Why didn’t Los Angeles have a huge San Andreas earthquake recently?

It’s a mystery that has blessed California and continues to raise fears for the future: Why hasn’t Los Angeles experienced a major San Andreas earthquake in generations? What does it mean when it comes?

A new study offers a possible answer — the dry Salton Sea, about 150 miles southeast of Los Angeles, has had no major flash flood waters flowing into it since it formed more than a century ago.

But one thing is certain. Seismic drought on the San Andreas Fault will not last. Draining the Salton Sea may help delay the next big earthquake, but this could lead to a stronger quake when it hits.

A study published Wednesday in the journal Nature by scientists at San Diego State University and the Scripps Institution of Oceanography at UC San Diego aims to help explain why the southern end of the San Andreas Fault, near the Mexican border, has not experienced an earthquake of magnitude 7 or greater. In about 300 years.

This is an unusually long gap. Other parts of San Andreas have been torn apart recently. One section between Monterey County and San Bernardino County was torn apart 166 years ago, and another was torn apart in the Great San Francisco Earthquake 117 years ago.

In this southern part of San Andreas, there were seven major earthquakes between the 10th and 18th centuries. Some of them may have been separated by only 40 or 50 years. For others, the hiatuses may have been as long as 280 years.

But it’s been about three centuries – sometime between 1721 and 1731 – since an earthquake of at least magnitude 7 struck the southern part of San Andreas.

“However, we know that this portion of the fault has accumulated enough tectonic strain to produce such an event,” said Riley J. Hill, a doctoral candidate in geophysics at Scripps Institution of Oceanography and San Diego State who was the lead author of the study. . “So this area actually poses the greatest seismic hazard in all of California because it can seriously damage the Los Angeles metropolitan area.”

A potential earthquake that begins in the Salton Sea and explodes through Palm Springs, continuing into Los Angeles County, is known as a ShakeOut scenario. That envisions a magnitude 7.8 quake that could kill 1,800 people and injure nearly 50,000 — the deadliest earthquake in Southern California’s recent history. More than 1,000 deaths could occur in Los Angeles County alone.

So what could explain the unusually long lull?

Before the formation of the Salton Sea began in 1905—as a result of human and natural causes—the Salton Low Basin, which lies below sea level, cycled over thousands of years between filling with water from the Colorado River and drying up.

The ancient predecessor of the Salton Sea is now referred to as Lake Cahuilla, which, when full, was 40 times larger in volume than its present-day remains. Lake Cahuilla was so vast that it stretched from the Coachella Valley south into Mexico and from as far west as Plaster City, California, to as far east as the Chocolate Mountains, according to study co-author Thomas Rockwell, a professor of geology at San Diego State.

Full Lake Cahuilla reached a height of about 40 feet above sea level, with a maximum depth of over 300 feet before it began to spill again. By contrast, the Salton Sea is currently about 240 feet below sea level, with a maximum depth of about 50 feet.

Researchers have found a pattern of Colorado River water flowing into Lake Cahuilla and accompanying large earthquakes before the lake periodically drains away. Lake Cahuilla is believed to have filled six times in the past millennium: roughly the periods 930 to 966, 1007 to 1070, 1192 to 1241, 1486 to 1503, 1618 to 1636, and 1731 to 1733.

“Looking at the history of the earthquake and its relationship to the lakes, we realize that most earthquakes occurred when the lake was present,” Rockwell said.

Of the seven major earthquakes the researchers found, six occurred when Lake Cahuilla was filling or was at peak level.

“Although previous studies were inconclusive about the temporal correlation of earthquakes and lake episodes over the past millennium, the new earthquake history strongly suggests that all lake-filling events were accompanied by major earthquakes,” the study said. “Such an association between earthquake and lake timing is probably not accidental.”

The main reason for the occurrence of earthquakes remains the same: stress accumulated for decades or centuries due to the movement of tectonic plates, and the fault suddenly bursts. Such an earthquake would inevitably have occurred whether or not Lake Cahuilla existed. But its periodic refilling may have been the tipping point that triggered earthquakes as they occurred.

However, the receding Salton Sea does not mean that Southern California will forever be protected from the San Andreas earthquake as long as this area is dry.

“The idea that letting the Salton Sea dry up will keep us safe from earthquakes is definitely wrong,” said Bill Filiposian, a USGS research geologist who was not involved in the study.

“Even if ancient Lake Cahuila or the Salton Sea never existed, these earthquakes would have happened eventually anyway. They happened a little sooner than they would have, due to the effect of filling and emptying the lakes over time,” said Philibosian. “Even if the Salton Sea dries up completely, the next San Andreas South Earthquake will happen. We still need to prepare for that.”

The study said scientists also noted other examples where filling reservoirs brought earthquake faults closer to collapse, which could lead to earthquakes. A report published in 1997 noted that seismic activity “is likely to be more widespread and deeper for a larger reservoir than for a smaller one.”

One documented example, cited in a study published in 1988, concerns the Koyna Dam in western India. The reservoir behind the dam began filling in 1962; Five years later, a 6.2-magnitude earthquake struck, damaging the dam and a nearby town, and killing 200 people.

Recently, some scientists have suggested that filling the Zipingpu Reservoir in China could plausibly help push the timing of the devastating 7.9-magnitude earthquake in Sichuan in 2008, which killed more than 69,000 people. It is unclear if the new reservoir directly caused the initial rupture of that earthquake. But the scientists noted that it could have helped a small tear along the fault to create larger cracks elsewhere.

In addition, earthquakes have been associated with the injection of wastewater deep into the earth – left over from fracking to extract oil and natural gas.

Such activities increase something called pore pressure – basically an increase in fluid between grains of rock. “If you have a rock and you completely saturate it with water, the pressure of the pores in it will increase because of the fluids inside,” Hill said. This weakens the earthquake fault, making it more susceptible to rupture.

Hill said that increasing pore pressure is like turning on an air hockey table. Low pore pressure can be like trying to push a disc when the table is off—it’s harder, but you can still move it. The higher pore pressure will be such as when the table is open, which means that it is much easier for the disc to move when given the same amount of thrust.

“That’s exactly what the lake does. It basically serves to de-stabilize” the two sides of the fault, which facilitates their movement, which leads to an earthquake, Hill said.

Not having a huge Lake Cahuila in this part of California anymore could mean more seismic strain builds up before it explodes in an earthquake. This could make it possible for an even larger event when an earthquake actually occurs.

“We’d probably have a much bigger event, because more tension can be released,” Hill said.

The recent large earthquake in this area of ​​the San Andreas caused one section of the other fault to exceed 12 to 14 feet, making it likely a magnitude 7.3 or 7.4 earthquake. Rockwell said he expects the next quake to be even bigger because it will be at least 300 years since the last quake, with one side of the fault likely to outpace the other by 16 feet or so, or make it a magnitude 7.5.

The open question is how far San Andreas will go. Infrequently, earthquakes from this area tear the rift not only through the Salton Trough and Coachella Valley regions, but spread farther north toward Riverside and San Bernardino.

“So one of the big questions is, if we have a large earthquake, is it likely to flow to the north and create a larger earthquake,” running through Los Angeles County and possibly all the way to the southern edge of Monterey County, Rockwell said.

The formation of the Salton Sea at the turn of the last century was small compared to the outflows that filled ancient Lake Cahuila. However, Hill said that a rapid increase in water flow into the newly formed body of water may have helped trigger the 6.1-magnitude earthquake in Brawley on April 18, 1906. Scientists had earlier suggested that the quake was triggered by the Great 1906 earthquake in San Francisco. , which occurred 11 hours after it occurred in Northern California.

The Salton Sea has long been fed by the waters of the Colorado River draining from the farmlands of the Imperial Valley. The lake has been shrinking since the early 2000s, when the local irrigation district began funneling part of its water to urban areas. Its waters are now about twice as salty as the ocean and continue to rise with evaporation, a shift that has caused significant declines in fish and bird populations.

Based on current trends, the lake is expected to continue shrinking in the coming years. Along the receding shores, state officials have worked on projects to control dust and build wetlands to provide habitats for wildlife.

A state-appointed committee recommended last year against the idea of ​​importing seawater to prevent the Salton Sea from shrinking. The commission concluded that California should not pursue such a plan, citing estimated costs in the tens of billions of dollars, damage to the coastal environment and a construction schedule that could take many years before any water reaches the lake.

Advocates of importing water into the Salton Sea sharply criticized the panel’s conclusions, arguing that the analysis was faulty.

Times staff writer Ian James contributed to this report.

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