SAN FRANCISCO (Reuters) – California will experience unthinkable damage when the next powerful quake strikes, probably within 30 years, even though the state prides itself on being on the leading edge of earthquake science.

Modern skyscrapers built to the state’s now-rigorous building codes might ride out the big jolt that experts say is all but inevitable, but the surviving buildings will tower over a carpet of rubble from older structures that have collapsed.

Hot desert winds could fan fires that quakes inevitably cause, overwhelming fire departments, even as ancient water pipelines burst, engineers and architects say.

Part of the lesson from the disaster that hit Japan on Friday is that no amount of preparation can fully protect a region such as California that sits on top of fault lines.

Even so, critics fear the state may have long skimped on retrofitting older buildings. Yet the cost of cleaning up after a big quake is likely to be much higher than the cost of even the most expensive prevention, they warn.

“Everybody is playing a gamble that something like this won’t happen,” said Dana Buntrock, associate professor of architecture, at the University of California, Berkeley.

Buntrock, like many others, sees California’s past as a present danger. The university is spending more than $300 million to retrofit and renovate its ancient sports stadium, tucked into the hills overlooking the bay.

“There are places where two walls that were aligned in the 1920s have moved a half meter apart,” she said.

But the steps that the school is taking are not as common in California as the overwhelming risks might suggest.

The concrete high-rises that rose in the years after the 1906 San Francisco earthquake were made without adequate reinforcing steel, while homes and apartment complexes that are built atop of ground-floor parking lots are among the most vulnerable structures in the state.

Japan’s 8.9 earthquake and the tsunami it unleashed destroyed entire villages and left 10,000 or more dead. That sends shivers up the spines of the engineers and architects who follow California’s strategy to withstand a big quake that experts say will surely hit the state one day.

“The question is not if but when Southern California will be hit by a major earthquake — one so damaging that it will permanently change lives and livelihoods in the region,” according to a 2008 study by the United States Geological Survey study.

It predicted 2,000 deaths and $200 billion in damage from a 7.8 southern California quake on the San Andreas Fault.

Geologists say a big earthquake in California would probably top out at a magnitude 8 as the state’s fault structures are different from Japan’s.

A quake of the 7.8 magnitude in the USGS study would have about 30 times less power than the one that struck Japan.

Forecasters in 2008 saw a 99 percent chance of a 6.7 magnitude quake within three decades, and 46 percent chance of a 7.5 or greater, with Southern California the likely center.

A monster California quake of magnitude 8 had only about a 4 percent probability — except in far Northern California and the Pacific Northwest. That area has a 10 percent chance of experiencing a magnitude 8 to 9 quake — Japan-sized — in the next 30 years.

A repeat of San Francisco’s 7.9 magnitude quake in 1906 could take up to about 900 lives, injure thousands and destroy 3,000 residential buildings, a recent report for the city found.

Even a smaller 7.2 quake would cause $30 billion in building damage, $10 billion more in additional costs — and if fires sweep the city, damage could rise by $4 billion, the report sponsored by the San Francisco Department of Building Inspection concluded. About 27,000 of the city’s 160,000 buildings would become unsafe to occupy.

One of the authors of the report, geotechnical engineer Thomas Tobin, reflected that the hot winds of Santa Ana winds blowing from the desert into Los Angeles could intensify a disaster created by a southern California quake.

“If it happens to be a large earthquake on a hot, dry day with the wind blowing, the losses could be huge,” he said.

COLLAPSING BOX

Tobin lists several types of “killer buildings” that would sustain the most damage in a California temblor, including older high rises and complexes featuring ground-floor parking:

* Most “tall, beautiful older buildings” built before 1980 that dot the San Francisco skyline were made without reinforcing steel, Tobin said.

* “Soft story” buildings with a ground-floor open garage or retail space also lack adequate bracing. The sturdier box of the upper floors likely would come crashing down on the “soft story.”

* “Tilt up” buildings of concrete slab that are pushed upright to create a big box, such as for a grocery store, are among the most vulnerable. Some localities have mandated relatively low-cost reinforcement. Tobin says San Francisco has not.

* Unreinforced brick buildings would collapse easily.

Some of the fixes for such structures are relatively inexpensive, such as tying together tilt-up buildings, he said. So-called “soft story” apartment buildings with five units would cost $10,000 to $20,000 per apartment to fix.

Local governments can offer powerful financial incentives to encourage landlords to make changes, he said, but most don’t.

That said, shoring up high rises is pricey, Tobin allows, and retrofitting has been inconsistent.

While California nuclear plants are built to withstand earthquakes and shut down when the earth shakes, the Union of Concerned Scientists, which supports nuclear energy to counter global warming, wants tougher safety measures.

PAST PERFORMANCE

Japan’s experience suggests that even the best preparations are no match for the power of nature.

Matthew Hornbach, a geophysicist research associate at the University of Texas Institute for Geophysics, said he was shocked by the scale of disaster across the Pacific.

“You don’t get better prepared than Japan and to see what’s going on there now is, I think, a real wake-up call, really, to the U.S.,” he said.

California and Japan tend to track each other in requirements for new buildings, but Japan tends to tear down and rebuild frequently, leaving them with a much smaller stock of older buildings, said UC Berkeley’s Buntrock.

California’s history of repairing quake damage varies.

Former California Governor Pete Wilson said that he waved environmental and other regulations after the 1994 Northridge earthquake – magnitude 6.7. As a result, he managed to get the Los Angeles U.S. 10 highway, the worlds’ busiest road, up and running in two months, versus some estimates of two years.

But the Bay Bridge, which partially collapsed in the 7.1 Loma Prieta quake that shook the San Francisco Bay Area five years earlier is still being replaced, said Randy Rentschler, director of legislation and public affairs at the San Francisco-area Metropolitan Transportation Commission.

“One lesson that we can give them (Japan) is how not to do it,” Rentschler said. “It took us how many years to do the Bay Bridge? – and it’s still under construction,” he added.

Aside from the obvious issue of cost, Rentschler said that debate over what to do slowed improvements.

“Local citizens’ groups were raising hell about all kinds of things, and they were permitted to get away with it,” he said. The replacement for the damaged span of the two-part bridge is set to open in 2013 and engineers say it is designed to withstand an event that occurs once every 1,500 years.

A final lesson from Japan is that the cities are not necessarily the most vulnerable areas. Rural areas of Japan closest to Friday’s quake were destroyed by tsunamis while Tokyo fared much better.

Frank Vernon, a geophysics professor and seismology specialist at the Scripps Institution of Oceanography at UC San Diego, described a similar fault far from San Francisco, hundreds of miles up the West Coast.

“The most important lesson in the U.S. and North America is the reminder that we have a similar subduction zone called Cascadia up on the coast of British Columbia, Washington, Oregon and very northern California which could do the same thing,” he said.

“Some day we will be having this same type of earthquake near our shores,” he said.

(For more environmental news see our Environment blog at http://blogs.reuters.com/environment)

(Additional reporting by Kevin Gray, Pascal Fletcher and Tom Brown in Miami and Dan Levine, Jim Christie and Braden Reddall in San Francisco, Steve Gorman in Los Angeles; Editing by Frank McGurty)