A new study is renewing concerns about the long-term earthquake threat facing Southern California, finding that key sections of the San Andreas and San Jacinto fault systems have accumulated more tectonic stress than at any time in the last 1,000 years.

The research, published last month in the Journal of Geophysical Research: Solid Earth, does not predict when a major earthquake will strike. But scientists say the prolonged lull in major seismic activity has allowed strain to continue building beneath one of the nation's most heavily populated regions.

"We keep accumulating that earthquake energy, and it has to be released. And the only way it gets released is through large earthquakes," U.S. Geological Survey geologist Kate Scharer, a co-author of the study, told the Los Angeles Times. "The small ones don't really do it."

Researchers focused on sections of the San Andreas and San Jacinto faults near Cajon Pass in San Bernardino County, using a computer model that combined a 1,000-year earthquake record with satellite observations of tectonic plate movement and measurements of Earth's crust.

Lead author Liliane Burkhard said the model "found that tectonic stress has now reached higher levels than at any point in that entire record."

"From the model, we see that the conditions that historically preceded large joint ruptures crossing both fault systems are now approaching," Burkhard told the Times.

According to the study, stress levels along portions of both fault systems now exceed those estimated before several major earthquakes over the past millennium.

Scientists also noted that Southern California has gone unusually long without a major rupture. Large earthquakes on parts of the San Andreas fault typically occur every 100 to 150 years, though longer gaps are possible. Along the southern section near the Salton Sea, the last major earthquake is believed to have occurred about 300 years ago.

The findings reinforce long-standing warnings from the U.S. Geological Survey that Southern California remains vulnerable to a major earthquake. USGS estimates project a 60% chance of a magnitude 6.7 or greater earthquake striking the Los Angeles region by 2045.

A hypothetical magnitude 7.8 earthquake on the southern San Andreas fault could produce violent shaking across Los Angeles, Orange, Riverside, San Bernardino, Ventura, and Imperial counties and result in roughly 1,800 deaths, according to the USGS' 2008 ShakeOut scenario.

Researchers stressed that the study is not an earthquake prediction and that all computer models have limitations. Still, Burkhard said the results indicate the fault systems are "more loaded than at any point in the 1,000-year record," which she argues "is a finding worth taking seriously."

She said the study's broader message is straightforward — "In the end, the most important message is a simple one: Let's make sure we are prepared."