News in Science

Big quake for California a 'certainty'

California will almost certainly be hit by a killer earthquake in the next 30 years, US scientists say.

New calculations reveal there is a 99.7% chance a magnitude 6.7 quake or larger will strike before 2038.

"It basically guarantees it's going to happen," says Dr Ned Field, a geophysicist with the US Geological Survey in Pasadena and lead author of the report.

The predictions were the result of a new system that combines information from seismology, earthquake geology, and precise measurements from the earth's surface, allowing the probabilities of a major earthquake to be forecast.

"This new, comprehensive forecast advances our understanding of earthquakes and pulls together existing research with new techniques and data," Field says.

The earthquake forecasts, known as the Uniform California Earthquake Rupture Forecast, were developed by Field's working group of scientists and engineers.

The group found that two of California's largest cities, Los Angeles and San Francisco, were more than 63% likely to face a 6.7 quake.

The 6.7 earthquake in Los Angeles in 1994 left 60 people dead and did an estimated US$10 billion damage, while a 6.9 quake in San Francisco in 1989 claimed the lives of 67 people.

Geologists say one of the biggest areas of concern in California is the southern section of the San Andreas fault in Riverside County, east of Los Angeles, which is described as being "10 months pregnant".

Hammered

"You have to realise this is a very long pregnancy, and it is way overdue," says Professor Tom Jordan, director of the Southern California Earthquake Centre at the University of Southern California.

"We can expect that we're going to get hammered by a big earthquake and we'd better be prepared."

Jordan says an earthquake of 7.5 magnitude might span 322 kilometres of fault length and have a displacement of 3.6 metres or more.

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

Jordan says the chance of a 7.5 magnitude quake hitting Southern California is 37%, compared with 15% in Northern California.

This is 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 says.