Major earthquakes on San Andreas happen more frequently than previously thought

Large earthquakes have rumbled along the southern section of the San Andreas fault more frequently than previously believed, suggesting that Southern California could be overdue for a strong temblor on the notorious fault line, according to a new study.

The Carrizo Plain section of the San Andreas has not seen a massive quake since the much-researched Ft. Tejon temblor of 1857, thought to have been about magnitude 7.9, which is considered the most powerful earthquake to hit Southern California in modern times.

But the new research by UC Irvine scientists, to be published next week, found major quakes occurred there roughly every 137 years over the last 700 years. Until now, scientists have believed big quakes have occurred along the fault roughly every 200 years. The findings are significant because seismologists have long believed this portion of the fault is capable of sparking the so-called “Big One” that officials have for decades warned will eventually occur in Southern California.

“That means it’s been long enough since 1857 that we should be concerned about another great earthquake that ruptures through this part of the fault,” said Ken Hudnut, a geophysicist at the U.S. Geological Survey in Pasadena, who was not involved in the study.

Many scientists thought the Carrizo area produced relatively infrequent earthquakes -– but ones on the massive scale of the Fort Tejon temblor.

The new work suggests the area produces more quakes but also ones of a smaller magnitude than Ft. Tejon, said Ray Weldon, a University of Oregon geologist who was not involved in the research, but reviewed the paper for the Journal of Geophysical Research. Still, experts warned, such temblors would likely be at least the size of the 1994 Northridge quake, which had a magnitude of 6.7.

“The most significant thing to come out of the work is recognition that there’s a greater variety of earthquake sizes that occur on the San Andreas,” Weldon said. “But even moderate earthquakes on the San Andreas can cause considerable damage, so the overall hazard and risk has gone up.”

-- Jia-Rui Chong

Photo: U.S. Geological Survey

Comments

[mathiastck]

Yeah Los Angeles just had one. At 7:47 pm 1/23/2009.

[LEE]

WOW WE JUST HAD AN EARTHQUAKE WOW

[Freddy Nager]

Nice timing! How did you know an earthquake would hit L.A. just 3 hours after you posted this?

[jeff]

while reading this article I just felt a jolting E.Q. I live in venice. It was only a 3.3 but centered in the Marina. Thank God it wasn't the big one. I hope I live to see the big one.

[Joe]

great timing.. read this 10 minutes before I felt a quake!! awesome

[Jared]

Aaaaah!

[Pete Viles]

not major but a 3.3/3.4 just rocked mar vista. lasted about two seconds but had a kick like a mule in our house.

[Sarah]

oh great.

[WTL]

Some basic facts missing from this article: The Carrizo Plain is in eastern San Luis Obispo County about 50 miles northwest of Fort Tejon/Gorman in the Grapevine. The earthquake was actually centered another 50 miles farther north between earthquake-happy Parkfield and Cholame.

The article confuses these locations when it talks about frequency of earthquakes. The Parkfield area registers a 6.0 roughly every 22 years (the last one was 2004, about 11 years behind schedule) - which means the fault is pretty much continuously relieving pressure. But the farther south you go, the less frequent the "moderate" sized earthquakes, which means the greater the likelihood of a giant one like the "Fort Tejon" one.

Coming up from the Salton Sea the fault takes a west turn at the San Bernardino Mountains and runs along the base of the San Gabriels, then heads northwest again at the Grapevine. The pressure builds up tremendously in this east-west jog because the two sides of the fault can't slip past each other so easily as they do farther north at Parkfield - they get jammed up, and the result is a potential for huge quakes, which haven't happened in this area for 300 years - not just 150 years.

So we are seriously overdue, more so than the article says - that Fort Tejon quake was NOT centered in the area we should be worried about. A 7.9 quake truly centered in the area from Fort Tejon/Grapevine to San Bernardino would hit the Inland Empire disastrously, and wreck a lot of LA, too, far more so than another 7.9 centered up near Parkfield.

This is how the article is misleading - it relies on the misnamed "Fort Tejon quake" to try to illustrate the danger to Los Angeles and the Inland Empire. The danger is actually far greater than the article says.

[M. Poutine]

Did they figure that out by subtracting 1857 from 1994? Someone needed a PhD for that?

[jennifer]

THIS IS SO COOL I NEED HELP ON MY SCIENCE HOMEWORK I NEED TO KNOW WHICH WAS THE TWO MAJOR EARTHQUAKES OF ALL? AND WHERE DID THEY HAPPEN?

PLEASE HELP ME LOVE JENNY OH AND IM IN FIFTH