SAN FRANCISCO — Sailors arriving in San Francisco in the 19th century used two giant redwood trees perched on a hill to help guide their ships into the bay. The redwoods were felled for their lumber at around the time of the gold rush, but San Francisco now has a new beacon: Salesforce Tower, the tallest office building in the West.

Clustered around the 1,070-foot tower are a collection of high rises built on the soft soil and sand on the edge of the bay. They represent a bold symbol of a new San Francisco, but also a potential danger for a city that sits precariously on unstable, earthquake-prone ground.

San Francisco lives with the certainty that the Big One will come. But the city is also putting up taller and taller buildings clustered closer and closer together because of the state’s severe housing shortage. Now those competing pressures have prompted an anxious rethinking of building regulations. Experts are sending this message: The building code does not protect cities from earthquakes nearly as much as you might think.

It’s been over a century — Wednesday marks the 112th anniversary — since the last devastating earthquake and subsequent inferno razed San Francisco. Witnesses on the morning of April 18, 1906, described the city’s streets as rising and falling like a ribbon carried by the wind.

The violent shaking ignited a fire that lasted three days, destroying 500 city blocks and 28,000 buildings.

Half of the population of around 400,000 was made homeless. Many were forced to flee the city.

1906 aerial photograph by George R. Lawrence/Library of Congress San Francisco Bay SAN FRANCISCO 2018 SAN FRANCISCO 2018 SAN FRANCISCO 2018 San Francisco Bay SAN FRANCISCO 2018 Diagram by The New York Times Millennium Tower 2009: 645 feet 340 Fremont 2016: 440 feet Salesforce Tower 2018: 1,070 feet 555 California St. 1969: 779 feet Transamerica Pyramid 1971: 853 feet Market St. San Francisco Bay SAN FRANCISCO 2018 Millennium Tower 2009: 645 feet Salesforce Tower 2018: 1,070 feet 555 California St. 1969: 779 feet Market St. SAN FRANCISCO 2018 Millennium Tower 2009 Salesforce Tower 2018 555 California St. 1969 Market St. SAN FRANCISCO 2018 Millennium Tower 2009: 645 feet 340 Fremont 2016: 440 feet Salesforce Tower 2018: 1,070 feet 555 California St. 1969: 779 feet Transamerica Pyramid 1971: 853 feet Market St. San Francisco Bay SAN FRANCISCO 2018 Diagram by The New York Times



An airship captured the devastation in San Francisco after the 1906 earthquake and fire. With the city leveled, Los Angeles quickly grew to capture San Francisco’s place as the premier city of the West. After decades of public hostility toward skyscrapers, the city has been advocating a more dense and more vertical downtown. San Francisco now has 160 buildings taller than 240 feet and a dozen more are planned or under construction.

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.

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“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.

San Francisco, once a low-rise city, is in the middle of a building boom and now home to the tallest office building west of the Mississippi. Jim Wilson/The New York Times

“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.

Millennium Tower has sunk almost a foot and a half and is leaning 14 inches toward neighboring high rises. Jim Wilson/The New York Times

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.

TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Likelihood that ground will liquefy in an earthquake NORTH BEACH PRESIDIO Bay Bridge VERY HIGH FINANCIAL DISTRICT Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT 19th Ave. Twin Peaks SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Likelihood that ground will liquefy in an earthquake NORTH BEACH PRESIDIO FINANCIAL DISTRICT Bay Bridge VERY HIGH Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT Twin Peaks 19th Ave. SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Likelihood that ground will liquefy in an earthquake NORTH BEACH PRESIDIO FINANCIAL DISTRICT Bay Bridge VERY HIGH Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT Twin Peaks 19th Ave. SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE Likelihood that ground will liquefy in an earthquake Golden Gate Bridge Golden Gate VERY HIGH San Francisco FINANCIAL DISTRICT PRESIDIO Bay Bridge RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park San Francisco Bay MISSION DISTRICT 19th Ave. Twin Peaks SAN FRANCISCO 1 MILE Likelihood that ground will liquefy in an earthquake VERY HIGH FINANCIAL DISTRICT SOUTH OF MARKET Golden Gate Park San Francisco Bay 19th Ave. MISSION DISTRICT SAN FRANCISCO 1 MILE Sources: California Geological Survey; United States Geological Survey; The City and County of San Francisco. TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Likelihood that ground will liquefy in an earthquake NORTH BEACH PRESIDIO Bay Bridge VERY HIGH FINANCIAL DISTRICT Market Street Buildings taller than 240 ft. RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT 19th Ave. Twin Peaks SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Likelihood that ground will liquefy in an earthquake NORTH BEACH PRESIDIO FINANCIAL DISTRICT Bay Bridge VERY HIGH Buildings taller than 240 ft. Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT Twin Peaks 19th Ave. SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Likelihood that ground will liquefy in an earthquake NORTH BEACH PRESIDIO FINANCIAL DISTRICT Bay Bridge VERY HIGH Buildings taller than 240 ft. Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT Twin Peaks 19th Ave. SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE Likelihood that ground will liquefy in an earthquake VERY HIGH FINANCIAL DISTRICT Buildings taller than 240 ft. SOUTH OF MARKET Golden Gate Park San Francisco Bay 19th Ave. MISSION DISTRICT SAN FRANCISCO 1 MILE Likelihood that ground will liquefy in an earthquake Golden Gate Bridge Golden Gate VERY HIGH San Francisco FINANCIAL DISTRICT PRESIDIO Bay Bridge Buildings taller than 240 ft. RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park San Francisco Bay MISSION DISTRICT 19th Ave. Twin Peaks SAN FRANCISCO 1 MILE Sources: California Geological Survey; United States Geological Survey; The City and County of San Francisco.

At least 100 buildings taller than 240 feet were built in areas that have a “very high” chance of 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.

San Francisco after the 1906 earthquake. Associated Press

The billions of dollars of infrastructure, headquarters of global industries and the denser, more vertical downtown make San Francisco a much more interconnected and vulnerable place.

The goal of the code, say proponents of a stronger one, should be the survival of cities — strengthening water systems, electrical grids and cellular networks — not just individual buildings.

“We’ve been sitting on our hands for decades about this problem,” said Keith Porter, a seismic engineer at the University of Colorado, who is hoping to spur greater public participation in a debate.

Dr. Porter’s research offers warnings on the economic consequences of a major earthquake in the San Francisco Bay Area. He has calculated that one out of every four buildings in the Bay Area might not be usable after a magnitude 7 earthquake, which although severe is not the worst the area could experience.

The map below shows areas where strong ground shaking is expected during an earthquake. At least 100 so-called soft-story buildings that require mandatory retrofitting are noncompliant — and most are in the area of anticipated strong shaking.

TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Areas expected to experience strong shaking in a large-magnitude earthquake. NORTH BEACH PRESIDIO Bay Bridge FINANCIAL DISTRICT Market Street Older, wood-frame multistory buildings that need to be retrofitted to comply with city regulations. RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT 19th Ave. Twin Peaks SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Areas expected to experience strong shaking in a large-magnitude earthquake. NORTH BEACH PRESIDIO FINANCIAL DISTRICT Bay Bridge Older, wood-frame multistory buildings that need to be retrofitted to comply with city regulations. Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT Twin Peaks 19th Ave. SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE TREASURE ISLAND San Francisco Golden Gate Bridge Golden Gate Areas expected to experience strong shaking in a large-magnitude earthquake. NORTH BEACH PRESIDIO FINANCIAL DISTRICT Bay Bridge Older, wood-frame multistory buildings that need to be retrofitted to comply with city regulations. Market Street RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park Pacific Ocean San Francisco Bay MISSION DISTRICT Twin Peaks 19th Ave. SAN ANDREAS FAULT SAN FRANCISCO HUNTERS POINT 1 MILE Areas expected to experience strong shaking in a large-magnitude earthquake. Older, wood-frame multistory buildings that need to be retrofitted to comply with city regulations. Golden Gate Bridge Golden Gate San Francisco FINANCIAL DISTRICT PRESIDIO Bay Bridge RICHMOND DISTRICT SOUTH OF MARKET Golden Gate Park San Francisco Bay MISSION DISTRICT 19th Ave. Twin Peaks SAN FRANCISCO 1 MILE Areas expected to experience strong shaking in a large-magnitude earthquake. Older, wood-frame multistory buildings that need to be retrofitted to comply with city regulations. Golden Gate Bridge San Francisco FINANCIAL DISTRICT PRESIDIO SOUTH OF MARKET San Francisco Bay Golden Gate Park 19th Ave. MISSION DISTRICT SAN FRANCISCO 1 MILE Sources: California Geological Survey; United States Geological Survey; Department of Building Inspection. Note: Soft-story noncompliant building data is updated as of April 12, 2018.

Dr. Porter calculates, hypothetically, that if buildings were built to a stronger standard, only 6 percent of buildings would be unusable.

Some structural engineers, however, say our modern crop of high rises are strong enough.

Ron Klemencic, the chief executive of Magnusson Klemencic Associates, the company that did the structural engineering for Salesforce Tower, says he agrees that water and sewage systems need higher strength requirements, but not high rises. “Buildings falling on top of other buildings — that’s not going to happen,” Mr. Klemencic said.

How much would stronger buildings cost to build?

At a time when the average price of a home in San Francisco is above $1.2 million, even a marginal increase in price tag is bound to meet resistance.

“We are already facing the concern of extremely high cost of housing and displacement,” said Brian Strong, the director of the office of resilience in San Francisco’s city government. In recent years the city has been focused on other seismic dangers, including older, low-rise apartment buildings with inadequate ground floor structures that could collapse, known as soft stories.

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.