Decomposing tatami mats, traditional flooring materials widely used in Japan, could have helped building debris ignite after the Tohoku earthquake and tsunami.

Decomposing tatami mats, traditional flooring materials widely used in Japan, could have helped building debris ignite after the Tohoku earthquake and tsunami.

Hiroshi Koseki, National Research Institute of Fire and Disaster

Why did rubble piles spontaneously combust in aftermath of Japanese quake?

Something strange happened in the aftermath of the 2011 earthquake and tsunami that pummeled Japan. Months later, mysterious fires began breaking out in piles of brick and wood from damaged buildings. Researchers puzzled over what sparked the fires, but a new study offers a possible explanation.

Throughout eastern Japan, shaking earth and roaring waves destroyed more than 127,000 buildings and damaged more than a million others, according to the country’s National Police Agency. In some places, cleanup crews could do little more than rearrange the debris—by some estimates as much as 25 million metric tons of it—into piles up to 10 meters high. The situation was already bad—and then the piles began to erupt into flames.

“After March 2011, we experienced about 40 fires,” says Hiroshi Koseki, a fire safety engineer at the National Research Institute of Fire and Disaster in Chofu, Japan, and co-author of the new report published in the current issue of Fire and Materials. They ignited as much as 6 months after the earthquake, he says, and some burned for days. The fires didn’t cause any additional damage to buildings, but one incinerated 18,000 cubic meters of material, enough to fill about seven Olympic-size swimming pools. Speculative news reports warned that radioactive material from the imperiled Fukushima Daiichi Nuclear Power Station could be to blame. That’s when local fire departments called in Koseki and his colleagues.

The team immediately suspected self-ignition, spontaneous combustion that occurs when mounds of organic material like mulch and hay heat up internally until they catch fire. They searched burn sites for materials that might have ignited, collecting samples of wood chips, industrial sludge, and fresh and rotten tatami, a commonly used floor covering made of rice straw. Using calorimeters and other thermal measuring devices, they observed how the samples responded to the temperature and moisture conditions likely to develop inside a rubble pile warming under the sun and exposed to the elements.

Rotting tatami posed the greatest fire risk, the researchers found, followed by fresh tatami and wood chips. The key is microbial activity, they determined. Under the right conditions, bacterial fermentation begins inside a mound of rubble, generating small amounts of heat. With poor air circulation, the biochemical reactions create a feedback loop, heating and reheating the mound until it ignites. Paradoxically, adding water increases a pile’s chances of flaring up by allowing microbes to simmer at temperatures that would kill them in dry piles.

The sludge samples themselves didn’t generate much heat, but the moisture they contained could still help other materials ignite, the researchers found. Within a certain range of temperature and moisture conditions—even after months of inactivity—a rubble pile rich in tatami debris can smolder back to life, they concluded.

“The topic is fascinating,” says Guillermo Rein, a mechanical engineer at Imperial College London who wasn’t connected to the study. He applauds the authors’ approach, but he cautions that they left out one important factor that changes radically from one site to the next in Japan: the size of the pile. “There is what is called a critical size,” he says. Because of how heat transfers, only piles larger than this critical size will ignite, meaning rearranging debris into smaller piles could prevent fires from breaking out.

Because disasters like this are so rare, the results shouldn’t influence how people dispose of their tatami mats, Koseki says, but they could help authorities prevent similar fires after future disasters. Some clear steps include monitoring the internal temperature of rubble piles and scattering them before heat from fermentation builds up to dangerous levels.

The cleanup from the disaster continues. Koseki says it could take years to remove the remaining rubble piles—and some are especially risky. “We have to take care, because there are huge amounts of materials around the Fukushima nuclear plant,” he says, concerned that radioactive materials in the debris could be released into the atmosphere all over again if ignited.