LOS ANGELES — The earth lurched without warning before dawn, jolting Los Angeles from its sleep. In a flash, freeway overpasses collapsed. Buildings were leveled or ruined. Fires spread.
Two decades after a 6.7-magnitude earthquake shattered Los Angeles and surrounding communities, buildings around the region remain vulnerable. While there has been progress to rebuild and shore up freeways and hospitals, there has been less attention paid to concrete buildings and housing with ground-floor parking.
‘‘That remains a significant problem,’’ said Jonathan Stewart, an earthquake engineer at the University of California, Los Angeles. “We really have not come very far.’’
At 4:31 a.m. on Jan. 17, 1994, the ground shuddered beneath the bedroom community of Northridge, rippling seismic energy outward. After the shaking stopped, sections of the city laid in fiery tatters. Several dozen people died, and 9,000 were injured. The quake caused $25 billion in damage, the costliest US natural disaster at the time.
The largest cluster of the 72 deaths occurred at the Northridge Meadows complex, where 16 people were killed when their first-floor apartments collapsed into the parking garage below.
The city does not keep count of how many so-called soft-story buildings exist and does not require repairs, even after 1994, because many such buildings survived the shaking.
The Los Angeles Department of Building and Safety recently proposed surveying 30,000 apartments built before 1978 to determine which might be compromised during violent shaking as a first step toward possible retrofitting. The department has yet to receive funding to start the work.
After the quake, the city required fixes to some 200 steel-frame high-rise buildings that suffered cracked welds and 2,750 concrete buildings that were poorly designed. Both types of repairs were relatively easy and did not bankrupt property owners.
Even so, computer simulations released this week by the California Institute of Technology and US Geological Survey found midrise steel buildings performed differently in the quake, depending on the type of welding.
Other retrofits were voluntary, including efforts to buttress concrete-frame buildings.
There are about 1,500 such buildings in Los Angeles County and between 16,000 and 17,000 statewide. Only about 10 to 15 percent are considered dangerous, said Craig Comartin, who led a study by the Concrete Coalition, a volunteer group of scientists, engineers, and government workers.
Earlier this week, the city announced it would partner with the US Geological Survey to develop a plan to address seismic safety, including ways to get privately owned buildings to be more quake-proof.
‘‘We’re as well prepared as any city in America, which is to say we’re unprepared,’’ Mayor Eric Garcetti said. ‘‘I don’t think anybody in America is very well prepared. There’s always going to be an earthquake we can’t be prepared for. ‘‘
Despite the lag in reinforcing potentially dangerous buildings, strides have been made in beefing up state-owned freeways and bridges.
Several freeway bridges that collapsed or were heavily damaged had been retrofitted decades earlier, but the efforts were limited to tying structures together using restrainers at the joints. Techniques have since evolved.
Since the quake, the California Department of Transportation spent more than $1 billion to further brace 1,155 bridges identified as at risk. Only two projects remained — a freeway overpass in Oakland that is scheduled to be completed this fall and the Schuyler Heim Bridge in Long Beach, scheduled to be finished in 2017.
Northridge was the last deadly quake to strike a US metropolitan area.
‘‘People forget,’’ said Tom Jordan, who heads the Southern California Earthquake Center at the University of Southern California. ‘‘It’s been ungodly quiet from a seismic point of view since Northridge.’’