Dec. 2, 2018 – A new study looks at the potential for a major earthquake along a stretch of the San Andreas Fault north of Los Angeles, site of one of the most powerful quakes ever recorded in the United States.
The U.S. Geological Survey research found earthquakes happen in the area known as the Grapevine on average every 100 years. The last major temblor occurred 160 years ago, rupturing 185 miles of the San Andreas fault.
The land on either side of the fault has been pushing against the other at a rate of more than 1 inch a year since 1857, the researchers said. That accumulating energy will be suddenly released in a major earthquake, when the land along the fault would move by many feet, according to the study.
“So, you expect that amount of accumulation of energy will be released in the future in a large-magnitude rupture, somewhere along the San Andreas,” lead author of the study, USGS research geologist Kate Scharer, told The Los Angeles Times.
A repeat of the 1857 earthquake could damage aqueducts that ferry water into Southern California from the north, disrupt electric transmission lines and tear up the 5 Freeway, where the Grapevine section runs on top of the San Andreas fault at Tejon Pass, The Times reported. It is a major north-south trucking route through California’s Central Valley.
Central Los Angeles could experience a couple of minutes of shaking, according to the study. The 1994 Northridge earthquake shook for roughly 15 seconds.
“This would be more broadly felt across the basin,” Scharer told The Times. “It would impact our ability to be a world-class city.”
Scientists observing trenches at this site discovered that earthquakes on this section of the San Andreas fault occurred on average once every 100 years, according to The Times. The 1857 quake, which occurred on Jan. 9, was centered about 45 miles northeast of San Luis Obispo.
Two deaths were reported in connection with the quake, which occurred at a time when the area was more sparsely populated, according to Caltech. The area of greatest shaking now includes the communities of Wrightwood, Frazier Park, Taft and Palmdale.
A repeat of the 1857 earthquake could damage aqueducts that ferry water into Southern California from the north, disrupt electric transmission lines and tear up Interstate 5, whose Grapevine section runs on top of the San Andreas fault at Tejon Pass.
Central Los Angeles could experience a couple of minutes of shaking, which could feel like a lifetime compared with the 1994 Northridge earthquake, which shook for roughly 15 seconds.
“This would be more broadly felt across the basin,” Scharer said. “It would impact our ability to be a world-class city.”
The quake will probably be something few Southern Californians have ever experienced. Of the 10 earthquakes Scharer and her colleagues found in the past 1,200 years, the most common magnitude was about magnitude 7.5.
Such an earthquake would tear up land along the fault’s length and displace it by an average of 9 feet.
The site studied by Scharer and her colleagues is next to Frazier Mountain at the top of Tejon Pass, close to the meeting point of Los Angeles, Ventura and Kern counties. Unlike other sections of the southern San Andreas, this region — about a 100-mile section of the fault — had not been studied in detail before, and scientists did not know what earthquakes occurred there before the 19th century.
They found 10 major earthquakes over a 1,000-year period. But “it doesn’t happen like clockwork,” Scharer said. There was once a gap of only 20 years between two major quakes. On the other end, there was a gap of about 200 years between quakes.
So, while this part of the San Andreas fault could be overdue for a large earthquake, it’s also possible it could be decades longer before the Big One strikes. Of the identified gaps between earthquakes, three took longer than 160 years to strike this part of the San Andreas again.
“Longer gaps have happened in the past, but we know they always do culminate in a large earthquake. There’s no getting out of this,” Scharer said.
There was one possible silver lining. The most common magnitude they found at this site, 7.5, means that the 7.9 earthquake experienced in 1857 was unusually strong.
In fact, of the newly discovered earthquakes, there was only one other estimated at 7.9 — one that struck around 1550.
The conclusion that 7.9 earthquakes there are relatively rare could be good news, said Caltech seismologist Egill Hauksson, who was not involved in the study.
“That means it released a lot of energy on the fault,” Hauksson said of the 1857 temblor. It also could explain why the southern San Andreas fault has been so quiet since then.
The difference between a 7.5 and a 7.9 earthquake is significant, Scharer said. A 7.5 earthquake produces roughly 15 times less energy than a 7.9. And while a 7.5 earthquake displaces land along the fault by an average of 9 feet, the bigger earthquake moves land by 20 feet.
“A road can handle a couple of feet of displacement,” Scharer said, “but when you start to get into a dozen feet, it’s a real challenge.”
The study was published online Thursday in the Journal of Geophysical Research, a publication of the American Geophysical Union.
The 1857 quake was so powerful that the soil liquefied, causing trees as far away as Stockton to sink. Trees were also uprooted west of Fort Tejon. The shaking lasted one to three minutes.
The San Andreas fault is 30 miles from downtown Los Angeles, but big earthquakes on the San Andreas fault are expected to heavily shake the nation’s second largest city.
A simulation of a possible magnitude 8 unzipping from Monterey County into L.A. County shows heavy shaking waves reverberating across a wide swath of the Los Angeles Basin and nearby valleys, whose soft soils can trap shaking energy like Jell-O.
In 2008, the U.S. Geological Survey simulated a magnitude 7.8 earthquake on the southern San Andreas that would begin at the Salton Sea and spread west toward the San Gabriel Mountains in Los Angeles County. Such a quake could cause more than 1,800 deaths, 50,000 injuries, $200 billion in damage and severe, long-lasting disruptions.
The collapse of five high-rise steel buildings, with 5,000 people inside them, was seen as possible in this USGS scenario, as would be the destruction of 50 concrete buildings; 800 people might be in completely collapsed concrete buildings, and 7,000 more in those that partially collapsed. About 900 brick buildings could be irreparably damaged.
Some cities, like Los Angeles and Santa Monica, have responded by passing laws requiring owners retrofit some of the most vulnerable buildings known to engineers. Santa Monica recently moved forward on a sweeping law that would require evaluations, and if needed, retrofits of wood apartments, brittle concrete buildings and flawed steel towers.