A fault system stretching nearly 70 miles along the coast of Los Angeles and Orange counties has the potential to trigger a magnitude 7.8 earthquake, according to a new study that is the latest to come. highlight the seismic threats facing Southern California.
Known as the Palos Verdes Fault Zone, the system extends deep beneath the Palos Verdes Peninsula. It was previously thought to be a segmented network of smaller faults, but further examination by scientists at Harvard University suggests it is a system of closely interconnected planar fractures. spaced stretching from Santa Monica Bay to the waters off Dana Point.
The analysis determined that the fault system, which passes under many neighborhoods as well as the ports of Long Beach and LA, has a much larger surface area that could rupture during the same seismic event, making it capable of a much more powerful earthquake than before. known.
Scientists found that the fault could produce an earthquake of a magnitude comparable to that of the San Andreas Fault. Earlier estimates indicated that the fault zone could generate an earthquake of magnitude 7.4, but the new study shows that it could produce an earthquake as strong as 7.8.
The difference may only be a few decimal places, but the energy of an earthquake is measured exponentially. According to the US Geological Survey, a magnitude 7.8 earthquake produces four times the energy of a magnitude 7.4 earthquake.
In the worst case, the Palos Verdes fault system could trigger an earthquake that combines the most destructive qualities of the 1994 Northridge earthquake, a magnitude 6.7 earthquake, and the earthquake of Ridgecrest’s 7.1 that hit in 2019, said John H. Shaw, a professor of structural and economic geology at Harvard University and one of three authors of the new study.
The Northridge earthquake, which killed 57 people, had a devastating combined side-to-side and up-and-down motion that proved particularly destructive to structures. This same combined lateral and vertical movement of faults is possible along the Palos Verdes network.
The Ridgecrest earthquake was an extended series along several interconnected fault lines, similar to those in the Palos Verdes system.
“Rather than a line on the map, we could see a rupture [series] it could happen over a wide area,” Shaw said of a major earthquake along the Palos Verdes fault zone. The aftershocks would be like falling dominoes, he said.
James Dolan, a professor of earth sciences at USC who reviewed the Harvard report, said the study is “by far the most detailed examination we’ve had of the internal structure and connectivity of the Palos Verdes fault system”.
Scientists had previously only been able to study parts of the Palos Verdes fault system, which operates mostly underwater. But Shaw and his co-authors Franklin D. Wolfe and Andreas Plesch collated earlier studies as well as information from oil company drilling and ground sensors, which Shaw and his colleagues used to create a new model of the area. fault.
Petroleum studies can be useful to earthquake scientists because they offer insight into what the Earth looks like below the surface where earthquake faults are.
“Historically, this defect was considered a segmented defect – lots of small pieces,” Shaw said. “It looked like a structure that wasn’t going to break in a single big earthquake.”
The new study, however, suggests the system is connected, stretching 68 miles and passing beneath southwestern Los Angeles County and the ports of Los Angeles and Long Beach and along the coasts of Los Angeles and the Orange County.
The study, published by the Bulletin of the Seismological Society of America, also found that the Palos Verdes Fault is sliding at a much faster rate than most active fracture zones in Los Angeles County, moving 1 to 6 millimeters per year. More than 50 active faults operate beneath LA County, and the majority are moving 1 millimeter or less per year, Shaw said.
As the faults slide, that energy is stored over time, Shaw said, adding that the built-up tension waits to break and eventually is unleashed in the form of earthquakes.
The effects of a magnitude 7.8 earthquake – which would be 45 times stronger than the Northridge earthquake – depend on the location of the fault.
An earthquake of this magnitude on the South San Andreas Fault, breaking between the Salton Sea near the Mexican border and passing through Palm Springs and into Lake Hughes, north of Santa Clarita in LA County, could claim 1,800 lives, including hundreds killed in building collapses, according to a simulation of such an earthquake published by the USGS in 2008.
Under the simulation scenario, highways connecting the region to Las Vegas and Phoenix could be destroyed, as could the aqueducts that bring most of LA County’s water. Vulnerable pipelines carrying fuel and natural gas and overhead power lines through Cajon Pass in San Bernardino County could explode, threatening the power grid.
Such a massive earthquake occurring in our lifetime is especially plausible because the San Andreas is the state’s fastest fault, sliding at a rate of 15 to 35 millimeters per year. Scientists have compared it to someone driving at dangerous speeds – they are the ones most likely to have a crash.
The last time Southern California was hit by an earthquake as powerful as 7.8 magnitude was in 1857, when the San Andreas Fault ruptured from Monterey County through the San Gabriel Mountains north of Los Angeles and into the Cajon Pass. Seismologists estimate that the South San Andreas Fault is capable of an 8.2 magnitude earthquake.
On a section of the San Andreas Fault along the Grapevine, scientists have found evidence that major earthquakes occur there on average every 100 years. But the gap between earthquakes can vary: over a period of 1,000 years, there was once a gap of 20 years between major earthquakes and once a gap of 200 years.
In contrast, the Palos Verdes fault zone did not produce a large earthquake during the same period. The area has recently generated magnitude 2-3 earthquakes, barely enough to be felt by most people.
Scientists have not fully explored the ramifications of a magnitude 7.8 earthquake on the Palos Verdes fault zone. But the USGS has been studying the implications of a less powerful 7.3 magnitude quake there, and it could still be destructive. Such an earthquake could kill more than 200 people and destroy more than 2,000 buildings. It could liquefy the artificial land beneath the country’s largest port complex and cause significant damage to infrastructure.
Caltech seismologist Egill Hauksson said a 7.8 quake on the Palos Verdes Fault would devastate Southern California. But Hauksson, one of the region’s most respected experts in the field, said it was also unlikely to happen in our lifetime.
“It would be pretty devastating, but it’s pretty unlikely,” Hauksson said. “It’s not a very active foul.”
Likewise, Shaw said the new information is not a major cause for concern, especially in earthquake-prone California.
“California is already a center for earthquakes, and the danger of a large earthquake constantly exists,” he said. “It helps us … understand that particular location in the fault system, where large earthquakes could occur. It helps us understand the hazards that those earthquakes could create so that we can better prepare for them.
“Location matters. And while future San Andreas Fault earthquakes are concerning and we should be prepared, smaller earthquakes that occur in an urban environment have shown to present as much, if not more, dangers.
Times writer Rong-Gong Lin II contributed to this report.