It's only a matter of time before a massive earthquake shakes Southern California to its core, scientists say.

Though dormant for more than 300 years, the southern end of the San Andreas Fault is long overdue for a giant upheaval, according to experts.

And the results of such a quake would be devastating.

"A large earthquake would likely kill thousands and cause billions of dollars in damages," said Lucy Jones, a seismologist with the U.S. Geological Survey.

A U.S. Geological Survey map shows earthquakes that hit the Los Angeles, California, area between August 6, 2007, and August 13, 2007.

Though the area is no stranger to mild- and medium-strength quakes, Southern California hasn't seen a major upheaval for more than 300 years. That means such a quake is long overdue, experts say - and "the scale of the disaster could be along the lines of Hurricane Katrina."

Jones serves as chief scientist of the Southern San Andreas Earthquake Scenario, a team of scientists assessing potential earthquake scenarios in an area of Southern California known as the Coachella Valley.

"The scale of the disaster could be along the lines of Hurricane Katrina," she said.

When, Not If

Historical data show that the average time between earthquakes in the southern end of the fault line is 150 to 200 years.

However, the last earthquake struck the area back in 1680.

While scientists can't explain this long gap between seismic activity, the experts are almost certain that it's a question of when, not if, the next one will strike.

"Even though there hasn't been an earthquake in a long time, the reality is that there will be one," explained Tom Fumal, research geologist with the U.S. Geological Survey's Earthquake Hazards Team.

The 1680 quake had relatively little effect, since it struck the then-uninhabited Palm Springs area.

But a lot has changed in the intervening time.

In one likely scenario, a magnitude 7.8 earthquake would strike near the Salton Sea in the Coachella Valley, which is located east of San Diego and Los Angeles, Jones said.

Extending north for approximately 200 miles (320 kilometers), the predicted quake would topple buildings, destroy roads and railways, ignite wildfires, and trigger landslides, she added.

In addition to causing numerous deaths, such a quake would trigger rampant destruction that would leave tens of thousands homeless and the area's transportation corridors to the rest of the country completely shut down.

"The largest growing area of Southern California is right next to the fault," Jones said. "There are way more people at risk from this earthquake than from hurricanes."

But the damage would not be limited to the Coachella Valley.

"If the earthquake nucleates at the Salton Sea and ruptures north, much of the seismic energy will be funneled into the Los Angeles Basin, causing very serious shaking and damage to the city," said Yuri Fialko, a geophysicist at the Scripps Institution of Oceanography.

If that happens, "this would be a very significant economic and social issue for the nation," Jones said.

Since no one can predict exactly when such an earthquake might strike - or do anything to prevent one - the best defense is proper preparation, Jones added.

Jones and her team's findings will eventually become the foundation for statewide emergency response demonstrations and preparedness efforts, she said.

"We want emergency preparedness to become a part of the Southern California consciousness. This earthquake is inevitable, but the disaster is not."