The two major earthquakes that have rocked the Japanese island of Kyushu since Thursday have renewed fears about the risks to a country dotted with nuclear plants, and given new voice to anti-nuclear campaigners.

The quakes that killed at least 42 people in Kumamoto prefecture prompted the closure of some main expressways and the suspension of local trains, including high-speed bullet trains.

These same roads and rail networks have been cited by senior officials as crucial evacuation routes in the event of a nuclear accident at the nearby Sendai Nuclear Power Station.

Satsumasendai city Mayor Hideo Iwakiri has previously said that the bullet trains could transport more residents faster should an accident occur.

"That [the transport shutdown] shows the city's evacuation plans are not workable at all," said Shoji Takagi, a leader of an anti-nuclear group in neighbouring Ichikikushikino city.

"It's totally wrong to restart nuclear reactors without viable evacuation plans," Takagi said.

The fear of an earthquake leading to a nuclear accident is still fresh in the minds of the Japanese following the country's worst nuclear accident five years ago.

"Just like the three reactors at the Fukushima Daiichi Power Station that suffered meltdowns [in 2011], the two reactors [at the Sendai plant] are ageing, more than 30 years old," Takagi added.

Operator Kyushu Electric Power reactivated the two reactors in August and October as the country still deals with the fallout of the meltdown at the Fukushima plant, caused by a massive earthquake - several degrees more powerful than the latest tremors - and resulting tsunami.

The disaster led to the shutting down of all of Japan's workable reactors in the months that followed. Nuclear-generated electricity made up about 30 per cent of the country's output before the accident.

By coincidence, the two Sendai reactors - located 130 kilometres south of Kumamoto prefecture where the quakes struck last week - were the first nationwide to be brought back online.

Japanese regulators have been unmoved by the critical voices, and said Monday it was not necessary to halt the two reactors.

"We will not make such a judgement [to suspend the reactors] unless there is a scientifically convincing basis," Nuclear Regulation Authority chairman Shunichi Tanaka told a news conference.

The government reiterated that there were no abnormalities at four nearby nuclear power plants, including the Sendai complex.

On Saturday, a group of prominent authors and journalists urged the authorities to exercise greater caution and halt operations at the plant.

"Based on the experience at the Fukushima Daiichi Nuclear Power Station, it is clear that it would be too late if you waited for some abnormality to occur," the group said.

The effect of the Kyushu quakes on the transport network, including severely damaged roads and landslides, have reminded locals of the risks from a nuclear accident, Kanna Misuta of Friends of the Earth Japan said.

"In case of a nuclear accident, evacuees are forced to travel a great distance. It's beyond comprehension," Mitsuta said. "They are once again frightened."

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