* Blast tore through mall an hour after earthquake

* Mall operator investigating possible gas leak

* Witnesses describe panic, dust and chaos before explosion

* Rescuers in "race against time" to find survivors

By Anton Bridge, Yuka Obayashi and Joseph Campbell

KUMAMOTO, Japan, July 29 (Reuters) - Workers in a mall in

southern Japan described being thrown to the floor or enveloped

in a cloud of dust by a massive 7.1-magnitude quake that rattled

the building on Tuesday. They were the lucky ones. An hour

later, the real disaster struck.

Hundreds of shoppers and store employees, quickly ushered by

staff to evacuate, watched on in horror as an explosion ripped

through the building, exposing the structure's steel beams and

catapulting debris across the parking lot.

First responders, initially fearful of entering the wreckage,

hauled eight people from the partially collapsed site early on

Wednesday, among them three dead. The status of four store

workers remains unknown, mall operator Aeon said,

adding that it was investigating a possible gas leak.

"I thought a bomb had gone off," Takateru Sonoda, 41, who

runs a combined hair salon and cafe near the mall, told news

agency Jiji Press.

"There were people calling on others to evacuate and sirens

were ringing out. Nearby roads were gridlocked, it was a state

of panic," Sonoda said.

Japanese Prime Minister Sanae Takaichi said on Wednesday it was

a "race against time" to rescue those still trapped in the mall

and others affected by the quake. Around 170 soldiers have been

deployed to help with rescue operations at the site.

MALL TOUTED SAFETY FEATURES

The mall, the biggest in the prefecture, had just reopened

last month as a new and improved shopping and entertainment

complex to commemorate its recovery from another deadly tremor a

decade ago.

After being badly damaged in the magnitude 7.3 Kumamoto

earthquake in April 2016, the operator highlighted its

seismically reinforced ceilings and overall improvements to

safety and resilience as part of its redevelopment.

Still, one worker manning one of the mall's 200 stores told the

Yomiuri newspaper that Tuesday's temblor shook the building for

10 seconds and brought him to his knees.

Another 22-year-old employee at the mall's cinema told the

newspaper that the quake threw up a disorientating cloud of

dust.

Despite the chaos, staff promptly followed protocol to evacuate

more than 200 shoppers and workers to a car park where many took

shelter in their vehicles in sweltering summer heat, the workers

said.

Some people, however, went back into the building prior to

the explosion, Kyodo news agency reported, citing customers.

Kazuya Tsurunaga was stunned by the blast while cleaning up

dishes and glasses broken by the earlier earthquake inside a pub

about 300 metres (328 yards) from the mall.

"A large cloud of smoke was billowing up, and because the

wind happened to be blowing, and the wind direction was toward

the shop, it was like volcanic ash was falling around us," he

told broadcaster TBS.

For the first few hours after the blast, police officers and

firefighters could not enter the wreckage because of the risk of

further building collapse amid a series of aftershocks, a police

official said. By the end of Tuesday, more than 100 aftershocks

had occurred in Kumamoto.

GAS WARNINGS

Video footage shot by a police officer who braved the wreckage

early on Wednesday showed large sections of the ceiling had

collapsed, store fronts had been blown open and furniture was

strewn across the floor.

Some rescue workers reported smelling gas, Japan's government

spokesperson Minoru Kihara told a press conference on Wednesday.

A TV correspondent reporting from the site said an emergency

vehicle repeatedly urged people not to get near the mall due to

a possible gas leak.

Kihara said the mall used LPG (liquefied petroleum gas), which

is transported in large canisters rather than via a pipeline.

Piped gas is automatically shut off by network operators if

seismic activity reaches a certain threshold or damage is

confirmed to the line, a spokesperson for the Japan Gas

Association said.

But LPG also has such safety protocols, an official with the

Japan LPG sales association said. Microcomputer-controlled

meters automatically shut off supply if they detect

abnormalities such as leaks or pressure fluctuations, and are

designed to cut supply during seismic activity of intensity 5 or

higher, the official said.

Gas suppliers and safety service providers can also remotely

shut off supply via radio transmission in emergencies, he added.

Aeon declined to comment on its gas supplier or gas safety

protocols.

"We would like to express our deepest condolences to those

who lost their lives," the firm said. "We also extend our

sincere apologies to the bereaved families and to those who were

injured."