San Bruno B.A.R.T.

  The earth shook violently and San Francisco caught fire 98 years ago Sunday, when, fortunately, there was no BART system or Transbay Tube.

  Experts now estimate there is a 62 percent chance of a catastrophic repeat of the "big one" in the next 30 years, and they predict the Transbay Tube won't survive. The tube carries two-thirds of all BART passengers and, during rush hour, the equivalent of one deck of the Bay Bridge.

  Key to the Transbay Tube's survivability during an earthquake rests on a pair of seismic joints that allow the tube to flex. Members of the media got a rare glimpse of one of these joints during an early morning tour on Friday sponsored by the Northern California Chapter of the Earthquake Engineering Research Institute.

  The tube is surrounded by a bed of loose gravel as it lies in a trench at the bottom of the San Francisco Bay. It's so loose that if the big one strikes the entire thing could pop out like an empty plastic soda bottle at the bottom of a swimming pool.

  Liquefaction will cause the loose gravel to shake like Jell-O, and the soil would become like quicksand, too loose to hold down the Tube.

  Even if the damage is less catastrophic, BART seismic experts do not know how much water could flood the tube, but one prediction is chilling.

  Jim Slossen, California's state geologist in the 1970s, once predicted that the water pressure would hurl a train out the end of the tunnel with the same force as a Boeing 747 on take-off. Slossen also predicted the Cypress Structure would pancake, which it did on Oct. 17, 1989, when the Loma Prieta earthquake ripped the region with a 7.1-magnitude shockwave.

  BART survived that quake largely unscathed because the epicenter was about 80 miles to the south of the tube. But BART was designed in the late 1950s engineered in the early 1960s and built in the late'60s, before the devastating 1971 San Fernando quake. Trains were already passing under the Bay when scientists divined new knowledge about ground motion from that fatal earthquake.

  The good news is BART has a plan to strengthen the Transbay Tube and enough money to get the work started. Last month Bay Area voters approved a $1 increase in bridge tolls, which will steer $143 million toward a seismic retrofit of the tube.

  The toll money continues a roughly $30 million effort to design and engineer those repairs -- an effort which is about 15 percent complete, said BART Chief Engineer James Dunn during Friday's tour of the tunnel.

  One of the seismic joints was the centerpiece of Friday's tour.

  Composed of 120 strands of woven steel wire cable and two thick teflon gaskets, the joints circle each end of the 3.6-mile tunnel and allow the entire underwater structure to flex, much like a segmented, wooden toy snake, but also rotate in any direction, like a shoulder joint. The joint can move about six inches in any direction, according to Tom Horton, BART's earthquake project manager.

  But unlike Caltrans' multibillion-dollar efforts to strengthen its Bay Area bridges, BART's seismic strategy will rely not on bolstering the structure, but the soil around it. A device will shake the gravel in a process called vibro-compaction, which settles the soil and silt and makes the ground around the tube denser.

  On the Posey and Webster tubes in Alameda, Caltrans stabilized the soil by injecting concrete grout into it.

  All told, Dunn thinks the Transbay Tube can be strengthened for $300 million. The toll-bridge money will pay for extensive soil samples and experiments to see how well vibro-compaction works under the Bay.

  BART's total seismic retrofit bill is currently estimated at $1.6 billion. BART Director Lynette Sweet hinted that in November, the transit district may ask voters for a second time to approve a seismic bond. They narrowly rejected the idea in 2002.

  "BART is the backbone of the Bay Area's transportation system. We want it to stay that way, even after a quake," Sweet said. "Unfortunately not everybody in the Bay Area is prepared to survive a massive quake and that includes BART."

  A majority of the nine-member elected board has embraced, but not voted for, a plan by Director Pete Snyder to increase fares 5 cents every year to pay for seismic and other big-ticket needs.

  Fifteen years after BART withstood the faraway Loma Prieta shockwave, the seismic doomsday clock keeps ticking. U.S. Geological Survey researcher David Schwartz presented new findings based on the historical record and trenching studies that traced ground motion back to 1600.

  "The effect of 1906 was to turn off all the other faults. It's called a 'stress shadow,' and we are emerging from it," he said, pointing to a chart that shows relative calm in the last century, but five major quakes on five district faults in a 40-year period in the mid-1700s.

  "We really do have large earthquakes in our near future," Schwartz said.

Contact Sean Holstege at sholstege@angnewspapers.com