Major quake almost inevitable for California - study

California will almost inevitably be struck by a major earthquake, and possibly a catastrophic quake, sometime in the next 30 years, scientists said on Monday in the most comprehensive geologic forecast for the state.

California faces a more than 99 percent chance of being hit by a magnitude 6.7 temblor - the size of the 1994 Northridge quake - in the next 30 years, according to a study using new data and analyzing earthquake probabilities across the state.

The analysis found a nearly 50 percent chance that California would be rocked by a magnitude 7.5 quake, which is capable of inflicting catastrophic damage if it is centered under a big city like Los Angeles or San Francisco.

"We can expect that we're going to get hammered by a big earthquake and we'd better be prepared," said Tom Jordan, director of the Southern California Earthquake Center at the University of Southern California.

"Magnitude 7.5, that's a really big earthquake," Jordan said. "If that were to hit on the San Andreas Fault it could be very destructive. You're talking about an earthquake that might span 200 miles (322 km) of fault length and a displacement of 12 feet (3.6 metres) or more.

"If that were to take place in say, the Los Angeles region, then you would have a big problem," he said.

Jordan said the chance of a 7.5 magnitude quake hitting Southern California was 37 percent, compared to 15 percent in Northern California, largely because the 1906 San Francisco earthquake relieved stress from the San Andreas Fault there.

The 1906 San Francisco quake was thought to have been a magnitude of around 7.8 or higher. The last temblor of that size in Southern California was in 1857, and the southernmost section of the San Andreas Fault has not seen such an event since 1680.

"Those faults have been accumulating stress all this time and that makes large earthquakes highly probable," Jordan said.

The Jan. 17, 1994, Northridge quake in Los Angeles killed 72 people, injured more than 10,000 and caused billions of dollars in damage.

The study was conducted by the U.S. Geological Survey, the Southern California Earthquake Center and California Geological Survey and is significant because it presents the probabilities statewide for the first time.

"This is the most comprehensive earthquake forecast ever for the state of California," Jordan said, adding that it was requested by the California Earthquake Authority and would be used by the agency as a basis for setting insurance rates.