The city is boosting seismic construction standards for new skyscrapers, but the biggest risk is to older high-rises and old brick buildings.

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One of the biggest unknowns about the next megaquake in the Pacific Northwest — aside from when it will hit — is what it will do to tall buildings.

The type of shaking expected when the offshore fault called Cascadia Subduction Zone ruptures is particularly hazardous to high-rises. But it’s been 318 years since the fault where the ocean floor collides with the continent last let loose, so scientists can’t rely on past experience to predict how cities in the Pacific Northwest will be rattled.

Now, results are in from the M9 project, a four-year effort to better estimate shaking in Seattle from a magnitude 9 Cascadia quake — and the news is not reassuring. Researchers found the sedimentary basin under the city can amplify the type of ground motion that’s hardest on high-rises by a factor of two to five — much more than previous estimates.

In response, Seattle and Bellevue are boosting seismic standards for new buildings 240 feet or taller, or roughly 20 stories or more. But the prospect of stronger shaking also raises concerns about older high-rises, many constructed long before the region’s earthquake hazards were fully understood.

“Older buildings are where the real risk is,” said University of Washington engineering professor Marc Eberhard. “We should be doing all kinds of studies and devoting a lot of resources to figure out which ones are most vulnerable.”

Building codes are constantly being upgraded, and structural engineers say modern high-rises are very unlikely to collapse in a powerful earthquake even if they don’t meet Seattle’s new standards. Those standards require tall buildings to be slightly stronger, and probably won’t result in major cost increases, say local engineers.

The changes come in the midst of a sizzling construction boom, with office towers and apartment buildings springing up from South Lake Union to Sodo.

The city had hoped to roll out the new rules last spring, but builders and engineers objected. Officials agreed to delay the rules until early this month so projects that hadn’t broken ground, but were already in the design pipeline, would not be affected, said Susan Chang, geotechnical supervisor for the city’s Department of Construction and Inspections.

That exempted 18 high-rises currently under review, including the 58-story building planned for the empty lot across from City Hall and the 54-story Altitude Sky Tower at Fifth Avenue and Stewart Street.

SEISMIC NEGLECT

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Even before the new shaking estimates, older high-rises were known to have a greater risk of serious damage and collapse than buildings that meet modern seismic codes. Some are supported by steel frames with a type of welded joint now known to be fracture-prone, while others were built of poorly-reinforced concrete.

“While I would not want to be in any tall building in Seattle in a Cascadia giant earthquake, I definitely would not want to be in one of those old concrete buildings of more than six stories built before 1975,” said Tom Heaton, professor of geophysics and civil engineering at Caltech. “And most of your (steel) high-rises built before 1994 are almost certainly constructed with defective welds.”

Many of Seattle’s prominent high-rises were built between 1960 and 1994, when the fracture-prone welds were commonly used. Without examining construction records, though, it’s impossible to know for sure which towers have the welds. The 1970s-era Rainier Tower, with its golf-tee shaped base, is one that does, according to the company that engineered it. Columbia Center, the city’s tallest building, has a sturdier type of construction, as do the Seattle Municipal Tower and Two Union Square.

In California, where seismic standards for tall buildings are also being reevaluated, Santa Monica and West Hollywood recently became the first cities to mandate upgrades for older steel high-rises. San Francisco commissioned an inventory of its 155 tallest buildings to identify those most in need of retrofits. Los Angeles and several other cities are also requiring retrofits for old concrete buildings and buildings with open ground floors vulnerable to collapse.

But in Seattle, where the threat to tall buildings is higher than in California because of the power and nature of subduction zone earthquakes, there have been no efforts to identify potentially dangerous high-rises. The city also does not require retrofits for old brick buildings, a seismic menace California tackled decades ago.

Ron Klemencic, whose engineering firm helped design many of Seattle’s and San Francisco’s tallest buildings, says he’s more concerned about earthquake damage to water systems and old brick buildings than the collapse of high-rises, which are generally designed to stand up to winds that exert more force than earthquake shaking.

“I think the tall building stock in Seattle is by and large safe,” said Klemencic, chairman and CEO of Magnusson Klemencic Associates. The company is headquartered in the Rainier Tower, despite its fracture-prone welds. “That should tell you that we’re not particularly worried,” he added.

A shaky base

The culprit behind Seattle’s increased seismic hazard estimate is the vast, sediment-filled basin that underlies most of the city and extends from Hood Canal to the Cascade foothills.

More than four miles deep in some places, the basin’s southern boundary is the Seattle fault — a wide swath of fractures that passes under Seattle’s SoDo area and extends from the Cascade foothills to Hood Canal. When jolted by an earthquake, the glacial-packed soils in the basin shake like a bowl of Jell-O, with seismic waves reflected and trapped within the basin.

Tacoma, Everett and Portland sit on or near similar basins.

To better estimate ground motions in Seattle, the M9 team examined records from subduction-zone earthquakes around the world, including Japan’s 2011 magnitude 9 Tohoku disaster. They also ran computer simulations of 50 different Cascadia quake scenarios. The three-dimensional simulations were the most detailed ever for the Pacific Northwest, covering more than a billion grid points and taking as much as three days of supercomputer time each.

“We really wanted to get an idea of the variability that we could expect from different earthquakes,” said U.S. Geological Survey researcher Erin Wirth, a leader of the National Science Foundation-funded project. “If you just look at just a few simulations you don’t know if you’re seeing a best-case, worst-case or average scenario.”

Among the more surprising findings is that shaking intensity in Seattle can vary by a factor of 10, depending on exactly how and where the 700-mile-long fault ruptures. The analysis also found that strong shaking will last for about two minutes, four times longer than most earthquakes, said USGS researcher Art Frankel, who conceived the project and led the ground motion estimates.

The fiercest pounding will be on the Pacific Coast, closest to the rupture zone. By the time the seismic waves reach Seattle the motion will be less like a jackhammer and more like a rolling sea.

Buildings under 10 stories and single-family homes can ride out that type of motion fairly well. But slow shaking matches the natural resonance of tall buildings, causing them to whip back and forth. And that’s the type of shaking that will be most intensified by the Seattle basin.

“There are many buildings in Seattle that would go through this Cascadia earthquake with relatively minor damage,” Heaton said. “But the type of building that’s most vulnerable is precisely the tall buildings.”

The magnitude 8.1 subduction zone earthquake that devastated Mexico City in 1985 occurred more than 200 miles away, but violent shaking amplified by the underlying basin collapsed nearly 400 buildings — mostly newer mid- to high-rises. In some places, the shaking was three times stronger than the building code anticipated.

The M9 analysis did not consider an earthquake on the Seattle Fault, but USGS scientists plan to run similar computer simulations for that scenario over the next year or so.

Beginning in 2009, Seattle was the first U.S. city to encourage engineers to factor basin amplification into the design of buildings over 240 feet as part of a special peer-review process. The new city rule requires buildings to be engineered for earthquake forces 20 to 25 percent stronger than before, Chang said.

The tougher requirement could add as much as $10 per square foot, or $5 million, to the cost of a 40-story residential tower, according to an estimate from Magnusson Klemencic. With construction costs for high-rises in Seattle averaging about $300 per square foot, that represents about a 3 percent increase.

“Walls would become a little thicker, rebar amounts would be a little higher, nothing very drastic,” said Andy Taylor, of KPFF Consulting Engineers.

Seattle’s authority to impose stricter seismic standards extends only to the tallest buildings, under the city’s oversight of high-rise construction. But amplified shaking from a Cascadia quake can also threaten shorter buildings of 10 to 20 stories.

Those structures only have to comply with the international building code, which does not require direct consideration of basin amplification.

The USGS incorporated basin effects for the Puget Sound region into its new, draft earthquake hazard maps this year, Frankel said. Those maps inform building codes, but the higher levels of shaking won’t be factored into the international code until its next iteration in 2024.

Fractured welds

Seattle has 116 existing buildings at least 240 feet tall, with about 20 under construction, according to the building industry website Emporis. Of the 25 tallest buildings in the city, Emporis lists 15 with steel frames built during the period when the fracture-prone welds were widely used.

Statewide, there are 53 suspect steel-framed buildings of 10 stories or more, occupied by perhaps 100,000 people, said Keith Porter, a research professor of structural engineering at the University of Colorado, who analyzed Emporis data. Some of the buildings may have been retrofitted and some may have superior design and construction, but there’s no way to tell without a detailed analysis.

The problem with older steel buildings was discovered in 1994, when relatively light shaking from the Northridge earthquake fractured welds in more than 60 high-rises around Los Angeles. None collapsed, but one came close.

A 2015 analysis concluded that an older skyscraper with fracture-prone welds is up to five times more likely to collapse in a very powerful earthquake than a modern building.

“We’ve known about it for a long time,” Porter said. “We have enough evidence from enough authoritative sources to say there is a reasonable chance of these tall buildings collapsing in a big earthquake.”

But Klemencic points out that many older, steel high-rises — like the Rainier Tower — are supported by multiple columns, beams and welds, so that even if several welds fracture, the building will remain standing.

Heaton, a leading skeptic of high-rise construction in earthquake country, isn’t convinced that even new skyscrapers are sound. Most on the U.S. West Coast are designed using computer models and haven’t been challenged yet by actual earthquakes, he said.

“It’s kind of like saying: I want you to get on a brand-new jetliner we haven’t had time to test yet, but the computer says everything is fine,” Heaton said. “In places like Seattle, (the next big earthquake) will be an enormous experiment and the population will be part of that experiment.”

The building code allows up to a 10 percent chance of collapse for new high-rises subjected to intense shaking from rare, extremely powerful earthquakes. The code doesn’t aim to prevent damage and loss of use, just loss of life.

But high levels of damage can be catastrophic to societies and economies, said Porter. For every modern building that collapses in an earthquake, about 13 will be so heavily damaged they will be unsafe to enter, and more than 50 will be partially uninhabitable, he said. For the San Francisco Bay Area, Porter calculated that one out of every four new buildings would be unusable after a magnitude 7 earthquake.

“Although new buildings are a very low threat to our lives, they may be a very high threat to our livelihoods,” he said. “Most people don’t understand that.”

As part of its review of high-rises, San Francisco is considering stricter construction standards to minimize damage and ensure that tall buildings will be usable after an earthquake. But legislation in California to boost building standards statewide was vetoed in October by Gov. Jerry Brown, who said the state shouldn’t duplicate the efforts of a federal agency that is also exploring the idea.

For Seattle, where the M9 results mean tall buildings aren’t as safe as previously assumed, Porter urges action to reduce the risk. “You don’t have to move out of Seattle for fear the big one is going to kill you tomorrow, but don’t be complacent either,” he said.

Retrofitting old steel and concrete high-rises is costly, but cities in California are giving owners 10 to 25 years to make the fixes.

“If you start down the road now and accept that a problem exists,” Porter said, “you have a good chance of having solved the problem … before it’s too late.”