A dozen rescue workers dug with shovels through rocks in Nagaoka, one of the cities most severely affected by the quake, to get into contact with the family trapped in their car.

"Specially trained rescuers sent from Tokyo have confirmed all of the three are alive," said Kazuhiro Noguchi, a local government spokesman, "but either the mother or the girl needs to receive life-saving operations."

The two-year-old boy was lifted into a helicopter hovering above and rushed to hospital, as national television broadcast the saga live.

Mother Takako Minagawa, 39, her daughter Mayu and her son Yuta were all in the car when it was buried by a landslide during the initial quake of 6.8 on the Richter scale Saturday.

At least 31 people have died since Saturday's initial tremor and from stress amid the hundreds of aftershocks.

Quake survivors grappling with the aftermath experienced another tremor measuring 6.1 on the Richter scale which was powerful enough to sway buildings in Tokyo, 200 kilometres (125 miles) to the southeast.

The tremor sent quake victims in shelters hurriedly lying down on the floor for their safety. About 86,000 people are still lodged in hundreds of shelters across Niigata province.

Public broadcaster NHK said some 20 people were injured in the latest tremor.

In Ojiya city, the hardest hit by the initial quake, an apartment building collapsed and damaged an adjacent clinic but two residents inside were rescued unharmed, a municipal official said.

The latest tremor also triggered at least one landslide and violently shook elevated rail tracks of Japan's bullet train -- which was derailed for the first time in its 40-year history by Saturday's quake.

Workers who had been trying to lift derailed train cars were seen scurrying back as the wagons shook with the new tremor. The Niigata train station was shut down for several hours Wednesday amid fears it would collapse, a local official said.

The meteorological agency called on Niigata residents to be fully alert.

"We want people to brace themselves for tremors of lower six or even upper six" on the Japanese method of measuring earthquakes which goes up to seven, said Masahiro Yamamoto, who heads the agency's earthquakes division.

The initial quake of 6.8 on the Richter scale hit the Japanese reading of upper six, the intensity under which people cannot stand but have to crawl.

Nearly 2,530 people have been injured by earthquakes in Niigata region since Saturday.

The earthquake was the deadliest to hit tremor-prone Japan since 1995, when 6,433 people were killed in the western city of Kobe.

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