Daniel Zepeda, a structural engineer who specializes in seismic retrofitting, works in a glimmering skyscraper on Bunker Hill. The steel-framed tower opened in 1985—nine years before the Northridge Earthquake would expose the dangers of modern high-rises.

“Us engineers, we wish we could go back in time and build our buildings with the knowledge we have now,” says Zepeda.

Shaking during the Northridge Earthquake lasted for less than a minute, but it was long enough to demonstrate that critical connections in welded steel moment frames—used to construct many of Downtown LA’s towers, as well as buildings of various heights across the region since the 1960s—were too brittle.

Welded joints fractured in about 100 LA buildings during the 6.7-magnitude quake in 1994, a revelation that took engineers by surprise.

They now know that in a larger earthquake, some of these steel moment frame buildings could collapse entirely.

Building codes changed quickly after the Northridge Earthquake to require stronger connections between the beams and columns. But 25 years later, an untold number of welded steel moment frame buildings constructed prior to 1994 have yet to be inspected, repaired, or retrofitted in the city of Los Angeles.

“We wish we could convince the public that we should look at every building out there and analyze and it make it stronger,” says Zepeda, a principal with Degenkolb Engineers. “But we have to be realistic. It’s probably not feasible.”

The city of Los Angeles has a robust retrofitting program that targets what Mayor Eric Garcetti has described as “known killers”: non-ductile concrete buildings that predate 1977, and “soft-story” wood-frame buildings erected before 1978, namely the city’s “dingbat” apartments where the first floor sits atop a carport supported by narrow columns.

When it comes to retrofitting, LA is much further ahead than some neighboring cities, including Long Beach, Torrance, and Pasadena, which do not have any mandatory seismic retrofit laws, according to the Los Angeles Times.

But the city of Los Angeles does ignore welded steel moment frames that predate Northridge.

In a city as big as Los Angeles, it would be an enormous undertaking to retrofit welded steel moment frames—not just because there are so many of them, but because the retrofits are more intense, more invasive, and more complicated than retrofits to soft-story and non-ductile concrete buildings.

Engineers say they understand why the city is prioritizing the riskiest buildings, the ones that threaten to kill the most people. (Approximately 49,000 apartment units in LA were destroyed or seriously damaged due to the Northridge earthquake; two-thirds of those were in soft-story buildings.)

But the Northridge Earthquake struck at 4:31 a.m., when most people were at home asleep. Experts question what would happen if a major earthquake, with a longer duration of shaking, struck during the day, when Angelenos were at work, in offices inside those gleaming steel skyscrapers.

It’s not just the threat of death and collapse that seismologists and engineers fear—it’s the threat of a major building being red-tagged, preventing Angelenos from going to work in the weeks or months after the quake.

“The consequences of the failure of one of those buildings would be catastrophic,” says Thomas Sabol, an adjunct professor in UCLA’s school of civil engineering, who specializes in seismic design and structural steel.

In 2008, scientists with the U.S. Geological Survey modeled a magnitude 7.8 earthquake on the southern San Andreas Fault, which they described as “a plausible event on the fault most likely to produce a major earthquake.”

Specific buildings are not named, but in that scenario, five steel moment frame high-rises that predate Northridge completely crumble—with 5,000 people inside.

A similar scenario was created in June for an earthquake striking the Bay Area. In the modeling for Northern California, scientists forecast that damage could render steel-frame high-rise office buildings “unusable for as long as 10 months.”

It also includes a list of 39 high-risk high-rises that could buckle in a major earthquake, including San Francisco’s iconic Transamerica Pyramid and the San Francisco Marriott.

The cities of Santa Monica and West Hollywood have recently taken inventory of their steel moment frame buildings. Santa Monica found 80 steel moment frame high-rises and is giving building owners 20 years to complete retrofits.

Los Angeles only has a partial list.

The city did take inventory in the months after Northridge, but it’s limited to areas of West Los Angeles and the San Fernando Valley that experienced the most ground acceleration.

It also mandated inspections to 239 welded steel moment frame buildings in those areas. In buildings where damage was found, the city required owners to make repairs, including the Bullock’s building in Sherman Oaks and dozens of mid-rise buildings along Ventura Boulevard. (In a 2011 presentation, the city’s building and safety department reported that 520 buildings were affected by the mandate and 519 had complied.)

Today, structural engineers are quick to point out the limits of that mandate. It did not include other sections of the city where welded steel moment frame are prolific, including Downtown Los Angeles. And while it required repairs, it did not require retrofits.

Researchers at Caltech, who have simulated how welded steel moment frame buildings would perform in various degrees of seismic ground motion, found that buildings with fracture-prone welds “are substantially more likely to collapse than buildings with sound welds.”

Connections that have not already fractured will not be immune to damage in the future, says Thomas Heaton, director of Caltech’s Earthquake Engineering Research Laboratory.

“If we do have a big earthquake, it could be an enormous problem,” he says. “There’s kind of a confusion that’s out there. People say: ‘If it’s an older structure, and it doesn’t have fractured connections, it’s not broken, and it doesn’t need to be fixed.”

But lab tests and Northridge prove that when “the connections are called on to do their job in a big earthquake—they won’t,” says Heaton. “They’ll break. Almost certainly.”

Not all moment frame buildings are as risky as the ones constructed from the 1960s through the early 1990s. Welding was employed during those three decades to save time and money, a “departure from the bolts and rivets used in previous generations of steel buildings,” according to the New York Times. There are other ways to join the beams and columns together, and while welding is common today, the technique is different.

In a report issued last year, Garcetti noted that the city eventually plans to develop recommendations for more types of structures, including steel buildings that predate Northridge. That could take 10 years to develop, according to Marissa Aho, the city’s chief resilience officer.

“We’re in the process of working with partners to get the next set of seismic work off the ground now,” she says.

Until then, Heaton says many Angelenos might not have any clue about the risks.

“If we don’t have a big earthquake, it’s not really a problem,” he says. “But if we do have a big earthquake, it could be an enormous problem. Frankly, I would not have an office in an older steel frame building.”

Zepada, who works in one of those older steel skyscrapers, is not as anxious.

“It is more vulnerable to damage in a seismic event than a new building,” he says. “[But] just because you have a deficiency in a building does not make it dangerous.”

Without a formal, citywide survey, however, there’s no way of knowing which buildings are dangerous.

“We would need to do an analysis of the building,” says Zepada, “to determine if we are willing to live with the risk.”