A fireman near Los Angeles searches for water in the aftermath of 1994's Northridge quake. Associated Press

The worst-case scenario means endless fires, impotent rescue services, and no water for months.

Scientists don't know when California's next great earthquake will strike, but they do know just about everything else about it, and the potential ramifications are terrifying: impotent rescue services, widespread fires, and no fresh water for months.

And yet California officials haven't done much to prepare for such an immense disaster, said Lucy Jones, a seismologist at the U.S. Geological Service, at The Atlantic's CityLab 2014 summit in Los Angeles. “There's so much that science knows about what's going to happen, and we're seeing that not being used," she said.

Jones is working with the L.A. government to create a seismic-resilience plan, and she had a dire warning for other California cities that don't brace for the worst from the San Andreas fault. Behold, a couple of the most terrifying run-on sentences in existence:

"When the San Andreas earthquake happens in Southern California—and that's the most-likely big earthquake in the United States—we know that all of the transportation life lines, the electric systems, the water systems, the gas lines, that cross the San Andreas fault, exactly where they'll break and what will happen when they break. That hasn't gotten anybody to do anything about them. So here in Los Angeles, we get 85 percent of our water from outside the region—that means across the San Andreas fault—in aqueducts that will break, [and] we could tell you how many times they're going to break and that it's going to take 18 months to get them fixed again and we have six-months' supply of water on this side of the fault—when we're not in a drought and the reservoirs are full."

Perhaps Californians aren't taking the San Andreas fault as seriously as they should because of the awful, but not civilization-stalling damage from the last mega temblor, the 1994 Northridge earthquake. But that disaster was piddling compared to what the "Big One" could bode, said Jones.

"When we have the San Andreas earthquake, that earth fault will probably be about 20 to 30 times larger than the fault that produced the Northridge earthquake," she said. (That quake killed 57 people and caused roughly $20 billion in damages.) But San Andreas is a much longer fault, and that means it can release a ton more energy. “And then instead of having a half-million people receiving a strong shaking, we're going to have 10 million people receiving the same level of strong shaking."

An event of this magnitude will likely trigger a catastrophic situation for emergency responders. "When you have that many fires starting—because the fires are proportional to the number of houses receiving strong shaking—we run out of fire engines," Jones said. "We know we will not have enough fire engines to handle the fires. And mutual aid is going to have to cross that broken San Andreas to get to us.

"When you get to the very biggest earthquakes you've seen, these fires get completely out of control," she continued. "They were a problem but manageable in Northridge. In the Great (Kanto) Earthquake that hit Tokyo in 1923, 150,000 people died because basically the city burned down.” (Note that in this modern age, that toll would be much lower.)

But Californians can console themselves with one comforting thought: At least their past experiences with quakes have led to a much better-prepared infrastructure than, say, certain older cities on the East Coast. Here's Jones with the last word:

"I'll tell ya, I'd rather be in a big earthquake here than in Boston. It's not just that [the buildings] are old; they're brick. They're unretrofitted; the roofs are being held up by brick walls; and when you shake them in an earthquake the mortar dissolves and they're gone. We know those buildings are going to kill people. Now the last significant quake they had was in the 17th century... But they had a significant earthquake in the 17th century. They will again."

The Atlantic, the Aspen Institute, and Bloomberg Philanthropies are hosting"CityLab 2014: Urban Solutions to Global Challenges," in Los Angeles on September 29 & 30. Find CityLab.com's full coverage here.

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