Scientists have determined the long-neglected part of the South San Andreas fault, which they say could be the biggest earthquake risk for the Greater Los Angeles area – and it has been in remission for nearly 80 years.
But there may be a silver lining. If his analysis is correct, experts say it is possible that when long-term forecasts and more catastrophic earthquakes occur, it will not damage the region as much as some scientists previously feared.
“If this is true then L.A. There is a significant reduction in risk, ”said Lucy Jones, a longtime seismologist who was not involved in the study, which was published Wednesday in the journal Science Advances.
The San Andres fault is an approximately 800-mile fracture that runs very long in California and is capable of producing a very frightening, large tumble called “terrible.”
As the plates of the Pacific and North American continents move past each other, the South San Andreas Fault carries half the resulting stress from that motion, 25 millimeters (about an inch) per year. Eventually, that stress is relieved by the earthquake.
However, each part of the fault has the same stress. In Southern California, the San Andreas Fault System is made up of many small “strands,” and it is difficult for seismic researchers to identify which parts of the Fault System are at risk of collapse.
In the case of the point: a bouquet of fault sears – Garnet Hill, Banning and Mission Creek – that crosses the Cochlea Valley. Scientists have long thought that the slip of the South Andrews Fault in the south is found along the Binging Strand and the Garnet Hill Strand; They said the Mission Creek strand doesn’t take too much stress at all.
But new findings turn the idea on its head.
Earthquake geologist Kimberly Blsniuk, of San Jose State University, was looking for evidence that the quake caused the groundform to turn to the surface. She met him at the Pushwala Canyon, next to the Mission Creek Strand in the Little San Bernardino Mountains.
There, beside the water-carved valley, he saw a series of three ancient “decapitated rivers” – a long desolation in the desert that seemed to have once been part of the original valley before the earthquake struck.
Blissniuk went to the area to better see these so-called signs of ancient breakdown. In each of the channels, she and her team dated the age of rocks and land.
The oldest channel, about 2 kilometers (more than a mile) from the current valley, was about 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 channel, 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 slip rate for the Mission Creek strand was approximately 21.6 millimeters (less than an inch) per year. At that rate, they realized, it was part of most of the tension with the South San Andreas fault.
In contrast, they calculated that the banning strand has a slip rate of only 2.5 millimeters per year.
“I was really excited,” BlissNuke said, adding that it took years for the ancient channels to create the data needed to make a convincing case that they once really connected to Pushwala Canyon.
“The San Andreas fault is the most well-studied defect in the world and there is still a lot we can do to better understand it,” he said.
Because the Southern San Andreas fault is likely to experience an Earth-collapse at an average rate of 215 years or more – and because the last Earth shaker in the southern part occurred in 1726 – we have about 80 years left, Blissniuk said.
The last one has since accumulated with an elastic stress defect of about six to nine meters, scientists say – which means that when it is finally released, about 20 to 30 feet of ground will probably change. Blissniuk said whether it takes an earthquake, or many of them, it remains to be seen the distance.
The discovery “seems to be a landmark study,” said Thomas Heat, an emeritus professor of engineering seismology at Caltech who was not involved in the research.
Jones, who was not involved in the study, has now retired from the U.S. Geological Survey. But, in 2008, she led a team of more than 300 scientists, engineers and other experts to study in detail the possible outcomes of the big one. The result was a shakeout earthquake scenario, which predicted that a magnitude 7.8 earthquake on the San Andres fault could result in more than 1,800 deaths, 50,000 injuries and નુકસાન 200 billion in damage and other damage.
Jones said the new findings could change the scenario and make it even more frightening. Here’s why: The big one can only be started by a big crash on the long strip of San Andres fault, which is on the order of 200 miles. If it ends up traveling along the ruptured banning strand – as the shakeout model anticipates – it will send energy to the east-west sloping San Bernardino Valley, San Gabriel Valley and finally the Los Angeles Basin.
But if the crash mission adheres to the Creek Strand, its further northwesterly direction will save some destruction, removing it from the LA basin.
Finally, Jones said, “This is an ongoing debate and has not yet been fully resolved – probably will not happen, until we have an earthquake.”
Heaton agreed.
“As a scientist, it would be almost a surprise to me if a real earthquake, when it happens, would play out in a way that is close to our imagination,” he said. “Earth has always amazed us – it always reminds us that we need a little humility in this business.”
