The greatest earthquake danger in LA is in the overlooked part of San Andreas, the study says. That could be good

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Scientists have identified a long-overlooked section of the San Andreas Fault in the south that they say could pose the most significant earthquake risk for the greater Los Angeles area – and should be laid off in about 80 years.

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

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

The San Andreas Failure is a break of approximately 800 kilometers that runs much of the length of California and is capable of producing a massive timbre that it fears, which is simply called “Great”.

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

However, not every part of the fault carries that voltage equally. In Southern California, the San Andreas Fault System consists of many smaller “threads,” and earthquake researchers find it difficult to determine which parts of the Fault System have the highest risk of rupture.

Case: a bouquet of fault threads – Garnet Hill, Banning and Mission Creek – crossing the Coachella Valley. Scientists have long thought that most of the San Andreas Fault slippage occurred along the Banning and Garnet Hill chains; the Mission Creek River, they said, did not withstand much strain at all.

But new discoveries are turning that idea around.

Kimberly Blisniuk, an earthquake geologist at San Jose State University, set out in search of evidence that the earthquakes caused the shape of the terrain to move across the surface. She found them in Pushawalla Canyon, a place along the Mission Creek chain in the Little San Bernardino Mountains.

There, right next to a canyon carved in the water, she saw a series of three ancient “cut canals” —long depressions in the desert that looked as if they had once been part of the original canyon before the earthquakes pushed them aside.

Blisniuk walked through the area to better see these landmark signs of ancient rupture. On each of the canals, she and her team dated the age of rocks and soil.

The oldest canal, which was about 2 kilometers (more than a mile) away from the current canyon, was approximately 80,000 to 95,000 years old. The second, about 1.3 miles (less than a mile) away, was about 70,000 years old; and the third severed canal, about 0.7 kilometers (less than half a mile) away, was about 25,000 years old.

Based on these three landmarks, the researchers calculated that the average sliding speed for the Mission Creek strand was about 21.6 millimeters (less than an inch) per year. At that rate, they realized, it made up the vast majority of strains along the southern San Andreas Fault.

In contrast, they calculated that the Banning strand had a slip of only 2.5 millimeters per year.

“I was really excited,” said Blisniuk, who said it took years to get the data needed for the convincing case that the ancient canals were indeed once connected to the Pushawall Canyon.

“The San Andreas Fault is one of the best-studied bugs in the world, and we can still do so much to understand it better,” she said.

Because an earthquake on the southern San Andreas Fault is likely to break out on average once every 215 years – and because the last such earthquake in the southernmost part occurred in 1726 – we are about 80 years late, Blisniuk said.

The scientists said about 6 to 9 meters of elastic stress had accumulated along the fault, meaning the ground would shift about 20 to 30 feet when it was finally released. Whether one earthquake or a larger number of them will be needed to cover that distance remains to be seen, Blisniuk said.

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

Jones, who was not included in the study, has now been withdrawn from the American 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 Great. The result was the ShakeOut earthquake scenario, which predicted that a magnitude 7.8 earthquake at the San Andreas Fault could lead to more than 1,800 deaths, 50,000 injuries and $ 200 billion in damage and other losses.

New discoveries could change that scenario and make it less gloomy, Jones said. Here’s why: The Great One can be triggered by a massive crack in the long stretch of the San Andreas Fault, just about 200 miles away. If those cracks ended up traveling along the Banning Chain – as the ShakeOut model assumed – its tilt east and west would send energy into the San Bernardino Valley, the San Gabriel Valley, and eventually the Los Angeles Basin.

But if the rupture followed a strand of Mission Creek, its northwestern orientation would divert some of that energy from the LA basin, saving it from some devastation.

Finally, Jones said, “This is part of an ongoing debate and has not yet been fully resolved – it probably won’t be until we have an earthquake.”

Heaton agreed.

“It would almost surprise me as a scientist that the real earthquake, when it happens, shows itself in a way that is really close to what we imagined,” he said. “The country always surprises us – it always reminds us that we need humility in this business.”


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More information:
Kimberly Blisniuk et al. Revised position of the primary chain of the Pleistocene-Holocene fault of San Andreas in Southern California, Scientific progress (2021). DOI: 10.1126 / sciadv.aaz5691

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