Gas leak triggered rare supersonic detonation in Kumamoto Aeon Mall blast, expert says

The massive explosion that killed at least six people at the Aeon Mall in Kumamoto was likely caused by a rare and destructive supersonic detonation of leaked gas triggered by the magnitude 7.1 earthquake, according to a Japanese fire expert. 

The blast occurred at around 6pm on July 28, roughly an hour after the powerful earthquake. 

Aeon Mall confirmed that staff helped guide roughly 200 customers to the outdoor parking lot to evacuate. However, several employees reportedly returned inside to inspect the premises or assist with cleanup, leaving them trapped in the rubble when the explosion occurred. 

Aerial footage and internal images released by local police and fire authorities showed severe damage to the mall’s south side. Large sections of the roof were blown off, and exterior walls collapsed along tens of metres, with shattered ceilings, loose wiring and scattered debris.

Yasuo Sato, former Hyper Rescue Chief of the Tokyo Fire Department who helped coordinate emergency response during the 2011 Japan earthquake, described the blast as a “detonation,” a rare, powerful form of explosion involving supersonic shockwaves. 

Citing the hydrogen explosions that devastated the Fukushima Daiichi Nuclear Power Plant in 2011, he said that stripped exterior walls and fractured columns at the mall match damage patterns caused by detonation.

On the cause, Sato pointed to a gas leak as the most likely trigger. Strong earthquake shaking could have fractured shared supply lines, releasing massive amounts of high-power propane used by food court restaurants.

Although commercial gas is typically mixed with odorants, detection may have been delayed by post-quake chaos and the mall’s vast size. With the gas trapped inside a tightly sealed, air-conditioned environment, it likely built up rapidly within about 30 minutes of customer evacuation before encountering an ignition source.

Police, firefighters, and the Self-Defense Force are currently clearing the site and probing into the incident.

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