The tremor struck at 1.25am local time and triggered a landslide that swept away homes and blocked a highway in the area of Minami-aso.
An active volcano then erupted a few hours later, but it was unclear whether this was linked to the earthquakes. The small eruption occurred at Mt. Aso, with smoke rising about 300 feet high.
The earthquake triggered a tsunami alert, although this was later lifted. There were no problems reported at three nuclear power plants in the area.
Yoshihide Suga, the chief cabinet secretary, said that nearly 80 people were believed to be trapped or buried in rubble. Soldiers would be sent to help the rescue effort, with up to 15,000 due to arrive in the area on Saturday, as well as more police, firefighters and medical personnel. "We are making every effort to respond," said Mr Suga.
The epicentre of the quake was near the city of Kumamoto and measured at a shallow depth of 6 miles, the US Geological Survey said. Almost 200,000 households were without power.
Heavy rain is forecast in the coming days, which could cause more landslides.
A magnitude 9 earthquake in March 2011 north of Tokyo touched off a massive tsunami and a meltdowns at Fukushima nuclear power station. Nearly 20,000 people were killed by the tsunami.
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