A major fault in California capable of producing a magnitude 8 earthquake began to move for the first time in a record, the result of this year's Ridgecrest earthquake sequence destabilizing near faults, Caltech scientists said in a new study released in Science. On thursday.
In the modern historical record, the 160-kilometer Garlock fault at the northern end of the Mojave Desert has never been observed to produce a strong earthquake or even drag on.
But new satellite radar images now show that the fault has begun to move, causing a bulging earth that can be seen from space.
"This is surprising because we have never seen Garlock fail to do anything. Here, suddenly, his behavior has changed," said the lead author of study, Zachary Ross, assistant professor of geophysics at Caltech. "We don't know what that means."
The creep illustrates how the Ridgecrest earthquakes – the largest in Southern California in two decades – have destabilized this remote California desert between the state's largest mountain range, the Sierra Nevada, and its lowest point, Death Valley.
It also pierces a lingering myth that circulates in California and beyond – that earthquakes like Ridgecrest tremors are somehow a good thing that makes future earthquakes less likely. In fact, earthquakes increase the likelihood of future earthquakes. Most of the time, the accompanying earthquakes are smaller. But occasionally they are bigger.
Not only did Garlock's fault begin to emerge in one section, but there was also a substantial swarm of small earthquakes in another section of the fault, and two additional groups of earthquakes elsewhere – one south of Owens Lake and one in the Panamint Valley. west of Death Valley.
It cannot be predicted whether destabilization will result in a major earthquake soon. In September, the US Geological Survey said the most likely scenario is that Ridgecrest earthquakes are unlikely to cause a major earthquake. However, the USGS said the July earthquakes increased the chances of a magnitude 7.5 or more earthquake at faults near Garlock, Owens Valley, Blackwater and Panamint Valley next year.
A major earthquake in the Garlock fault has the potential to send heavy aftershocks into the San Fernando Valley, Santa Clarita, Lancaster, Palmdale, Ventura, Oxnard, Bakersfield and Kern County, one of the country's most productive regions for agriculture and oil.
Major military facilities could also be hit hard, such as Edwards Air Force Base, China Lake Naval Weapons Station and Fort Irwin National Training Center. The fault is crossed by two of Southern California's most important imported water supplies – the California and Los Angeles aqueducts – and critical roads such as Interstate 5, state routes 14 and 58 and US 395.
A major earthquake caused by Garlock could in turn destabilize the San Andreas. A severe earthquake on a 400-kilometer stretch of the southern San Andreas Fault could cause the worst disruption in Southern California since 1857 and send destructive tremors through Los Angeles and beyond.
A plausible scenario surrounds the Ridgecrest earthquakes, causing a tremor in Garlock's failure, which then triggers a seismic event in San Andreas. The chances of this event happening are slim. Another plausible, unmapped scenario involves a fault rupture southeast of the Ridgecrest earthquakes.
(Jon Schleuss / Los Angeles Times)
A ground fault caused by a nearby earthquake does not necessarily mean that a major earthquake is coming. The southernmost tip of the San Andreas fault has traditionally dragged in response to distant earthquakes, including the magnitude 8.2 earthquake off the southern coast of Mexico in 2017, some 3,000 kilometers away. "But that doesn't mean that San Andreas has exploded," said geologist Kate Scharer, a USGS researcher who did not participate in the study.
What is unusual now, Ross said, is that Garlock's failure has been seismically silent in the historical record so far. And while it's unclear what the shaking and aftershocks might mean for the near future, the newly recorded move highlights the potential risk that Garlock's failure poses to California if it breaks too badly.
The research was authored by some of the country's leading earthquake science experts at Caltech in Pasadena and at NASA's Jet Propulsion Laboratory at La Cañada Flintridge – which is operated by Caltech.
The findings confirm what some scientists expected from the Ridgecrest earthquakes. The largest earthquake in the sequence, the 7.1 magnitude event on July 5, was broken over 56 kilometers by a series of unidentified faults within 22 seconds. Its southeast ends ended a few miles from Garlock's fault.
Garlock's fault builds up at one of California's fastest rates. According to geophysicist Morgan Page, a USGS researcher who did not participate in the study, the average time between earthquakes of at least magnitude 7 in the central part of the fault is about 1,200 years. But there is a huge variation; sometimes only 200 years can pass between major earthquakes in the fault; so, however, it may take 2,000 years before an encore. The last time a major earthquake is believed to have hit Garlock's fault is 465 years ago, a century or so.
For some scientists, the physics of the 7.1 magnitude earthquake on July 5 immediately suggested that Garlock's failure would be more likely to rupture as a result. Here is a possible explanation: The southwest side of the fault that broke on July 5 spread northwest. This had the effect of moving a block of ground away from Garlock's fault, releasing it and facilitating the movement of earth blocks that accumulate seismic stress on both sides of Garlock's fault – as if a cyclist had decided to release the brakes they had been holding. the tire firmly.
Satellite radar imagery shows that the part of Garlock's fault that began to crawl is about 32 kilometers long, with the earth on the north side of the fault moving west while the other side moving east. Radar images show that one side of the fault moved to its greatest extent about four-fifths of an inch from the other.
Helping scientists has been cutting-edge observations with incredible high-resolution details that were not possible in any previous major earthquake in California.
Ridgecrest earthquakes occurred in an area that has a particularly extensive network of earthquake sensors near the seismically active Coso Volcanic Field from Inyo County, which uses magma heat to power a plant. More seismic stations have been installed since the last major earthquake in Southern California in 1999, and now there are frequent satellite radar images taken from the earth's surface.
In addition to Garlock's failure, there are also reasons to focus on the risks of other nearby failures.
There is a line of potentially mature fault zones along the so-called Eastern California Shear Zone, one of the state's most important seismic zones, that carries much of the earthquake load needed to accommodate the movement of tectonic plates as the earthquake moves. Pacific plate slides to the northwest. the American dish.
These generally include an uninterrupted segment about 48 kilometers long between faults that occurred in the 1872 Owens Valley earthquake and the Ridgecrest earthquakes, and a 120-kilometer gap across the fault system. Blackwater between faults caused by Ridgecrest earthquakes and magnitude 7.3 Earthquakes Someday, these fault segments will eventually need to break to keep up with the movement of the tectonic plates, but it is not known if this will happen in our lives.
Earthquake scientists not affiliated with the study called the fluency discovery triggered by Garlock's failure scientifically interesting that it should be better understood, but emphasize that its implications are unclear. Although Garlock has not been observed before in response to major earthquakes, other faults that have arisen have not been seen as breaking into major earthquakes.
"In fact, it's probably very common, and if that's the case … it doesn't necessarily mean you're announcing something terrible," said Page of the USGS.
In addition, the observed creep probably occurred only in a relatively shallow area. "What really interests us is what happens deep within where earthquakes occur," said USGS seismologist Elizabeth Cochran, who did not participate in the study. Earthquakes usually occur between one mile and 10 miles deep; calculated creep probably occurred at the shallowest hundreds of meters below the surface.
More research needs to be done on whether the release of seismic energy in the form of a creep near the surface advances or lessens a subsequent earthquake, said USGS Scharer. In this particular case, the amount of creep and its superficiality do little to affect when the next earthquake hits Garlock's fault, Scharer said.
Sometimes large earthquakes can lead to others; A classic example was in 1992, when the magnitude 6.1 Joshua Tree Temple in April was followed two months later by the magnitude 7.3 Landing earthquake, which in turn triggered a few hours later the magnitude 6.3 Big Bear earthquake; seven years later one magnitude 7.1 earthquake hit Hector Mine. But at other times, a single major earthquake and its associated aftershocks can lead to decades of seismic silence, such as magnitude 6.9 Loma Prieta Earthquake from 1989.
Despite the uncertainty, what is happening in this region is thoroughly examined, considering that Garlock's failure is a major and important failure for Southern California. There are few major earthquakes that have been observed in California in modern times, and just because something has not been observed in the past does not mean that it cannot happen.