Because the region is so huge and most damage from earthquakes and fires typically is relatively localized, most of the region's planning is based on the assumption that damage will be confined to one or two areas, several officials said.
Detailed plans to deal with a massive emergency -- one that displaces more than 300,000 people -- have not been developed since the end of the Cold War, said Stephen Sellers, head of Southern California operations for the state Office of Emergency Services.
Sellers and others say a tragedy on that scale goes beyond many worst-case scenarios and would include a chemical or nuclear attack, or a catastrophic earthquake on one of the faults that run directly under Los Angeles, such as Newport-Inglewood or Puente Hills.
Computer models released in May by the Federal Emergency Management Agency and the Southern California Earthquake Center found that a magnitude 7.5 quake along the Puente Hills fault could kill as many as 18,000 people, injure up to 268,000 and displace as many as 735,000 families. A study by the state Division of Mines and Geology found that a 7.0 temblor on the Newport-Inglewood fault would block freeways, sharply curtail flights at LAX, reduce the number of hospital beds by a third and knock out major power plants for days.
The difficulty of dealing with the volume of displaced people and downed services after Hurricane Katrina has caused some officials of emergency response agencies to think anew.
Katrina "just shattered all of our planning assumptions," said Sharon Grigsby, who heads bioterrorism response for the Los Angeles County Department of Public Health. "We certainly should be planning for a larger-scale event than we have focused on up to now."
If major portions of the city have been damaged and huge numbers of people displaced or hurt, it could take several days -- perhaps up to a week -- for government and charitable agencies to respond, several officials said. For that reason, it is all the more important that residents keep survival kits at home, said Constance Perett, director of the Office of Emergency Management for Los Angeles County.
In the past, officials had recommended that residents keep three days' worth of food, water, first aid and other items in a safe place in or near their homes. But in the wake of Katrina, Perett is recommending that everyone keep seven days' worth of supplies.
During the most disruptive natural disaster in recent memory, the 1994 Northridge earthquake, the Red Cross was able to house 20,000 displaced people in tent cities, gyms, hotels and other locations. But the agency, which local officials rely on to provide shelter in emergencies, does not have agreements with large private venues like Staples Center that can hold many thousands of people.
Taking care of hundreds of thousands, said Kevin Leisher, the Red Cross response officer for Los Angeles, would be too much for the agency to handle on its own immediately after a catastrophic event.
"I can't tell you that tonight I could open shelters for 300,000 people," Leisher said. "I can't give everybody a cot and a blanket."
Damage from a major quake in the heart of Los Angeles -- where buildings and infrastructures are older and urban faults like Newport-Inglewood and Puente Hills stretch for miles through densely populated areas -- would dwarf the destruction from Northridge, said Lucy Jones, scientist in charge of Southern California for the U.S. Geological Survey.
"How do you get people to understand that Northridge was actually a little earthquake?" she said. "Puente Hills would be so much worse than Northridge."
Los Angeles police have participated in dozens of disaster drills, said LAPD Chief William J. Bratton, and the department is developing a response plan meant to be a model for other municipalities.
But the LAPD has just 1,000 officers on the beat at any given time, Bratton said. During Monday's power outage, when traffic lights went out throughout the city, Bratton said, he could not spare enough officers to properly direct and manage traffic at all the places where the signals were out -- and that was a relatively minor incident.
"We would be stretched very thin to do the multiplicity of things you have to do in a disaster -- assist the injured, secure facilities that are critical to you, prevent disorder," Bratton said.
Such sober assessments come despite statements made by city and state leaders in the wake of Hurricane Katrina that California and the Los Angeles area is relatively well-prepared for a major disaster. On Thursday, Los Angeles Mayor Antonio Villaraigosa announced the formation of a panel to examine the city's emergency plans, proclaiming that Los Angeles "is as prepared as any city in the nation."
The region's disaster response effort -- like the rest of the state's -- would be highly decentralized: The Sheriff's Department is responsible for evacuations; the Red Cross provides emergency shelters; a corps of volunteer doctors and nurses have promised to help people who are sick or injured.