Los Angeles’ biggest earthquake threat is in a neglected part of San Andreas, study finds. It can be good

Scientists have identified a long-neglected part of the southern San Andreas fault that they believe may pose the greatest earthquake risk for the Greater Los Angeles area – and its release has been expected for about 80 years.

But there might be a silver lining. If their analysis is correct, experts say it’s possible that when a long-predicted and much more devastating earthquake does occur, it won’t do as much damage to the region as some scientists previously feared.

“That’s a significant reduction in risk for LA if this is true,” said longtime seismologist Lucy Jones, who was not involved in the study published in the journal Wednesday. Scientific progress.

The San Andreas Fault is an approximately 800 mile fracture that spans much of the length of California and is capable of producing a much feared massive quake known simply as the “Big One.”

As the continental plates of the Pacific and North America move, the southern San Andreas fault supports about half of the stress resulting from this movement, up to 25 millimeters (about 1 inch) per year. Eventually, this tension is released by earthquakes.

However, not all parts of the fault carry this tension equally. In southern California, the San Andreas fault system is made up of many smaller “strands”, and it is difficult for seismic researchers to identify which parts of the fault system are most at risk of rupture.

Example: the clump of fault strands – Garnet Hill, Banning, and Mission Creek – that runs through the Coachella Valley. Scientists have long believed that much of the South San Andreas Fault slide occurred along the prohibition strand and Garnet Hill strand; the Mission Creek strand, they said, didn’t take much strain at all.

But the new findings reverse that idea.

Kimberly Blisniuk, a seismic geologist at San Jose State University, looked for evidence that the earthquakes caused landforms to shift across the surface. She found them at Pushawalla Canyon, a site along the Mission Creek Strand in the Little San Bernardino Mountains.

There, right next to the canyon carved out of the water, she saw a series of three ancient “decapitated canals” – long depressions in the desert that once seemed to be part of the original canyon before earthquakes brought them down. spread.

Blisniuk scoured the area to get a better look at these telltale signs of an old rift. In each of the canals, she and her team dated the ages of the rocks and soil.

The oldest canal, located about 2 kilometers (over a mile) from the present canyon, was between 80,000 and 95,000 years old. The second, about 1.3 kilometers (less than a mile), was about 70,000 years old; and the decapitated third canal, about 0.7 kilometers (less than half a mile), was about 25,000 years old.

Based on these three benchmarks, the researchers calculated that the average slip rate for the Mission Creek strand was about 21.6 millimeters (less than an inch) per year. At this rate, they realized that this made up the vast majority of the stump along the southern San Andreas fault.

In contrast, they calculated that the ban strand had a slip rate of just 2.5 millimeters per year.

“I was really excited,” said Blisniuk, who said it took years to produce the data needed to convincingly demonstrate that the old canals did indeed connect once to the Pushawalla Canyon.

“The San Andreas fault is one of the best studied faults in the world, and there is still a lot that we can do” to better understand it, she said.

Because the San Andreas fault to the south is susceptible to earthquakes that break ground at an average rate of one every 215 years or so – and because the last earthquake in the uppermost section south took place in 1726 – we’re about 80 years behind. , Blisniuk said.

About 6 to 9 meters of elastic deformation has likely accumulated along the fault since it was last, the scientists said – meaning that when it is finally released, the ground will likely move around 20 to 30 feet. It remains to be seen whether it takes a single earthquake, or more of them, to travel this distance, Blisniuk said.

The discovery “looks like it could be a landmark study,” said Thomas Heaton, professor emeritus of engineering seismology at Caltech who was not involved in the research.

Jones, who was not involved in the study, is now retired from the US Geological Survey. But in 2008, she led a group of more than 300 scientists, engineers and other experts to study in detail the potential consequences of the Big One. The result was the ShakeOut earthquake scenario, which predicted that a 7.8 magnitude earthquake on the San Andreas fault could cause more than 1,800 deaths, 50,000 injuries and $ 200 billion in damage and other losses. .

The new findings could change that scenario and make it less grim, Jones said. Here’s why: The Big One can only be triggered by a massive rupture on a long stretch of the San Andreas Fault, something on the order of 200 miles. If that break ended up moving along the Banning Strand – as the ShakeOut model assumed – its east-west tilt would send energy into the San Bernardino Valley, the San Gabriel Valley, and ultimately the Los Angeles Basin. .

But if the break were to follow the Mission Creek strand, its more northwest orientation would divert some of that energy from the Los Angeles Basin, sparing it some of the devastation.

Ultimately, Jones said: “This is an item in an ongoing debate and not yet fully resolved – it likely won’t be until the earthquake.”

Heaton agreed.

“It would almost be a surprise to me as a scientist if the real earthquake, when it does occur, unfolds in a way that is really close to what we imagined,” he said. “The earth always surprises us, it always reminds us that we need a little humility in this profession.”

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