LOS ANGELES (AP) - 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 magnitude-6.7 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. We really have not come very far," said Jonathan Stewart, an earthquake engineer at the University of California, Los Angeles.
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 dozens died and 9,000 were injured. The quake caused $25 billion in damage - the costliest U.S. natural disaster at the time.
The largest cluster of deaths occurred at the Northridge Meadows complex where 16 people were killed when their first-floor apartments crashed onto the parking garage below.
The city doesn't keep count of how many so-called soft-story buildings exist and doesn't require mandatory 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 Northridge, the city required fixes to some 200 steel-frame high-rise buildings that unexpectedly suffered cracked welds and 2,750 concrete tilt-up buildings - erected with concrete walls and lifted into place by cranes - 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 U.S. Geological Survey found mid-rise steel buildings performed differently 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 governments.
Earlier this week, the city announced it would partner with the USGS 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 in an interview with the Associated Press. "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. "
At least 57 people died, according to the state. A 1995 study put the death toll at 72, including heart attacks.
Despite the lag in reinforcing potentially dangerous buildings, strides have been made in beefing up state-owned freeways and bridges by wrapping support columns with jackets of steel.
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 bridge joints. Techniques have since evolved.
Since Northridge, 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's scheduled to be completed this fall and the Schuyler Heim Bridge in Long Beach that's due in 2017.
Two hospitals suffered major structural damage, prompting lawmakers to require hospitals statewide to strengthen buildings so they can still treat patients after a disaster. Out of 1,313 hospital buildings deemed at risk for collapse, 69 percent have been retrofitted, replaced or demolished. The work continues despite extended deadlines.
Northridge was the last deadly quake to strike a U.S. metropolitan area.
Wendy Yost lived through it. A student and resident adviser at California State University, Northridge, Yost awoke to a messy dorm room and a waterlogged bathroom after the toilet came off its base.
Though the school was on winter break, some students stayed behind. With a candle in hand, Yost dodged broken glass in the hallway, knocking on students' doors to get them to safety.
"This wasn't my first rodeo when Northridge happened," said Yost, who also survived the Loma Prieta quake that rocked the San Francisco Bay area during the 1989 World Series.
Those born after 1994 have never experienced a major temblor. And for many who lived through Northridge, 20 years of relative seismic calm may lull some into a false sense of security.
"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."
To prevent amnesia, scientists and emergency responders have held an annual preparedness drill since 2008 to educate Californians and residents in other quake-prone states to "duck, cover and hold on" during shaking.
Northridge exposed a hidden fault. Afterward, scientists scoured for similar geologic structures lurking around the Los Angeles basin. Five years later, they discovered the Puente Hills fault that runs underneath downtown Los Angeles that could cause far greater damage. Budget cuts through the years have slowed efforts to map potentially dangerous fault lines.
By seismic standards, Northridge wasn't a monster in terms of its strength or the destruction it wrought. Northridge didn't cause mass casualties like the 2010 quake in Haiti did, but a future quake certainly could.
If a segment of the 800-mile-long San Andreas Fault unzipped and unleashed the "Big One" as scientists call it, it would cause far greater chaos to lives, properties and lifelines including roads, pipelines and telecommunication systems. Despite intense study, there's no magic bullet that allows scientists to predict when a quake would hit.
"We're chipping away at it," UCLA's Stewart said of the retrofit process. "We're not where we would like to be, but we are making progress toward hardening the system."