LOS ANGELES (Reuters) - When the seismic "big one" hits California, scientists doubt it will be quite as powerful as the earthquake that struck Japan last week although it could do plenty of damage.
The colossal California quake considered inevitable and long overdue is most likely to strike along the southern end of the famed San Andreas Fault and register a magnitude of 7.5 or greater, many times less powerful than the 9.0 temblor that rocked Japan on Friday, geologists say.
Still, an earthquake damage forecast prepared in 2008 for the U.S. Geological Survey by geophysicists and engineers envisions a calamity that would leave 2,000 people dead, 50,000 injured and 250,000 homeless.
That scenario is based on the premise of a magnitude 7.8 quake rupturing the San Andreas in the desert east of Los Angeles and radiating with catastrophic fury into the nation's second-largest metropolitan area.
Such a quake could be expected to topple 1,500 buildings, badly damage another 300,000 and sever highways, power lines, pipelines, railroads, communications networks and aqueducts. Property losses of more than $200 billion are projected.
The hypothetical quake also would ignite about 1,600 fires, some growing into conflagrations that would engulf hundreds of city blocks.
Experts predict the biggest long-term economic disruption would come from damage to water-distribution systems that would leave some homes and businesses without running water for months.
"The lesson is you don't need a magnitude 9 to cause extensive damage," said USGS spokeswoman Leslie Gordon.
The quake scenario for the southern San Andreas does not foresee damage to the nearest of the state's two nuclear power plants, the Southern California Edison-owned San Onofre station between Los Angeles and San Diego.
Both Edison and Pacific Gas & Electric, owner of the Diablo Canyon plant to the north at San Luis Obispo, say their facilities are built to withstand quakes far greater than nearby faults are capable of producing.
And unlike Japan, California faces little if any risk of tsunamis from its own quakes.
But substandard construction poses a bigger problem in California, said Lucy Jones, a USGS geologist who co-authored the agency's quake scenario.
"The Japanese have done a better job than we have done of retrofitting older buildings," she said on Tuesday.
USGS studies put the probability of California being hit by a quake measuring 7.5 or more in the next 30 years at 46 percent, though the extent of damage will depend on where in the state it occurs. The likelihood of a 6.7 quake, comparable in size to the temblors that rocked San Francisco in 1989 and Los Angeles in 1994, is 99 percent statewide.
The Los Angeles basin is especially vulnerable to violent shaking from earthquakes because the area is heavily populated and built on motion-sensitive sediment that runs four miles deep before hitting bedrock, USGS geologist Erik Pounders said.
The 9.0 quake that struck Friday off Japan's northeast coast, unleashing a deadly tsunami and a nuclear power crisis, was the biggest in that island nation's modern history. The death toll is expected to surpass 10,000, and the quake ranks as the fifth most powerful in the world for the past century.
Its force was roughly equivalent to the power of 30 quakes like the one imagined in the 2008 USGS scenario.
Geologists believe a 9.0 quake is virtually impossible along the San Andreas, a network of "strike-slip" faults smaller and more fragmented than the great chasm that exists where two continent-sized plates of the Earth's crust meet along the Japanese islands.
This subduction zone beneath the Pacific, where one tectonic plate is thrust up over another, is capable of producing the biggest quakes on Earth, on an order of magnitude higher than any recorded in California.
Offshore quakes generated from subduction zones, also found along Alaska's Aleutian Islands chain, can produce tsunamis because of the tremendous volume of water they suddenly displace on the sea floor.
The horizontal ruptures of California's seismic faults, even those offshore, displace little or no water, and thus pose no tsunami threat, except in cases when they trigger underwater landslides. Even those tsunamis, however, are small compared with the ones caused by subduction quakes at sea.
At the high end of quake magnitudes considered possible in California was the massive rupture of the San Andreas Fault in northern California which caused the devastating 8.3 quake that laid waste to San Francisco in 1906.
The last "big one" of equivalent size to strike south of the San Gabriel Mountains, near Los Angeles, was some 300 years ago, and the average interval between such quakes in that region is 150 years.
Editing by Dan Whitcomb and Jerry Norton