Taiwan prosecutors Wednesday questioned the developer of an apartment complex that collapsed during an earthquake as prosecutors detailed flaws in the construction of the building where nearly 100 people remain trapped.

The prosecutors in the southern city of Tainan launched an investigation into Saturday's disaster after photos showed cans and foam had been used to fill parts of the complex's concrete framework.

The district court, which earlier ordered the developer's arrest, said some reinforcing bars in the concrete of the 16-storey structure were too thin or too short.

The court took developer Lin Ming-hui, and two other people connected with the development, into formal custody late Tuesday on charges of professional negligence resulting in death.

So far 43 people have been confirmed dead and 95 are still missing after the collapse of the Wei-kuan complex.

It was the only high-rise in Tainan to crumble completely when the 6.4 magnitude quake struck before dawn Saturday.

"The suspects are being questioned further today, but as it is underway, details are not likely to be made public," Tainan court spokeswoman Kuo Jen-shiow told AFP. Prosecutors were not immediately available for comment.

The city government said it would seek a court injunction next Monday to stop the developer disposing of his assets while facing lawsuits.

According to a court statement prosecutors found that the beams and pillars of the first five floors on the east side of the building had too few reinforcing bars, and the bars were thinner than they were supposed to be.

It said samples from the third floor of the site also showed that the bars were shorter than required.

"Apparently in this case, there were indeed flaws in the construction of the building," the statement said.

Rescuers were using diggers and other heavy equipment to remove giant concrete slabs to access the deeper layers of rubble where scores of people are feared buried.

The 72-hour "golden window" for finding survivors passed early Tuesday. But near midnight Tuesday Tainan mayor William Lai offered fresh hope, saying rescuers had detected signs of life.

"After detecting signs of life, the rescue team then banged in the direction of the signs three times, and they got a response three times. This shows there's a living person inside," the mayor told a press conference.

But more than 17 hours later, no one has been pulled out.

Some 175 people have been rescued, including an eight-year-old girl and three others pulled from the rubble Monday.

Cranes, drills, ladders, sniffer dogs and life detection equipment are being used to locate those buried, with emergency workers and soldiers shoring up the rubble to avoid further collapses.

The quake struck two days before Lunar New Year, when many people would have been visiting relatives for the biggest celebration of the Chinese calendar.

Premier Chang San-cheng ordered the Taiwanese flag flown at half-mast Monday to pay tribute to the victims.

Taiwan lies near the junction of two tectonic plates and is regularly hit by earthquakes.

The island's worst tremor in recent decades was a 7.6 magnitude quake in September 1999 that killed around 2,400 people.