Los Angeles’ biggest seismic threat is in an overlooked part of San Andreas, according to a study. That can be good

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Scientists have identified a long-overlooked portion of the southern San Andreas fault that they believe could represent the most significant earthquake risk for the Los Angeles metropolitan area, and should be delayed about 80 for publication. years.

But there could be a silver lining. If their analysis is correct, experts say it is possible that when a much more devastating and long-predicted earthquake hits, it may not cause as much damage to the region as some scientists previously feared.

“That’s a significant reduction in risk for Los Angeles if this is true,” said seismologist Lucy Jones, who was not involved in the study published Wednesday in the journal. Progress of science.

The San Andreas Fault is an approximately 800-mile fracture that runs through much of the length of California and is capable of producing a much-feared massive earthquake known simply as “the big one.”

As the continental plates of the Pacific and North America move over each other, the San Andreas fault in the south bears about half of the stress resulting from that movement, up to 25 millimeters (about 1 inch) per year. . Eventually, that tension is released through earthquakes.

However, not all parts of the fault carry this stress equally. In Southern California, the San Andreas fault system is made up of many smaller “threads,” and it is difficult for earthquake researchers to identify which parts of the fault system are at greatest risk of rupture.

Case in point: the fault chain branch – Garnet Hill, Banning and Mission Creek – that crosses the Coachella Valley. Scientists long thought that much of the southern San Andreas fault landslide occurred along the Banning chain and the Garnet Hill chain; the Mission Creek chain, they said, didn’t take much stress at all.

But the new findings turn that idea around.

Kimberly Blisniuk, an earthquake geologist at San Jose State University, went looking for evidence that earthquakes had caused landforms to move across the surface. He found them in Pushawalla Canyon, a site along the Mission Creek range in the little San Bernardino mountains.

There, right next to the water-carved canyon, he saw a series of three ancient “decapitated canals,” long depressions in the desert that appeared to have been part of the original canyon before earthquakes pushed them aside.

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

The oldest channel, which was about 2 kilometers (more than a mile) from the current canyon, was approximately 80,000 to 95,000 years old. The second, about 1.3 kilometers (less than a mile) away, was about 70,000 years old; and the third decapitated canal, about 0.7 kilometers (less than half a mile) away, 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 that rate, they realized, it accounted for the vast majority of the stress along the southern San Andreas fault.

Rather, they calculated that the Banning chain 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 necessary data to make a convincing case that ancient canals actually did connect to Pushawalla Canyon once.

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

Because the San Andreas fault in the south is likely to experience ground-breaking earthquakes at an average rate of one every 215 years or so, and because the last of these tremors in the southernmost section took place in 1726, we are roughly 80 years behind, Blisniuk said.

About 20 to 30 feet of elastic stress has likely built up along the fault since the last one, the scientists said, meaning that when it is finally released, the ground is likely to move about 20 to 30 feet. feet. It remains to be seen whether it takes a single earthquake, or many of them, to reach that distance, Blisniuk said.

The discovery “looks like it could be a landmark study,” said Thomas Heaton, an emeritus professor of seismological engineering 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, he led a group of more than 300 scientists, engineers and other experts to study in detail the possible 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 result in more than 1,800 deaths, 50,000 injuries and $ 200 billion in damage and other losses.

The new findings could alter that scenario and make it less bleak, Jones said. Here’s why: The Big One can only be triggered by a massive rupture in a long stretch of the San Andreas fault, something on the order of 200 miles. If that break ends up traveling along the Banning chain, as the ShakeOut model assumed, its east-west tilt would send power to 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 northwesterly orientation would divert some of that energy from the Los Angeles basin, sparing some of the devastation.

Ultimately, Jones said: “This is a piece in an ongoing debate and it has yet to be fully resolved; it probably won’t be until we have the earthquake.”

Heaton agreed.

“It would almost be a surprise to me as a scientist if the actual earthquake, when it occurs, unfolds in a way very close to what we imagine,” he said. “The earth always surprises us, it always reminds us that we need some humility in this business.”

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More information:
Kimberly Blisniuk et al. A revised position for the main chain of the Pleistocene-Holocene San Andreas Fault in Southern California, Progress of science (2021). DOI: 10.1126 / sciadv.aaz5691

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Citation: Los Angeles’ biggest seismic threat is in an overlooked part of San Andreas, according to a study. That may be good (2021, March 25) retrieved March 25, 2021 from https://phys.org/news/2021-03-la-biggest-quake-threat-overlooked.html

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