San Francisco’s Big Seismic Gamble

California has strict building requirements to protect schools and hospitals from a major earthquake. But not skyscrapers. A five-story building has the same strength requirements as a 50-story building.

Yet skyscrapers cast a much broader shadow of risk across a city and their collapse or impairment could cause a cascade of consequences.

How safe are San Francisco’s skyscrapers? Even the engineers who design them can’t provide exact answers. Earthquakes are too unpredictable. And too few major cities have been tested by major temblors.

“The profession does the best job we can to model and predict, but there are a number of uncertainties,” said Ron Hamburger, one of the country’s leading structural engineers. “We don’t have as many records, particularly for large magnitude earthquakes, as we would like.”

Previous earthquakes have revealed flaws with some skyscrapers. A widely used welding technique was found to rupture during the 1994 Northridge earthquake in Los Angeles. (Many buildings in San Francisco and Los Angeles have not been retrofitted.)

California has made significant strides in earthquake preparedness over the past century. Freeway overpasses, bridges and some municipal buildings have been strengthened. Many Californians live in single-family wood frame homes, which have been found to hold up relatively well during earthquakes.

But until recently, high-rise buildings were not a focus of San Francisco’s seismic safety.

Newer high rises across California, which are typically built around a concrete core, are designed using computer modeling.

This raises concerns among experts such as Thomas H. Heaton, the director of the Earthquake Engineering Research Laboratory at the California Institute of Technology and perhaps the most prominent skeptic of building high rises in earthquake zones.

“It’s kind of like getting in a new airplane that’s only been designed on paper but nobody has ever flown in it,” he said.

Last September, San Francisco’s former mayor, Edwin M. Lee, responding to a scandal about a skyscraper that was sinking and leaning, ordered city officials to strengthen building codes for high rises and requested an independent study on their safety.

Known as the Tall Buildings Study, it will for the first time create a detailed database of the more than 160 high rises, classified by building type. Ayse Hortacsu, the structural engineer who is leading the study, has deployed Stanford graduate students to pore over blueprints and records at the San Francisco Department of Building Inspection.

“It would have been great to do this before this building boom in San Francisco,” Ms. Hortacsu said. “But we are going to seize the moment and make the best out of it.”

For years the city restricted building height to 500 feet in most neighborhoods. The objection to high rises was largely cultural and aesthetic — critics deplored “Manhattanization” and said high rises were not in keeping with the ethos of the city.

But by 2004, city officials had put in motion a plan to redevelop a neighborhood of warehouses and vacant lots that today are the heart of downtown.

The city pushed for the construction of a tall, iconic building — the future Salesforce Tower, which can be seen in the right half of this photograph, shimmering over its neighbors.

“We saw that as a symbol of the new San Francisco and we wanted the building to be at least 1,000 feet in height,” said Dean Macris, a key figure in conceiving the new high-rise San Francisco who led the planning board under four mayors.

Now retired, Mr. Macris said the issue of seismic safety of high rises was “never a factor” in the redevelopment plans of the South of Market area, or SoMa, as it’s known.

What shifted the debate on seismic safety was the sinking and tilting of the 58-floor Millennium Tower.

When it was completed in 2009, the building won numerous awards for ingenuity from engineering associations, including Outstanding Structural Engineering Project of the Year by the San Francisco office of the American Society of Civil Engineers.

The developer and city officials knew of the building’s flaws for years, but kept them confidential until 2016, when news leaked to the public. The latest measurements, taken in December, show that the building has sunk a foot and a half and is leaning 14 inches toward neighboring high rises. It is across the street from Salesforce Tower and right next to a transit hub for buses, trains and eventually high speed rail that is being touted as the Grand Central of the West.

With the Millennium Tower, San Francisco got a foretaste of what it means to have a structurally compromised skyscraper. If the city is hit by a severe earthquake, experts fear there could be many more.

The area around Millennium Tower is considered among the most hazardous for earthquakes. The United States Geological Survey rates the ground there — layers of mud and clay — as having a very high risk of acting like quicksand during an earthquake, a process known as liquefaction.

In light of the problems with the Millennium Tower, there are now increasing calls in California for a reassessment of earthquake risks, much of it focused on strengthening the building code.

Right now the code says a structure must be engineered to have a 90 percent chance of avoiding total collapse. But many experts believe that is not enough.

“Ten percent of buildings will collapse,” said Lucy Jones, the former leader of natural hazards research at the United States Geological Survey who is leading a campaign to make building codes in California stronger. “I don’t understand why that’s acceptable.”

The code also does not specify that a building be fit for occupancy after an earthquake. Many buildings might not collapse completely, but they could be damaged beyond repair. The interior walls, the plumbing, elevators — all could be wrecked or damaged.

“When I tell people what the current building code gives them most people are shocked,” Dr. Jones said. “Enough buildings will be so badly damaged that people are going to find it too hard to live in L.A. or San Francisco.”

In January a Southern California assemblyman, Adrin Nazarian, introduced a bill in the State Legislature that would require the building code to make new buildings strong enough for “functional recovery” after an earthquake. The bill passed its first hurdle, a committee hearing, last week.

Driving the push to change the code is the notion that California has so much more to lose than it did in 1906.

Charles Richter, the earthquake pioneer who invented the scale used to measure their power, had strong opinions about skyscrapers. Don’t build them in California, he said.

In the years since Mr. Richter’s death in 1985, construction materials have become stronger and engineering more precise.

Yet Hiroo Kanamori, an emeritus professor of seismology at the California Institute of Technology who developed the earthquake magnitude scale that replaced Dr. Richter’s, says the vast power and mysteries of earthquakes should continue to instill a deep humility.

In recent decades scientists have recorded violent ground motions that were previously thought impossible. A soon-to-be-published paper by Caltech engineers showed that an earthquake with a similar intensity of the one that struck Chichi, Taiwan in 1999 would bring down or render unusable numerous steel frame high rises in Los Angeles.

“People say, ‘Don’t worry about it, it’s an outlier,’” Dr. Kanamori said. “This is the problem with earthquakes. By nature of the process there are a lot of unpredictable elements.

“And a single event can be catastrophic,” he said.

Sources: City and County of San Francisco (building footprints); Skyscraper Center, Council on Tall Buildings and Urban Habitat (building heights)

Additional work by K.K. Rebecca Lai. Thomas Fuller reported from San Francisco, and Anjali Singhvi, Josh Williams and Ms. Lai from New York.