CHRISTCHURCH, New Zealand — Workers continued to sift through the rubble for survivors of Tuesday’s 6.3-magnitude earthquake here, but officials said hopes had dimmed that those buried would be found alive.

Despite that gloomy assessment, volunteers pulled two people from the ruins of one building Wednesday after hearing the cries of a woman in the wreckage. The official death toll remained at 75, with scores missing.

As Christchurch settled in for another uneasy night, family members of the missing waited for word under a steady drizzle and cold temperatures. Police imposed a 6:30 p.m. curfew in the city center as rescue teams continued their search, and violators faced arrest.

On Wednesday, international flights returned to Christchurch, but more people scrambled to get out of the city than in, with dozens camped out at the airport waiting for flights. Many tourists traveled without luggage or even passports, after they fled hotel rooms without any belongings.

At Canterbury Television, where 200 people worked, searchers at 1 a.m. Thursday brought in floodlights and a digger to sift through the still-smoking ruins. On Wednesday, officials had declared that no one would be found alive there but later returned to search some more. They said more than 100 people may have been lost in the CTV building and that the devastation was not survivable, so they were concentrating on recovery of bodies rather than rescue.

Earlier Wednesday, officials said some trapped victims were text messaging for help.

Others could be heard tapping out signals as aftershocks continued to hit the devastated city, the island nation’s second largest, causing thousands of skittish residents to crowd into temporary shelters, schools and community halls. Many tourists who had abandoned their hotels after the temblor struck at midday Tuesday huddled in hastily pitched tents under a drizzling rain.

Prime Minister John Key said at a news conference that only 55 of the 75 confirmed dead had been identified. He declared a national emergency, giving the government wider powers to respond to the crisis.

Working under floodlights Tuesday night, rescuers used dogs to locate the injured and dead and heavy equipment to pick away at piles of concrete, brick and crumpled metal. Using only their hands, they threw aside heavy slabs to free some victims who were scarcely a bruise. Others had to have limbs amputated to be extricated, police Supt. Russell Gibson told reporters.

Among nearly a dozen buildings where survivors were believed still trapped, two in particular suffered major damage and were the focus of rescue efforts.

Authorities said 22 people had been rescued from the Pyne Gould Guinness Building and another 22 were believed still to be under the rubble. Eight were pulled from the Canterbury television building, with a large number still unaccounted for.

Rescuers reported hearing screams for help from trapped victims, some of whom spoke with friends and relatives by cellphone. One woman called her children to say goodbye.

Authorities said the death toll was evolving.

“”We know it will be significant,”” Gibson said, “”and we know there are a lot of people going through the nightmare ordeal of waiting for news of their loved ones.””

Much of the city remained without water, although power and telephone service had been restored to many areas. With wastewater pumping stations out of action, raw sewage was being pumped into the Avon River.

More than 235 police volunteers arrived in the city from throughout New Zealand and the region, including specialized disaster-victim identification teams, urban search-and-rescue staff and police-dog units.

Meanwhile, frantic calls from relatives seeking information on family members were pouring into crisis hotlines staffed by local volunteers.

Seismologists said the shaking produced by the shallow quake was some of the strongest on record. The damage to reinforced buildings, some of which were constructed following standards used by quake-prone areas such as California and Japan, will prompt a reevaluation of current building codes, experts said.

Jason Ingham was at a Christchurch hotel preparing for a seminar on earthquake building standards Tuesday when the hotel began to shake. He ran outside and saw people climbing out of the top floors of a hotel across the street, which was tilting to one side. The scene downtown quickly turned to chaos.

“”There was a mass exit of people in every direction,”” Ingham said. “”The first mode was just to survive.””

Prime Minister Key said Christchurch had suffered “”utter devastation.””

“”We may well be witnessing New Zealand’s darkest day,”” Key said.

More than 400 rescuers from around the world were expected to join the search effort, including a 74-member team from the Los Angeles County Fire Department.

The quake was centered just southeast of Christchurch, which sits midway down the east coast of New Zealand’s South Island. The shaking ripped down walls of older masonry churches and historic buildings in addition to damaging newer structures.

The highest ground acceleration recorded was close to twice the force of gravity, said Susan Hough, seismologist with the U.S. Geological Survey in Pasadena.

“”That’s quite extreme shaking”” and stronger than the movement in Haiti last year and Northridge in 1994, she said.

The damage to earthquake-resistant buildings may indicate “”we still have some things to learn”” about improving building codes, Hough said.

The quake also provided dramatic evidence of the threat a major earthquake poses even after the jolt may have begun to fade from memory. Seismologists said Tuesday’s quake was an aftershock to a magnitude 7.1 quake that hit New Zealand on Sept. 4. That quake was deeper and caused no fatalities.

Thomas Jordan, director of the Southern California Earthquake Center at USC, said the most destructive quake can be an aftershock to the main quake. Aftershocks of New Zealand’s September quake have been moving eastward in recent months, closer to Christchurch.

“”Earthquakes don’t happen individually, but in sequences. We have to be prepared that when we get large earthquakes in California that we recognize that that means the seismic hazard has gone up, not down. One earthquake does not mean the end of story,”” Jordan said.

New Zealand’s worst earthquake, on the North Island, hit in 1931 and killed more than 250 people.