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Earthquake faults crisscross LA. Is it safe to tunnel here?

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In earthquakes, subway tunnels are not immune to damage—but they’re probably safer than skyscrapers

The underground portion of the soon-to-open Crenshaw/LAX Line is located in a liquefaction zone, where engineers must contend with damp, sandy soil that can destabilize in an earthquake.
By Liz Kuball

In December, tech billionaire Elon Musk and his tunneling startup, The Boring Company, unveiled a mile-long tunnel beneath Hawthorne—a prototype of an underground transportation system called “Loop.”

Musk has repeatedly suggested that a network of subterranean thoroughfares would solve Los Angeles’s well-known traffic woes. But the day after showing off the test tunnel, he addressed a different LA concern: earthquakes.

“Are tunnels safe in an earthquake?” Musk wrote on Twitter. He attached a brief primer on seismology and tunnel design from the Boring Company website.

According to that blurb, tunnels are “some of the safest places to be during an earthquake.”

Jean-Philippe Avouac, geology professor at Caltech, more or less agrees.

“Structures which are underground are less vulnerable to shaking than structures at the surface,” he says. “That’s just the effect of inertia.”

During an earthquake, explains Avouac, ground shaking causes buildings to rock back and forth, putting stress on them that can cause structural damage or even collapse. A tunnel, on the other hand, moves within the ground rather than atop it.

“When you have a subsurface structure, it’s less vulnerable, because you don’t have that inertia effect,” Avouac says.

But that doesn’t mean tunnels are invulnerable to damage during earthquakes.

“Probably the most common source of difficulty with underground construction, is from liquefaction,” says Jonathan Stewart, UCLA professor of geotechnical engineering.

Liquefaction happens when damp, sandy soil loses its load-bearing strength during an earthquake. It can cause the ground surrounding tunnels to shift, with potentially severe consequences.

In 1995, the deadly Great Hanshin earthquake caused liquefaction that contributed to the collapse of multiple underground structures, including a subway station in Kobe, Japan.

Zones where liquefaction is likely to occur cover large swaths of Los Angeles County, including much of the land around Dodger Stadium—where the Boring Company aims to build a high-speed tunnel. The underground portion of the soon-to-open Crenshaw/LAX Line and the site of a future Metro Purple Line station at the intersection of Wilshire and La Cienega boulevards are also located in liquefaction zones.

Kobe subway collapse
The collapse of a Kobe subway tunnel after the Great Hanshin earthquake created a noticeable dip in the roadway above.
AFP/Getty Images

Construction of the station is part of Metro’s Westside Purple Line extension, a project that residents and elected officials have closely scrutinized.

An environmental report from Metro indicates that the agency plans to address potential liquefaction at key points along the Purple Line route through a process called “ground improvement” in which grout or other material is literally mixed into the ground soil in order to strengthen earth in these areas and make it less prone to displacement during a major earthquake.

As simplistic a solution as this may seem, Stewart says it’s likely to be effective as long as structural engineers and geologists have properly assessed the risk.

Metro reports indicate that ground improvement in liquefaction zones along the Purple and Crenshaw/LAX lines will be coordinated on a case-by-case basis, following state guidelines.

Subway tunnels in LA are also lined with 12-inch thick concrete walls capable of bearing additional stress caused by ground movement during an earthquake. In fault zones and areas where additional hazards may be present, tunnels are lined with airtight material to guard against water and gas leaks.

Liquefaction zones
Areas where liquefaction is likely to occur (shaded green) cover much of Los Angeles.
California Geological Survey

The presence of methane beneath Los Angeles is another major concern for tunnel builders in the area. It’s a key reason why Metro’s Red Line travels through East Hollywood.

Originally, transportation officials proposed a Red Line route that would have extended along Wilshire Boulevard to Fairfax Avenue, cutting north at that point before doubling back east along Hollywood Boulevard to the Cahuenga Pass.

That was before a 1985 methane leak triggered a fiery explosion at a Ross store in Fairfax. Worried officials insisted on a different route for the project that would avoid the gassy soils around the La Brea Tar Pits.

According to a 2011 safety report, contractors working on other Metro projects have since gained experience tunneling in areas where methane gas is present, and developed techniques for safely ventilating gas released during drilling.

Metro’s Purple Line extension will bring subway service to areas where the agency once shied away from digging—train service to Fairfax is expected to be up and running in the next four years.

Research suggests that earthquakes can cause methane leaks, and that’s a possibility that Metro has also evaluated, according to environmental reports prepared for the Purple Line project. In the event that methane enters a subway tunnel, high-powered fans at nearby stations will begin working in tandem to clear the air.

This map from 2011 shows where the Purple Line extension would pass the Santa Monica Fault and West Beverly Hills Lineament (part of the Newport-Inglewood Fault). The map shows various Westside station locations, which had not yet been finalized.
Courtesy of Metro

A trickier problem for tunnel builders to deal with in Los Angeles is the presence of major seismic faults, many of which run directly through densely populated neighborhoods and business districts.

Some of those faults are capable of producing a surface rupture, in which plate movement below ground is significant enough, and close enough to the Earth’s surface, to cause land to visibly shift at ground level. These ruptures can move roads and tear apart fences.

More than a dozen active faults run below the surface of Los Angeles, and it’s possible that others may not have been identified yet. (The 1994 Northridge earthquake occurred along a fault that scientists had yet to discover.)

“The odds that there’s an undiscovered fault somewhere in the greater Southern California area that could produce a magnitude 6.5 earthquake are pretty good,” says Stewart. But he says that such a fault would probably be buried deep below ground and unlikely to cause surface rupture (like the Northridge Fault, which did not rupture the surface in 1994).

That gives tunnel engineers more ability to predict what kind of stress to expect in certain areas in the event of an earthquake.

Both Metro’s existing Red Line route and the future Purple Line extension pass through faults capable of producing surface-level ruptures. On the Purple Line, Metro has accounted for that by planning wide tunnels with flexible steel linings to compensate for ground movement.

Still, Stewart says damage to a tunnel intersecting with such a fault would be difficult to avoid in the event of a surface rupture.

“At a fault crossing, it’s not really possible to design [a tunnel] in such a way that, once that earthquake actually happens, the day after the earthquake, everything’s going to be fine, and the trains are going to be running,” he says.

Because a surface rupture moves the ground itself, tunnels passing through the fault would be permanently offset.

“That bend in the track would always be there in the future,” says Stewart. “You would just try to reduce the curvature as much as possible.”

In 1994, the Northridge Earthquake gave Los Angeles’s nascent subway system its first major test. Metro’s Red Line, which had begun service through Downtown LA just a year earlier, sustained no damage during the quake and service was not affected.

The Boring Company explainer tweeted out by Musk uses the subway’s resilience in 1994 to illustrate the safety of tunnels during major earthquakes.

Asked how the company would address specific seismic threats, such as liquefaction, in building its Loop system, a Boring Company representative linked Curbed to a Google search for “tunnels in earthquakes” and said that “designing for seismic loads and installing emergency ventilation systems is a standard part of structural tunnel engineering and is accounted for in Boring Company designs.”

Avouac says no structure is impervious to damage in a major earthquake. But, he says, tunnels are more likely to withstand shaking that would threaten surface structures.

“I’d be less nervous about being in the subway than I would about being in a high-rise in Downtown LA,” he says.