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LOS ANGELES – It has been 20 years since Southern California experienced a major earthquake, a powerful 6.7-magnitude temblor that rolled through Northridge, killing 57 people. But this stretch of seismic calm, though welcome in obvious ways, has undermined efforts to force Los Angeles to deal with what officials describe as potentially lethal deficiencies in earthquake preparation.

That may be changing. Since two back-to-back earthquakes Friday evening – a relatively small one with a magnitude of 3.6, followed by a long and rolling 5.1 quake – Los Angeles has been shaken by nearly 175 smaller aftershocks. It is the first time this area has suffered an earthquake in excess of 5 magnitude since 1997, and it comes two weeks after a 4.4 earthquake jolted residents awake.

None of these quakes caused injuries or widespread damage, other than broken water pipes and some homes that have been declared at least temporarily uninhabitable. But geologists see them as the predictable end of a cycle: a return to what might be an uncomfortable normal in which 5-magnitude earthquakes become routine events.

“The last 17 years has been the quietest time we have ever seen,” said Lucile M. Jones, a seismologist at the U.S. Geological Survey. “Maybe we’re starting to turn back to more normal levels.”

The burst of seismic activity has come at a critical time for this part of the country, as Los Angeles confronts growing criticism that it has lagged in taking steps to minimize earthquake casualties. Its new mayor, Eric M. Garcetti, has seized on this as a defining issue. He appointed Jones as a special adviser to draw up an earthquake preparedness plan, following the lead of San Francisco and other cities.

Garcetti and Jones said the biggest obstacle in pushing Los Angeles to take crucial steps – most importantly, reinforcing hundreds of old concrete buildings at risk of collapse – had been the lack of a sense of urgency resulting from the long period of seismic dormancy.

“When earthquakes happen that are big enough to feel but small enough not to cause great damage, they help us provide the awareness we need in Southern California,” Garcetti said in an interview Sunday. “It’s what everybody is talking about. The place is abuzz about earthquakes. I’m going to take that and use that for our efforts.”

Vito Teti, 73, a part-time security guard who lives in Fullerton, said that when he returned from work Friday, his home looked like a war zone, with toppled furniture, broken lamps and pictures that had fallen from walls.

“If you’re a native of California, the problem is you get too complacent,” Teti said. “You hear about earthquakes in the news, but it’s always someplace else.”

Garcetti said he was preparing a plan to deal with what Jones, a familiar figure in California known as the Earthquake Lady, identified as the three most serious shortcomings in the city’s earthquake preparation. The first challenge is to fix unreinforced buildings that could collapse in a big quake, an issue that promises to stir debates about acceptable risk and who should pay for that very expensive endeavor.

The other two are to plan for catastrophic collapses in water supply and basic communication, as many of the aqueducts and Internet cables that supply Los Angeles cross the San Andreas Fault.

Garcetti’s decision to bring Jones to City Hall on a yearlong appointment, with weekly meetings devoted to earthquake presentation, came amid repeated reminders of the region’s vulnerability. About three months ago, an ambitious program to rebuild large parts of Hollywood, including the construction of tall buildings, was suspended after state geologists found that a major fault runs through the heart of the redevelopment area.

The reaction to the recent quakes – a combination of mordant humor, sang-froid and get-me-out-of-here alarm – has been intensified by social media and the Internet, which barely existed when the Northridge quake hit in 1994.

Twitter exploded Friday night with people sharing reports, pictures and links to maps awash in blazes of red and orange indicating the danger areas.

The earthquakes began in Orange County, about 30 miles from Los Angeles. The shaking was felt across much of the Los Angeles basin. In contrast to the quake two weeks ago, which arrived with a large jolt, this one produced prolonged rolling.

Jones said she suspected that the absence of severe geological activity since the Northridge earthquake, and the 7.3-magnitude Landers quake that took place in the remote Mojave Desert two years earlier, accounted for the lull.

“It was a very big earthquake that sort of relaxed the whole LA area,” she said. “It took the energy out of the crust.”

Jones said she thought the recent quakes would make it easier to rally political support for measures like forcing owners to pay to rehabilitate older buildings.

“When you have damage, it’s a lot easier to talk to people about what you need to do to avoid damage,” she said.

“There are real lives at stake,” Jones said. “We absolutely know that there are buildings that will kill people when they collapse.”

Robert Silvey, the owner of a carpet cleaning business in Fullerton, said he was shocked at the jolt of the earthquake, but more disconcerted by the aftershocks.

“They just keep coming,” he said. “There’s probably been a hundred, and we’ve felt a couple dozen. Every time the windows shake, we grab the dog and go outside. We’re a little jumpy.”