USGS updates forecast of when a major quake might strike LA region

You've heard this before: "The Big One" can strike any day in Southern California. We're talking about a really big earthquake along the southern end of the San Andreas Fault that measures magnitude 7.5 or greater.

The U.S. Geological Survey has released a new study that says we could be overdue for a major earthquake from the San Andreas Fault along the Grapevine — in the Tejon Pass near Frazier Mountain in northeastern Kern County — that would shake the Los Angeles basin for several minutes.

USGS geologist Kate Scharer led a team that investigated the timing of sand, mud and gravel deposits that were episodically ripped apart by earthquakes along the San Andreas Fault over the last 1,200 years. They found evidence of 10 ground-rupturing earthquakes between 800 A.D. and the last rupture in 1857.

"In the area of the Grapevine we had a 100-mile or so stretch where we didn’t have good information on the timing of earthquakes back for the last thousand years or so," Scharer said. "And our goal was to find out when they occurred and what the average time period between those earthquakes is. ... What we find is, on average, that earthquakes have occurred every 100 years, but some of the intervals between earthquakes are as short as just 20 years and other intervals can be as long as about 200 years."

The Tejon Pass earthquake in 1857 measured an estimated 7.9 magnitude and caused a ground rupture about 210 miles long, according to the study's findings. Previous quakes appear to have measured between 7.0 and 7.5 magnitudes.

Based on data from the study, the USGS is forecasting that there is a 16 percent chance of a 7.5-magnitude or larger earthquake occurring on this section of the fault in the next 30 years. 

"There’s certainly going to be several minutes of ground shaking in the L.A. basin for a large-magnitude rupture like a 7.5 or greater," Scharer said.

A repeat of the 1857 earthquake could damage aqueducts that deliver 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.

When the USGS released results of a similar study in 2006, it said such a quake could result in thousands of deaths and billions of dollars in damage in L.A.

Meanwhile, another study released Tuesday says an earthquake fault running from San Diego to L.A. is capable of producing a magnitude-7.4 temblor.

The study, conducted by the Scripps Institution of Oceanography at the University of California, San Diego, looked at the Newport-Inglewood and Rose Canyon systems. They had been considered separate — but the study concludes they are one long fault running offshore from the San Diego Bay and on land through the L.A. Basin.

The study's lead author, Valerie Sahakian, said the fault is never more than four miles offshore, and even a moderately large quake could have a major impact on the region.

In 1933, a magnitude-6.4 quake on the fault struck the Long Beach area, killing 115 people.