The quake measuring 5.2 percent on the Richter scale took place 20 kilometres (12 miles) underground in Niigata prefecture, around 200 kilometers (125 miles) northwest of Tokyo, the meteorological agency said.

The area was hit by a 6.8 magnitude quake on October 23 and hundreds of aftershocks, resulting in 38 deaths and forcing thousands to abandon their homes to live in temporary shelters.

Tokyo Electric Power said a reactor at the Kashiwazaki-Kariwa nuclear power plant automatically shut down as Thursday's quake hit.

The reactor remained shut hours after the tremor but there was no radioactive leak, a company spokesman said, adding five other reactors at the plant continued operations with another reactor closed for regular check-ups.

A bullet train ground to a halt during the quake but no passengers were injured.

The meteorological agency warned residents to brace themselves for more strong quakes.

"They need to be on the alert at least for the coming month," said Masahiro Yamamoto, who heads the earthquakes division of the agency.

More than 50,000 people evacuated or left homeless by last month's quake were still at shelters Thursday, according to the Niigata government.

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