SAN FRANCISCO -- Imagine driving on the Golden Gate Bridge when your car starts shaking and the roadway ahead makes a groaning noise and collapses. That's probably what would have happened several years ago if an earthquake the magnitude of the 1906 San Francisco earthquake had struck the Bay Area. But that's before the bridge district began a $660 million project to seismically retrofit the bridge. When the Golden Gate was built in 1937, engineers weren't thinking much about quakes. There was "more concern about wind," said Ewa Bauer, chief engineer of the bridge district. "Earthquakes were such an elusive thing. They came and (went) from time to time," she told reporters Wednesday in advance of next month's 75th anniversary commemoration of the bridge. Spurred by the scare of the 1989 Loma Prieta earthquake, however, the district drew plans three years later to redesign the bridge to withstand a magnitude 8.3 earthquake. By comparison, seismologists estimate that the devastating 1906 quake was a 7.9. For the last decade, engineers have retrofitted and reinforced the bridge. Starting with the vulnerable north and south viaducts, engineers have moved in different phases ranging from replacing entire towers, to cladding support pylons with steel and replacing road sections piecemeal -- all without closing the bridge or altering its appearance. They plan to install wind deflectors. "We have to modify it to a great extent, but to the public we have to leave the bridge looking like nothing happened," Bauer said, She compared the work to brain surgery. "You go inside a very complex brain, you have to repair it, but you can't change it," she said. When contractors replaced the towers supporting the north viaduct one by one, they used a reusable tower that acted as a high-tech carjack. It lifted the bridge 1½ inches, giving enough clearance for contractors to demolish the underlying tower. "When I presented this work to the public, especially to the locals who use the bridge, they said, 'You did what?' '' When they told her they hadn't noticed, Bauer said, she took it as "the biggest compliment." The U.S. Geological Survey forecasts that in the next three decades, there's a 62 percent chance that the Bay Area will be hit with a magnitude 6.5 earthquake. But not all of those earthquakes would damage the bridge. In fact, the bridge was unscathed in the 6.9 Loma Prieta quake, centered in Santa Cruz County. Earthquakes that occur in the northern part of the Bay Area tend to be bigger, said Peggy Hellweg, a seismologist at UC Berkeley. But they're less frequent. "The big ones seem to happen every couple of hundred years," she said. But scientists admit it's a bit of a guessing game -- particularly since there weren't accurate records of the Bay Area until the 20th century. "I think we should retrofit what we can," Hellweg said. "The big earthquake could happen tomorrow or even today." The bridge district is now working on the third phase of retrofitting, in which crews will reinforce the north anchorage housing and the pylon. The next, final phase of construction, expected to be bid on next year and completed in 2017 or 2018, will reinforce the main towers of the suspension bridge, and the south pier and fender. It still requires congressional approval of $200 million in funds. The cost of the retrofit will be paid from tolls and regional, state and federal funding. But the $660 million is a lot cheaper than the estimated cost of a new bridge -- about $3 billion. Besides, Bauer agrees with most Bay Area residents that the bridge really can't be replaced. "We can build a new one right next to it,'' she said, "but it won't be the Golden Gate Bridge."