Tokyo: Rescuers were frantically searching on Wednesday for survivors of a 7.1-magnitude earthquake that shook southern Japan, killing at least 13 people, knocking out power to thousands of homes and rupturing roads across the region.

Eight people were pulled alive from the rubble of a partially collapsed shopping mall near the city of Kumamoto that was torn apart by an apparent explosion about an hour after Tuesday’s earthquake, killing two women in their 20s.

Around 20 to 30 workers at the Aeon mall in Kashima, just outside Kumamoto, were unaccounted for on Tuesday, public broadcaster NHK said. Emergency fire rescue teams, police and about 170 army personnel were focusing on areas of the building where calls for help had been received.

“Even now, there are people waiting to be rescued, and this is a race against time. We will marshal all available resources on the ground to save and rescue as many people as possible,” Japanese Prime Minister Sanae Takaichi told reporters in Tokyo.

Large-scale damage includes building collapses and fires, she added, urging people in the area to be wary of heatstroke, with temperatures forecast to hit about 33 degrees.

Senior government official Minoru Kohara said power and water supplies had been disrupted in some disaster-affected areas, and about 4600 military personnel were engaged in rescue and support operations.

The cause of the mall blast was still under investigation, Kihara told reporters at a briefing. According to Japanese media, authorities are investigating a possible gas explosion, with rescuers reportedly smelling gas inside the building.

A spokesperson for the mall operator said customers and employees were evacuated after the initial quake and that the exact cause of the explosion was unclear.

One side of the mall, which houses around 200 shops, was torn away, exposing steel beams and strewing debris across a car park, footage showed.

Kazuya Tsurunaga told TBS television he was stunned by the blast while cleaning up dishes broken by the earthquake inside a pub about 300 metres away.

“It was a huge impact,” he said. “A large cloud of smoke billowed up, and because the wind happened to be blowing, and the wind direction was towards the shop, the area around us was in a state like volcanic ash was falling.”

Takaichi said about 450 rest centres have been opened to shelter more than 7000 evacuated people, and measures were being taken to provide food, water, power generators and air conditioning amid the searing temperatures.

Seven people are also missing after a chimney collapsed at a Nippon Paper Industries factory, while four others were seriously injured, a local government official said.

Hospitals were reported to be treating dozens of patients. One hospital in the city of Uki, near the epicentre, said a power cut caused by the earthquake meant it was unable to function.

“It’s become like a field hospital,” the head of the administrative department told NHK.

Another hospital in the city said it had suspended admissions as it could no longer handle additional emergency cases after accepting 86 injured people following the quake, including three with serious injuries.

Across the prefecture, which also suffered a deadly earthquake a decade ago, some 260,000 people were initially instructed to go to evacuation centres, authorities said.

Police said they received 386 emergency calls after the quake hit at 4.27pm Japanese time (5.30pm AEST), about 20 kilometres south of Kumamoto, central Kyushu’s largest city with a population of around 700,000.

Authorities have warned residents in the areas that felt the biggest tremors to be on alert for further strong quakes for about a week, as well as for the risk of landslides.

Some hospitals overstretched

Several passengers aboard high-speed trains at the time of the quake were also injured, said operators, who immediately suspended all services.

Some major companies with plants in the area, including Sony, Tokyo Electron, and Honda, said they would suspend operations at their plants. TSMC, the world’s largest contract chipmaker, evacuated workers from its local plant as a precaution after the quake but said it had begun resuming operations late on Tuesday.

No irregularities were reported at the nuclear power stations in the area, Japan’s Nuclear Regulation Authority said. Kyushu Electric Power said 48,000 homes had lost power due to the quake.

Some roads were badly damaged, with large cracks appearing in major highways and causing severe traffic tailbacks on Tuesday evening.

Located on the “Ring of Fire” of volcanoes and oceanic trenches partly encircling the Pacific Basin, Japan accounts for about 20 per cent of the world’s earthquakes of magnitude 6.0 or more.

A massive earthquake in Kumamoto 10 years ago killed 275 people and injured a further 2739, according to an official count, and damaged thousands of buildings, including the walls of the city’s castle, a top tourist site.

The castle was also among several historic buildings damaged in Tuesday’s quakes, alongside shrines with heavy tiled roofs that partially collapsed and a 17th-century tea house.