If a giant earthquake were to shake up Palo Alto, it would likely devastate more than 200 buildings, cause about $2.4 billion in damage and wipe out about four years' worth of construction.

Buildings with concrete shear walls, which bear a structure's weight, or constructed using the tilt-up method -- in which concrete is poured into the ground, cured and lifted -- would likely feel the brunt of a 7.9-magnitude quake, according to a new risk assessment that the city has just completed.

About 100 structures in these two categories would face extensive damage and possible destruction.

Wood-frame homes with "soft stories" -- garages or commercial space on the ground floor, which are not as structurally robust -- could also be in danger of collapsing, according to the assessment. In most cases, these are apartment buildings that were built in the 1960s or 1970s.

Palo Alto has 294 wood-frame buildings with soft stories, yet none of them are covered by the city's building code. Neither are the tilt-up structures, the concrete soft-story structures or, for that matter, the majority of the buildings constructed before 1977 and that are now deemed vulnerable.

To better brace for the inevitable Big One, Palo Alto is considering following in the footsteps of cities such as Berkeley and San Francisco, which recently revised their building codes to encourage -- and, in some cases, require -- retrofits to soft-story structures and other vulnerable building types.

In doing so, however, Palo Alto officials are confronting a series of complex and politically thorny questions, including: What types of buildings should be regulated? Should the city rely on carrots or sticks in seeking compliance? And how much time should a building owner be given to make the needed repairs before the city takes further action?

The risk assessment, prepared by the consulting firm Rutherford + Chikene, is intended to inform the debate. The study relied on tax-assessor files, GIS data, Fire Department surveys and an "extensive sidewalk survey" to examine more than 2,300 buildings in Palo Alto.

It also identifies new building categories for the city to consider for seismic upgrades, surveys the efforts in other jurisdictions and lays out the factors that the city should consider in developing its own program, which could involve new requirements for disclosures, surveys and/or retrofits.

Given the variety of vulnerabilities in the city's building stock, a 7.9-magnitude tremor would cause about $1.7 billion in building damage and another $700 million in "content damage," which includes furniture, equipment and other items not integral to the structure, the report states. A 6.7-magnitude quake would result in about $800 million in building and $400 million in content damage, the study found, noting that this does not include "the effects of lives lost, business disruptions or ripple effects in the local economy or real estate market."

One thing the report does not do is recommend a specific course of action. Different cities, it notes, have crafted, often over a decade or more, a "unique package of measures suited to their own local economic, social, political and risk realities." Palo Alto, the report states, must do the same.

In pursuing seismic improvements, some cities have relied on disclosure requirements and community outreach, while others have taken more aggressive actions and mandated buildings be retrofitted.

Richmond, for instance, developed an inventory of vulnerable buildings, hosted a community meeting and created a "very low-cost voluntary approach to (retrofitting) soft-story wood-frame buildings," according to the report.

Compared to jurisdictions that have done nothing, Richmond's program achieved "meaningful progress," the report stated. However, the program has been stymied by the city's -- and the property owners' -- limited resources, the report states.

Other cities have taken more aggressive action. Berkeley's program addressed retrofits of soft-story structures in phases but did not address the city's roughly 50 tilt-up concrete structures. It is also now producing a comprehensive assessment of earthquake vulnerabilities, an approach similar to the one taken by Los Angeles and San Francisco.

San Francisco also required retrofits of soft-story buildings in phases, a process that initially targeted large institutional buildings and later encompassed roughly 500 apartment buildings with 15 or more units. The next deadline will come in September, when roughly 3,500 soft-story buildings with five to 15 units are required to begin their retrofits. (These requirements come with their own logistical and enforcement challenges. According to the San Francisco Chronicle, owners of more than 1,800 of these buildings have yet to start the process, prompting concerns about whether there are enough seismic retrofitters out there to handle the workload.)

If the Palo Alto council opts to follow in the footsteps of San Francisco, Los Angeles and Berkeley, it would oversee the most significant revamp of Palo Alto's seismic rules since 1986, when the city adopted an ordinance that identified three types of buildings as particularly vulnerable, instituted mandatory reporting requirements and created incentives for downtown property owners to upgrade deficient buildings. Those who retrofitted their buildings became eligible to add to their buildings without being required to provide additional parking.

That ordinance specifically targeted downtown structures constructed of unreinforced masonry (these are called Category I buildings), those constructed before 1935 (Category II) and those that were built before 1976 and that have 300 or more occupants (Category III).

The 1986 program has seen some success, according to Development Services staff. Owners of all 89 buildings that fell into these categories complied with the ordinance and submitted engineering reports about structural deficiencies and potential improvements. Furthermore, 67 of these buildings were strengthened, demolished or proposed to be demolished, a new staff report states.

One such soft-story building on University Avenue houses the six-story Hotel President, whose ground-floor retailers include Yogurtland and Pluto's. It was retrofitted in 2004 by adding three steel interior frames on the first floor and a wall along Cowper Street that extends from below the basement grade up to the bottom of the second floor, building owner Chris Dressel told the Weekly in 2015.

Only 22 buildings in the original three categories remain in place today, according to the new assessment.

View map of Palo Alto's 22 most vulnerable buildings.

Yet officials also acknowledge that the program has significant gaps, particularly when it comes to soft-story buildings and old concrete ones. As the Rutherford + Chekene report notes, problems with soft-story construction became more apparent after the 1994 Northridge earthquake in the Los Angeles area. The event, which saw the collapse of a Bullock's department store, a Kaiser medical office and other concrete structures, prompted changes in construction-industry standards.

In Palo Alto, the recent effort to upgrade seismic regulations was sparked by a 2014 assessment of threats to the city, which identified earthquakes as one of the city's leading dangers. The study noted that Palo Alto's land-use decisions have "not always taken hazards into consideration."

"Moreover, older buildings and infrastructure reflect the construction and engineering standards of their era, which in most cases fall short of current standards for seismic safety," the study stated. "As a result, a portion of the city, including 130 soft-story structures, would be at some risk in the event of a major earthquakes."

Later that year, the council directed staff to come up with a new inventory and consider additional ways to make local buildings more resilient. During a December 2014 meeting, Councilman (and now Mayor) Greg Scharff said he's been through several earthquakes in his life, including the 1971 earthquake in Los Angeles that sent his bed "flying across the room."

"And I'm thinking to myself, 'It's one thing for the bed to fly, it's another thing if a building came pancaking down like that,'" he said. "I do think we need to take care of this."

Scharff also said at the time that he is in favor of a mandatory program, particularly one that targets offices and homes (retail would be a slightly lower priority).

When compared to other jurisdictions, Palo Alto's existing rules for seismic retrofitting are somewhere in the middle of the pack in terms of scope and safety requirements, the report states. Though the council set some important policy precedents in the 1980s, the voluntary program that came out of the effort addressed only a small subset of the city's vulnerable building stock, Rutherford + Chikene found.

"By investing in data collection and community discussions this year, Palo Alto is now poised to move forward into a new position of seismic policy leadership," the study states.

If the goal is to prevent the greatest amount of damage and loss, the report states, Palo Alto should focus on building types that are both potentially hazardous and that exist in large numbers. The most effective programs, the report states, "use a package of measures to tip the balance away from the status quo by publicizing and increasing the consequences of not retrofitting while also publicizing, easing the costs, and increasing the benefits of retrofitting."

Among the options on the table are expanding the existing program to encompass more building types, "nudging" building owners through new disclosure methods (including notifications to tenants of the buildings' vulnerability or signage requirements), adding incentives (San Francisco, for instance, allows buildings owners with soft-story retrofits to add accessory-dwelling units) and different combinations of incentives and requirements to achieve retrofits.

Peter Pirnejad, director of the Development Services Department, said that given the large volume of data and the array of options, staff decided to pursue change in three steps. The new report represents the first step, he said. Next, the city plans to hold a study session in which the council members will offer feedback about their preferred direction. Staff would then use this feedback to return to the council for a public hearing with proposed language for amending the municipal code, Pirnejad said.