Los Angeles gets 88% of its water from three major aqueducts, flowing from the Colorado River, Owens Valley and the Sacramento-San Joaquin River Delta.
But as they make their way into the region, the aqueducts cross the San Andreas fault a total of 32 times.
Officials have long warned that a massive temblor on the San Andreas could destroy key sections of the aqueducts, cutting off the water supply for more than 22 million people in Southern California.
L.A. officials are for the first time taking concrete steps to address the problem. Making L.A.'s water supply less vulnerable in a huge quake will probably cost billions, and it remains unclear where that money would come from.
Mayor Eric Garcetti has asked for proposals aimed at better protecting the water supply and developing alternatives in case a quake blocks the aqueducts. The ideas range from strengthening the waterways to developing an emergency supply for firefighters using ocean water and reclaimed water.
Los Angeles is behind the San Francisco Bay Area in this effort.
The East Bay Municipal Utility District has built backup tunnels, stronger pipes and new waterways to ensure water continues flowing from the Sierra Nevada even if one of its three main aqueducts is blocked. The efforts have cost more than $350 million, paid for by water customers, bonds and government grants.
The San Francisco Public Utilities Commission built a new water tunnel under San Francisco Bay and even installed a specially designed pipe that crosses the Hayward fault. The pipe is connected by accordion-like joints that allow it to flex and swing, even as a quake moves the earth in two directions. The projects are part of a $4.8-billion effort funded by a surcharge on water bills.
Compared with other large cities, Los Angeles is critically dependent on water sources far from the city center, said U.S. Geological Survey seismologist Lucy Jones, the mayor's science advisor on earthquake safety.
"We're the first city that's really bet its life on outside water," Jones said. "We have to cross the faults. There's no way to not go over the fault."
Garcetti acknowledged that the high cost of the water projects, along with his proposals to retrofit thousands of vulnerable buildings and preserve access to the Internet after an earthquake, would have a total price tag in the billions. But failing to act now will exact an even greater toll economically after the Big One hits.
Water is "one of L.A.'s greatest earthquake vulnerabilities," Garcetti said. "If it were to take six months to get our water system back … residents and businesses would be forced to relocate for so long that they might never come back."
A first step would be for the three agencies that manage the Los Angeles, California and Colorado River aqueducts to work together to come up with an earthquake retrofit plan.
Another would be to add fire hydrants to a recycled water pipe system already being planned for certain parts of the city to irrigate golf courses and parks. The "purple pipe" system would be connected to treatment plants that sanitize wastewater. Equipped with quake-proof pipes, the system could act as a backup water system like the one San Francisco built after the 1906 earthquake.
After the 1994 Northridge quake, hydrants ran dry in large swaths of the San Fernando Valley, forcing firefighters to switch to water-dropping helicopters using swimming pool water to fight blazes.
None of the aqueducts have gone through a large San Andreas earthquake in Southern California. The last time the fault unleashed a Big One in the Southland was in 1857, when a magnitude 7.9 earthquake hit.
The Los Angeles Aqueduct, built a century ago by William Mulholland, crosses the San Andreas fault through the five-mile Elizabeth Tunnel under the mountains north of Santa Clarita. Experts said the San Andreas fault can move as much as 33 feet in a big earthquake and could slice the tunnel, dam it up and collapse some of its concrete sections.
The most expensive solutions include building a wider, stronger tunnel; another is to use electricity to pump water over the mountains toward L.A. But the Department of Water and Power hasn't had the resources to extensively study various retrofit options until now, said Craig Davis, an earthquake engineering expert for the utility.
The DWP has already begun sketching out an interim solution — placing a 3-foot-wide, strong plastic pipe through the tunnel. Even if the tunnel collapses, it's hoped the plastic pipe might keep enough of the passageway intact to keep some water flowing through, Davis said.
Davis warns that the Edmund G. Brown California Aqueduct could be pulled apart by the San Andreas fault in the Palmdale area, allowing large volumes of water to escape.
The 1940s-era Colorado River Aqueduct has a different problem. A section of the aqueduct in the mountain pass west of Palm Springs could be lifted 13 feet in a San Andreas quake, stopping the water flow. There's no backup pumps there to keep the water moving toward Los Angeles, Davis said.
Representatives for the California and Colorado River aqueducts said they have been studying the seismic vulnerabilities of their systems.
"There should be a serious dialogue among the agencies that are responsible for the three sources of water to Southern California," said Thomas O'Rourke, a Cornell University engineering professor and longtime seismic consultant for the DWP. "Sometimes it's very difficult to go beyond those institutional barriers.... Somebody just has to take it up."
Although authorities have been studying the seismic risks for many years, it was only in about 2008 that water officials began to understand that all three aqueducts could be choked off in the same magnitude-7.8 San Andreas earthquake, based on the research Jones and a large research team published that year.
O'Rourke said the consequences of failure would far exceed the bill to prevent calamity.
"The price tag for protecting the incoming water supply might not be as high as people think," he said, pointing to the projects already completed in the Bay Area.
Garcetti last week instructed city officials to give him a plan by July on many of these issues. He expects the reports to come with proposals for strengthening the Los Angeles Aqueduct, a bond measure to fund water-related seismic safety projects and prioritizing which of L.A.'s century-old pipes should be replaced first with quake-proof pipes.
The mayor also asked for the managers of all three major aqueducts — the DWP, state Department of Water Resources and Metropolitan Water District of Southern California — to begin working together to find a solution.
If nothing is done, Southern California could be left with less than six months of stored water on the L.A. side of the San Andreas fault. It could take more than a year to get all three aqueducts flowing again after a major quake.
"This is a regional issue, with significant infrastructure costs," Garcetti said. "We all know how precious water is these days with our historic drought.... Water is also one of L.A.'s greatest earthquake vulnerabilities, too."