Grapevine area may be overdue for major quake

The San Andreas fault has been building up seismic stress for 160 years, researchers find.

SAN FRANCISCO — Southern California could be overdue for a major earthquake along the Grapevine north of Los Angeles, according to a sobering new study by the U.S. Geological Survey.

The research found earthquakes happen there on average every 100 years. The last major temblor occurred 160 years ago, a catastrophic geological event that ruptured an astonishing 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, accumulating energy that will be suddenly released in a major earthquake, when the land along the fault would move by many feet.

“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,” said the lead author of the study, USGS research geologist Kate Scharer.

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.”

There has been a long drought of major earthquakes on the southern San Andreas fault, which has slowly been accumulating strain as the Pacific plate grinds northward against the North American plate. The seismic stress has been building up without relief since the presidency of Franklin Pierce.

The quake will probably be something few Southern Californians have ever seen. The most common magnitude of earthquakes that Scharer and her colleagues found in the last 1,200 years 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.

So Scharer and other scientists undertook the most intensive study of its kind in this section of the San Andreas fault, where researchers could hear the roar of traffic from Interstate 5. Trenches were dug deeper than ever before at this location to better understand the history of quakes there.

“To get 1,200 years of records, we have to do lots of excavations and go quite deep,” Scharer said.

They found 10 major earthquakes over a 1,000year 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 in the Journal of Geophysical Research, a publication of the American Geophysical Union.