New calculations reveal there is a 99.7 percent chance a magnitude-6.7 quake or larger will strike in the next 30 years. The odds of such an event are higher in Southern California than Northern California, 97 percent versus 93 percent.

The last time a jolt this size rattled California was the 1994 Northridge disaster, which killed 72 people, injured more than 9,000 and caused US$25 billion in damage.

California is one of the most seismically active regions in the world. More than 300 faults crisscross the state, which sits atop two of Earth's major tectonic plates, the Pacific and North American plates. About 10,000 quakes each year rattle Southern California alone, although most of them are too small to be felt.

The analysis is the first comprehensive effort by the USGS, Southern California Earthquake Center and California Geological Survey to calculate earthquake probabilities for the entire state using newly available data. Previous quake probabilities focused on specific regions and used various methodologies that made it difficult to compare.

Scientists still cannot predict exactly where in the state such a quake will occur or when. But they say the analysis should be a wake-up call for residents to prepare for a natural disaster in earthquake country.