A quiet California fault could rupture and cut water to millions

The San Andreas Fault is famous mostly for the ground it can shake, but one of its most disruptive threats has little to do with collapsing buildings and everything to do with the taps in millions of homes. The fault slices across the aqueducts that carry water from Northern California and the Colorado River into the thirsty cities of the south, and a major rupture on its southern reach could sever those lifelines all at once, leaving a huge population short of water for months.

Southern California is a semi-arid region that survives by importing most of its water from hundreds of miles away. That dependence turns the quiet, slow-creeping fault into more than an earthquake hazard; it makes it a chokepoint for the region’s most basic resource, one that planners have modeled in unsettling detail.

A plate boundary running through California

The San Andreas Fault marks the boundary between the Pacific and North American plates, which grind past each other horizontally along a line stretching more than 1,200 kilometers through the state. Its southern segment, running near the Los Angeles region, has not produced a great earthquake since 1857 and has accumulated centuries of strain, which is why seismologists regard a large rupture there as overdue rather than unlikely.

Unlike the offshore megathrusts that drop coastlines, the San Andreas moves side to side, offsetting roads, pipelines and streambeds. It is precisely that lateral motion, tearing across anything laid over the fault, that puts the region’s imported-water infrastructure at risk.

The southern segment’s long quiet is itself a warning sign. Sections of the fault to the north relieve stress in smaller, more frequent earthquakes, but the stretch nearest Los Angeles has stayed locked for well over a century and a half, accumulating slip it has not yet released. Geologists studying the fault’s prehistoric record have found evidence of repeated large ruptures over the past two thousand years, spaced closely enough that the gap since 1857 is now comparable to the longer intervals between past events.

Where the aqueducts cross the fault

Los Angeles draws the bulk of its supply through a handful of major aqueducts, and every one of them has to cross the San Andreas somewhere to reach the coastal plain. According to reporting on the region’s vulnerability, the aqueducts serving Los Angeles cross the fault zone in dozens of places, meaning a single rupture could break several of them at once rather than just one. The California Aqueduct, the Los Angeles Aqueduct and the Colorado River Aqueduct all thread through the same seismic gauntlet.

When a fault offsets a pipe or canal by several meters in seconds, no ordinary joint can absorb the movement. The crossings are therefore among the most closely studied weak points in the entire water system, and repairing them after a quake is expected to be slow and difficult work in terrain that the same earthquake will have torn up. Access roads may be blocked by landslides, the pipes themselves may be buried under collapsed hillsides, and crews may have to fabricate custom sections to bridge ground that has physically shifted, all of which stretches repair timelines from days into months.

What a magnitude 7.8 rupture would do to water

To plan for the worst, the U.S. Geological Survey and partners built a detailed scenario around a magnitude 7.8 rupture on the southern San Andreas, an event comparable to 1857, known as the ShakeOut scenario. It projects that such a quake could cut the aqueducts feeding Southern California and interrupt the majority of the water the region relies on. Some links might be restored within months, but others could stay out of service for a year or more.

A prolonged loss on that scale would ripple through everything from firefighting to sanitation to industry. Water utilities have used the scenario to justify stockpiling repair materials, hardening critical crossings and planning for emergency supplies, precisely because the modeling shows how quickly a water shortage would follow the shaking.

The same scenario spells out how the loss would cascade beyond the water mains. Highways, fuel pipelines and electrical transmission lines cross the southern San Andreas in many of the same places the aqueducts do, so a rupture that breaks the water supply is likely to sever power and fuel deliveries at once. Restoring water often depends on electricity to run pumps, which means the different failures are tangled together, and planners treat them as a single interlocking emergency rather than separate problems to solve in sequence.

Reducing the risk before the shaking starts

Because the fault cannot be prevented from moving, resilience efforts concentrate on the infrastructure. Regional wholesalers such as the Metropolitan Water District have studied ways to make the system more earthquake-resilient, including flexible fault-crossing designs, backup storage closer to the cities, interconnections that let one system feed another, and local reserves that can carry communities through weeks of disruption.

The strategy accepts that a great southern San Andreas earthquake is a question of when, not if, and aims to shorten the time between the rupture and the return of running water. Some utilities encourage households and businesses to keep several days of drinking water on hand precisely because the regional network could take far longer than that to recover. For a region built on imported supply, the quiet fault’s real menace is not only what it can knock down, but how long it could leave millions of people dry.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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