentially stuck at their edges due to friction, building pressure. When the pressure becomes greater than the frictional forces, the two masses can suddenly slide over each other releasing energy in waves and causing vibration.

The USGS estimates a high probability of an earthquake of magnitude 6.7 or greater within the next 30 years in the Los Angeles area.

The new study, published Wednesday in the journal Nature, investigated seismic activity along the southern San Andreas fault over the past 1,000 years. Hill and his colleagues collected field data from rocks near the fault, and found that earthquakes occur about every 180 years, give or take 40 years, and coincide with rising water levels in nearby ancient Lake Cahuila.

“If earlier earthquakes occurred every 180 years, plus or minus forty years, why are we sitting on 300 years without an earthquake?” Hill said. “This has had many scientists scratching their heads for many years. Understanding the history of this fault and what may have caused cracks in the past helps inform our understanding of what might happen in the future.”

The team created a computer model to simulate how an entire lake would affect the fault. They found that rising water levels in Lake Cahuilla stimulated activity along the fault on two sides. First, the weight of the lake water caused the crust underneath to flex, opening up the plates so they wouldn’t touch as much. Lake water also seeped into cracks and pores in the crust below, increasing fluid pressure within the fault and increasing plate separation.

Think of the scenario like playing air hockey, Hill said. Without air, a disk doesn’t slide as easily across a table as it does 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 each other.

“The correlation between the stress rate of lake loading … and the observed earthquake probability is striking,” Götz Boeckelmann, a geophysicist at the University of Vienna who was not involved in the study, said in an email. “If this continues, it will be very important.”

There is likely to be some controversy after this study because previous studies in the area have reached different conclusions, Böckelmann said, but he found this mechanism to be “quite plausible.” The new study contradicts previous research that found that influences from Lake Cahuilla were not as significant as found here and that looked at secondary faults within the Salton Sea as additional contributors to stress.

“Our study shows that the lake itself was sufficient enough to trigger events on the southern San Andreas fault — and large events,” Hill said. “This applies pretty much anywhere where there is hydrological loading, whether natural or man-made.”

Hill points to other areas where additional stress from the water played a role in triggering an earthquake. The Wenchuan earthquake that struck China in 2008, with a magnitude of 7.9, was associated with the sequestration of the Zipengpu Dam. Changes in the water level have also been associated with historical earthquakes at the Dead Sea.

But could changes in the current Salton Sea help trigger an earthquake? maybe.

The good news, Hill said, is that the prospects of refilling the Salton Sea to the size of ancient Lake Cahuila are “virtually impossible, fortunately.” Lake Cahuilla was about 32 times larger than the current Salton Sea and was also fed by the Colorado River. Today, he said, the Colorado River is primarily drained from water use before it reaches the body of water

The bad news, however, is that Hill and his colleagues found that it’s not necessarily volume that can increase pressure, but the lake’s filling rate. This is problematic due to plans to restore and possibly refill the Salton Sea, which has been increasingly affected by climate change and the region’s toxic dust.

“If you suddenly quickly increase the fullness of the lake, it could actually trigger earthquakes.” Hill said. “That would be really bad for this area, because we already know that a lot of pressure has been built up on this bug.”

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