Researchers believe there is a 62 percent chance another earthquake rivaling the "great one" in 1906 will hit the San Francisco Bay Area within the next 30 years.

Judging from the timing of past earthquakes in the region, two of the main fault lines -- the Hayward and San Andreas faults -- are due for a major quake of a magnitude 6.7 or higher, according to a working group of U.S. Geological Survey scientists, universities and private institutions.

"We're sitting on a tectonic time bomb," said USGS research geologist David Schwartz. Efforts are under way to prepare the bay area, such as rebuilding and strengthening bridges, water systems and buildings, but the question is whether the area has enough time, he explained.

The San Francisco Bay Area is vulnerable because it is built partly on rock and partly on sand dunes. If a major earthquake hits, the sandy areas would act as a liquid, causing unreinforced buildings to topple. Many of the old brick and concrete structures, built pre-1970s construction codes, would not survive.

Particularly vulnerable are apartment buildings -- built pre-1970s when stricter buildings codes were put in place -- situated atop parking garages and retail stores, which are considered "soft" first stories because of their spindly support columns and basically empty space.

Underground water pipelines that cross fault lines also could bend or break, cutting off water supplies and making it difficult to fight fires. Splintered and leaking gas pipelines could ignite flash fires.

Localities have developed emergency plans to deal with the aftermath of a destructive quake, but some problems remain. For instance, San Francisco's pricey real estate prevents many fire and other emergency personnel from living in the city, as became apparent during the 1989 earthquake.

About one-third of the San Francisco Fire Department's 1,800 paid firefighters live in San Francisco, although a "high majority" live within the nine bay area counties, said the department's Chief Joanne Hayes-White.

"It's something I have some concerns about," Hayes-White said, adding that she plans to address the issue in future contract talks.

Still, in an exercise conducted in October 2005 aimed at getting emergency workers into the city, the bulk of personnel were able to respond, she said.

Firefighters and police officers also have begun training together in non-emergency settings so they will be better able to join forces in the event of an actual emergency, she added.

Another issue the San Francisco Fire Department had to address was its unique fire hydrants. If firefighters from other municipalities ever had to assist with post-earthquake fires, their hoses wouldn't be able to attach to the hydrants without special adaptors.

Many of San Francisco's fire stations now have adaptors to distribute if needed, and caches of adaptors are located at four entry points into the city, said Hayes-White.

The San Francisco Fire Department has never had to request assistance since it came into existence in 1867, she said, although "it would be burying my head in the sand to say we'd never need mutual aid."

Water supplies and bay bridge reconstruction

San Francisco's Hetch Hetchy water system, which supplies 2.4 million residents with drinking water, crosses three fault lines: the San Andreas, Hayward and Calaveras. The city has undertaken a $4.3 billion, 10-year effort to upgrade the aging system to make it more resistant to earthquakes, said Anthony Irons, deputy general manager of the San Francisco Public Utilities Commission.

The plan entails repairing, replacing and strengthening the system's pipelines, reservoirs, tunnels and dams, and creating redundancy in case the pipelines burst.

Currently, if a major pipeline breaks, it might take 60 days to repair. The goal of the redesign is to get at least some water to residents in 24 hours and be able to restore water to full supply in 30 days, said Irons.

The need to maintain a reliable water source for residents after disaster strikes was never as clearly illustrated as after Hurricane Katrina. When Katrina hit the Gulf Coast in August 2005, it stranded thousands of New Orleans residents without drinking water.

"Water is a principle issue following disasters, as Katrina showed," Irons said.

Another major reconstruction effort involves the double-decker San Francisco-Oakland Bay Bridge, part of which collapsed during the 1989 Loma Prieta earthquake.

The city and state decided to replace the 2.2 mile-long eastern span with separate support systems for the upper and lower decks at a cost of about $6.3 billion, according to Bart Ney, California Department of Transportation spokesman on the bay bridge retrofit project. Construction is expected to be completed in 2012.

The western approach has already undergone a less expensive upgrade, which included additional bolts and rivets, support foundations and steel supporters.

Vulnerability assessment

Although the 1989 Loma Prieta earthquake served as a reminder of the bay area's vulnerabilities, it is not considered by most residents to be "the big one" because it was located 60 miles south of San Francisco in the Santa Cruz Mountains. Residents were not hugely affected, and future quakes are expected to hit stronger and closer to populated areas, according to the USGS.

Meanwhile, San Francisco is still assessing the damage another major earthquake could inflict.

In 2001, the city contracted a group to assess neighborhoods' vulnerability, establish criteria to fix and retrofit earthquake-damaged buildings and create an incentives program for the most vulnerable parts of the city.

The Community Action Plan for Seismic Safety was still in draft form in April 2003 when the City Building Commission canceled it. Due to media and public pressure, the city released the draft report a year later.

The Applied Technology Council is reviewing the report and will decide whether to expand it, said ATC Executive Director Christopher Rojahn.

For example, the city may develop guidelines on evaluating and repairing earthquake-damaged buildings, Rojahn said. "After a disaster occurs, it's the wrong time to develop the rebuilding policy," he explained.

Another hazard mitigation measure the city may develop is an incentives program for retrofitting buildings. And portions of the report itself may be rewritten to help the general public better understand how conclusions were reached, he added.

-- By Larisa Epatko, Online NewsHour