Beyond the sunshine, the palm trees and Hollywood, if there is one certainty in California, it's that a massive earthquake will strike at some point.

But when the Big One hits, a recent report says, the western state is ill-prepared and local officials as well as major businesses need to face that reality to "prevent the inevitable disaster from becoming a catastrophe."

Drafted by a group of business and policy leaders, the report identifies several key areas that need to be addressed before a quake as strong as a magnitude 8 happens, notably aging infrastructure, water supplies and the risk of catastrophic fires.

One of the biggest vulnerabilities, the report states, relates to the Cajon Pass, a narrow mountain pass where the mighty San Andreas Fault intersects with key lifelines, including freeways, railway lines, gas and petroleum pipelines as well as electric lines.

A major earthquake on the San Andreas, one of California's most dangerous faults, would cut most lifelines in and out of southern California, preventing critical aid from reaching some 20 million people and hampering recovery efforts, experts say.

The quake would also rupture flammable pipelines, triggering explosions and fires that could burn out of control.

"Anything that comes into southern California has to cross the San Andreas Fault to get to us - gas, electricity, water, freeways, railways," said seismologist Lucy Jones, who acted as adviser for the Southern California Disaster Risk Reduction Initiative committee, which issued the report.

"Most of the water that we get has to cross the fault to reach us but when the earthquake happens, all of the aqueducts will be broken at the same time," Jones, known as California's "earthquake woman," told AFP.

She said one way to get around this dependency was to look at alternative water sources, including from contaminated aquifers beneath the Los Angeles area that could be cleaned up, albeit at a massive cost.

"The best defense against a broken aqueduct is to not need an aqueduct," Jones said.

Installing automatic shut off valves on natural gas and petroleum gas pipelines that run near the San Andreas Fault could also help prevent major fires, according to the report.