Thousands of older wood and concrete apartment buildings vulnerable to collapse in a major earthquake would get costly upgrades under sweeping retrofitting rules passed Friday by the Los Angeles City Council.

The mandate would affect as many as 13,500 so-called soft-first-story buildings, which are typically wood-frame structures with large spaces such as parking lots on the ground floor. As many as 1,500 brittle concrete buildings would also require upgrades.

The measure passed on a 12-0 vote.

"There's no question that we're going to have an earthquake. The question is, when?" Councilman Gil Cedillo said. "In here we've laid out the groundwork for the seismic retrofitting that needs to be done."

Before the vote, representatives for residential landlords and commercial building owners signaled their approval of the plan — while expressing concerns about potential costs.

City leaders will now have to agree on how the estimated $5,000-per-unit retrofitting would be split between tenants and landlords. The law currently allows owners to increase rents up to $75 per month to pay for a required earthquake retrofit, but both sides say such a hike is too steep. One proposal is to divide the costs 50-50 and cap possible monthly rent increases at $38.

To help pay for the upgrades, apartment groups are looking for certain financial support, such as breaks on property and state income taxes and business license and building permit fees for owners who retrofit.

The proposed quake retrofitting mandate is part of an effort by Mayor Eric Garcetti to make the city resilient to major earthquakes. His plan released in December focuses on rapidly identifying and retrofitting at-risk residential and commercial buildings, fortifying major water systems that would be severed by a huge quake and keeping telecommunications systems operating.

The goal of the mayor's broad plan is to keep the region sufficiently functional to avoid a long-term economic collapse despite what seismologists say is an inevitable jolt on the order of a magnitude-7.8 quake caused by a 200-mile-long rupture of the mighty San Andreas Fault.

Wood apartments will be given seven years to complete construction once an owner is ordered by the Department of Building and Safety to retrofit the building. Owners of brittle concrete buildings will have 25 years to do the work.

Estimates for upgrades for soft-first-story structures range from $60,000 to $130,000 per apartment building. Taller concrete buildings can cost millions of dollars to strengthen.

Studies estimate that a massive earthquake in the Los Angeles area could kill up to 18,000 people and cause some $250 billion in damage. Sixteen people were killed in the collapse of a soft-first-story building during the Jan. 17, 1994, Northridge earthquake. The magnitude-6.7 jolt was the last significant seismic disaster in the Los Angeles region.

U.S. Geological Survey seismologist Lucy Jones, the mayor's earthquake science adviser who was a consultant for the council, was on hand for the vote. She pushed for passage of the plan, saying lives would be saved.

"It's not every day we have the opportunity to save lives," Council President Herb Wesson said after the vote. "Today we had that opportunity."