(Corrects paragraph 2 to show while eight people were pulled from the rubble two were confirmed dead)
By Joseph Campbell, Kaori Kaneko and Mariko Katsumura
KUMAMOTO, Japan, July 29 (Reuters) - 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 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, among them two dead women in their 20s.
Around 20 to 30 mall workers were unaccounted for on Tuesday, public broadcaster NHK said.
"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," Prime Minister Sanae Takaichi told reporters in Tokyo.
Authorities are investigating a possible gas explosion at the Aeon mall, media reported. Rescuers reported smelling gas inside the building but the cause of the incident was still under investigation, Japan's top government spokesperson Minoru Kihara said.
Emergency fire rescue teams, police and around 170 army personnel were focusing on areas of the building where calls for help had been received. More than 4,500 soldiers have been deployed to help with recovery efforts across the quake zone.
Seven people are also missing after a chimney collapsed at a Nippon Paper Industries factory, while four others are seriously injured, a local government official said. Hospitals were reported to be treating dozens of patients.
Across the prefecture, which also suffered a deadly earthquake a decade ago, around 260,000 people were instructed to go to evacuation centres, authorities said. Police said they received 386 emergency calls after the quake hit, including reports of collapsed buildings and people seeking rescue.
The quake’s epicentre was about 20 kilometres (12 miles) south of Kumamoto City, 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.
With more than 36,000 homes still without power as temperatures soar to around 34 degrees Celsius (93 degrees Fahrenheit) on Wednesday, authorities also warned about the risk of heat stroke.
SOME HOSPITALS OVERSTRETCHED
One hospital in Uki city, near the epicentre, said a power outage 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.
Several passengers aboard high-speed trains at the time of the quake were also injured, said operators, who immediately suspended all services after the jolt.
Public attention was focused on the collapse of the Aeon mall, the largest in the prefecture.
One side of the mall, which houses around 200 stores, was torn away, exposing steel beams and strewing debris across a parking lot, footage showed.
A spokesperson for mall operator Aeon said customers and employees were evacuated right after the initial quake and that the exact cause of the ensuing explosion was unclear.
Aeon shares fell nearly 2% in early trading in Tokyo. The benchmark index was up 0.8%.
Some major companies with plants in the area including Sony, Tokyo Electron and Honda said they would suspend operations at their plants through Wednesday.
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.
Some roads were also badly damaged, with large cracks splintering major highways and snarling traffic on Tuesday evening.
Located on the "Ring of Fire" of volcanoes and oceanic trenches partly encircling the Pacific Basin, Japan accounts for about 20% 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 2,739, 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.
(Reporting by Joseph Campbell in Kumamoto, Kaori Kaneko and Mariko Katsumura in Tokyo; Writing by John Geddie; Editing by Stephen Coates and Kate Mayberry)
