Will SF's Golden Gate Bridge survive the next big earthquake? USGS details new threat

SAN FRANCISCO (KGO) -- California is long overdue for a major earthquake. In fact, new data from the United States Geological Survey suggests the next one to hit the Bay Area will likely be larger and more impactful than previously thought. As far as when, geologists warn the threat could be imminent.

The ABC7 News I-Team is digging into the preparation underway to see which areas are most at risk - starting with the Golden Gate Bridge.

The retrofit

"There is only one Golden Gate Bridge and we're going to protect it," said U.S. Transportation Secretary Pete Buttigieg in Jan.

The question is - how will the bridge be protected?

"It's about retrofit, but we also have new technology," said Speaker Emerita Nancy Pelosi, who fought for years to strengthen the Golden Gate Bridge.

Most recently, Pelosi and her congressional colleagues secured $400 million to protect the bridge from seismic threats. It's a part of the historic infrastructure law passed in 2021.

"That was the real battle and we were successful in getting that done," Pelosi told ABC7 News.

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As the MyShake app was put to the test during the 5.1 earthquake near San Jose, experts also discuss when a 7.5 quake could rock the Bay Area.

According to Pelosi, there are 43,000 bridges in the country that could be vulnerable to earthquakes and this law will benefit 15,000 of them. Yet, she was able to secure the most funding for the Golden Gate.

The Golden Gate Bridge is the only bridge in the state of California that hasn't been fully seismically retrofitted since the Loma Prieta earthquake. The first three phases of the Golden Gate Seismic Retrofit were completed in 2014. This included retrofitting the North and South anchorages and viaducts that were found to be the most vulnerable. It also involved replacing the steel supports and new foundation that is 10 times stronger.

"We also put seismic isolation bearings on top," said Denis Mulligan, the general manager of the Golden Gate Highway and Transportation District. "So when the ground starts to shake, that tower doesn't have to drag the whole weight of the bridge."

The retrofit also funded dozens of energy dissipation devices that will be installed to reduce the amount of energy transferred on the bridge in the event of a big earthquake.

"We found these devices work amazingly well," said Mulligan.

The next phase of the retrofit begins in 2024

Even though the bridge didn't sustain any damage during the 1989 Loma Prieta earthquake, Mulligan expects the next one will be worse.

"It's important to realize that the earthquake was 60 miles away - a Santa Cruz earthquake and while the magnitude never sounds big, it's actually a moderate earthquake; an 8.3 magnitude quake would be a 100 times more energy," Mulligan said.

Since then, the district has been preparing.

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The latest phase involves retrofitting the suspension bridge, which is set to begin this year. This involves the replacement of hundreds of floor beams with new bracing. The process isn't expected to be complete until 2029.

"So what happens if we have a big earthquake before that?" ABC7's Stephanie Sierra asked.

"The bridge is safe, it will not collapse. No one will die on the bridge," Mulligan said. "However, there will be damage. There will be significant damage that will necessitate potentially having to replace the bridge."

Mulligan says that could cost more than $10 billion.

Earthquake reality check

So what are the Bay Area's chances of another massive earthquake?

According to USGS, they are high.

"So overall, the region faces this 72% chance of a really large earthquake a magnitude 6.7 plus in the next three decades," said USGS research geologist Austin Elliott.

Elliott says most of that threat comes from the Hayward fault, which has a one in three chance of producing an earthquake within that window. According to USGS, the margin of error is decades - meaning it could happen tomorrow or it could happen 20 years from now.

USGS added while it's difficult to predict precise numbers, the latest estimate from the national scale forecast suggests the Hayward fault could see at least a magnitude 7.1 to 7.3 up to a 7.5. Meanwhile, the San Andreas Fault could produce up to a magnitude 8.0.

"But in recent research, people have done some more specific modeling of the Hayward in particular that suggests that the magnitude could be larger, especially if it links with other faults," said Elliott.

USGS recently identified a structural connection between the Hayward fault south of San Pablo Bay and the Rogers Creek fault north of San Pablo Bay.

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California geologists say a once-in-a-lifetime tsunami could inundate more of the Bay Area than previously thought.

"So essentially, it's one big continuous structure and it may host a single large earthquake," Elliott said. "What's important for people to understand about this is - that it doesn't necessarily mean that the ground shaking at any individual spot would be stronger, but the earthquake would last longer and the area that's affected would be much larger."

As Elliott pointed out, magnitude alone doesn't tell the whole story. Modeling by USGS shows parts of the threat is tied to where on the fault it begins.

"It depends on where an earthquake really starts and the nature of the fault slip itself that determines ground motion and the intensity of shaking," said Elliott. "If it starts on the south end, you would get this northward concentration of energy."

Bringing with it shock waves, threatening infrastructure across the Bay Area - including the Golden Gate.

"You know, if you're on the roadway, it's secure. The part of the bridge that will twist and have damage are below the roadway," Mulligan said.

The retrofit of the suspension bridge will start later this year. The Golden Gate Highway and Transportation District says impacts to commuters will be during overnight hours where two lanes will be temporarily closed for construction. While the bridge could suffer damage if a significant earthquake hits the Bay Area before it's done, Mulligan stressed once it's completed, the bridge will be fully protected.

Take a look at more stories by the ABC7 News I-Team.

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