SAN FRANCISCO — The picturesque Victorians and brightly painted apartment buildings where thousands of city residents live and work are especially vulnerable during earthquakes, according to a report issued Friday by the San Francisco Department of Building Inspection.
The report said that an earthquake with a magnitude of 7.2 or higher could render unlivable as many as 85 percent of the city’s “soft-story” apartment buildings — those that are less structurally sound because their ground floors are open space, often used as retail stores or garages. At least 65,000 people live and work in the 2,800 most vulnerable buildings studied in the report.
The cost to retrofit those wood-framed buildings would be about $260 million. The expense would be borne by the landlords and the city, which is facing a $576 million budget shortfall.
“A big earthquake is overdue in the region, and we’re not naïve to that reality,” said Mayor Gavin Newsom, who ordered the report in July and is working on legislation to make earthquake safety upgrades mandatory on soft-story buildings. “We cannot wait five years. We should have done this 35 years ago, 100 years ago.”
Mr. Newsom said that he recognized the economic realities facing the city and its 744,000 residents and that he did not want retrofitting to put building owners “at risk of insolvency.”
Still, building owners say they are nervous about the cost of earthquake damages and the cost of mandatory changes. Few apartment owners in the city carry earthquake insurance, the report said.
“We want to keep our tenants safe, but we’re fearful in this economy,” said Janan New, director of the San Francisco Apartment Association, a rental property owners association. “No one is going to get financing for construction in this market.”
There is a 20 percent chance of a magnitude 7.2 earthquake on the San Andreas Fault just west of the city sometime in the next 30 years, according to the United States Geological Survey. That probability jumps to 63 percent for a magnitude 6.7 tremor. And seismologists say many of the fault lines running veinlike across the state could begin shaking anytime.
Predictions about earthquakes and the potential wreckage wrought are not taken lightly in San Francisco, where a quake in 1906 left much of the city in ruins and started a fire that lasted three days, killing more than a thousand people. A 1989 earthquake, which had a magnitude 6.9 on the Richter scale, resulted in dozens of deaths and billions of dollars in damage.
Some neighborhoods, particularly those along the water, were once wetlands and sand dunes that had to be fortified. Particularly precarious are the soft-story building atop the artificial fill because, “the ground becomes liquid and buildings lose their ability to stand and then they begin sinking into the ground,” said Thomas Brocher, a chief scientist for the Geological Survey’s Western Earthquake Hazards Team.
Building department employees walked block by block through the city, tallying the number of multiunit, soft-story buildings constructed before 1973, when changes to the city’s building codes mandated more structurally sound buildings. The count was 4,400. The study released Friday by the building department’s Community Action Plan for Seismic Safety considers only the most dangerous of those.
The price tag to fortify the city against the grinding fault lines flanking it on all sides is likely to climb as the building department continues to study other at-risk structures over the next 18 months.
“This report shows the potential for soft-story buildings to collapse,” said Vivian Day, director of the building department. “But in earthquake country, almost any kind of building can collapse. It just depends on the size of the earthquake.”