Torrential downpours, unstable ground and tardy evacuation advisories all contributed to the tragedy that had claimed the lives of at least 39 people in Hiroshima as of Thursday afternoon.
Experts agree unusually heavy rains overnight overwhelmed the fragile mountain soil on which many homes were built, triggering mudslides Wednesday in the city of Hiroshima.
“The biggest reason was torrential rain,” said Susumu Yasuda, a professor of geotechnics at Tokyo Denki University. “Moreover, the ground in the affected areas is decomposed granite, which crumbles easily” when saturated.
A record 217.5 mm of rain fell from 1:30 a.m. to 4:30 a.m. in Asakita Ward, one of the hardest-hit areas. But the Hiroshima Municipal Government didn’t issue its first evacuation advisory until 4:15 a.m., which, a city official admitted, was too late.
Firefighters said they started getting information by phone that landslides had occurred at around 3:20 a.m., Kyodo News reported.
Hiroshima Mayor Kazumi Matsui voiced regret over the late advisory, saying the city will look into what can be done to improve the system.
Kensaku Kawabe, a section chief at the land ministry’s Otagawa River Office in the city, agreed with Yasuda’s assessment. He noted that Hiroshima Prefecture is the most landslide-prone area in the country.
According to land ministry figures, the prefecture has 31,987 areas at high risk of landslides. Next on the list is Shimane Prefecture with 22,296 areas and Yamaguchi with 22,248.
“Most of the ground in the city of Hiroshima, Hatsukaichi and Otake (under the office’s jurisdiction) is made up of granite, which poses a risk of crumbling if weathered,” Kawabe added.
Mitsuharu Hiura, an official at the Sediment Control Division of the prefectural government, said 40 to 50 percent of the prefecture is believed to be covered by decomposed granite.
Hiroshima Prefecture is no stranger to such disasters. In June 1999, mudslides triggered by rain left 32 people dead or missing.
After that, the prefectural government tried to prevent further disasters by, among other tactics, building embankments in valleys to stem mudslides and lining cliff faces with cribwork, Hiura said.
But Hiura admitted that more work remains to be done.
“We couldn’t finish the infrastructure work due to a large number of dangerous locations and limited budget,” Hiura said.
Yasuda of Tokyo Denki University said there is a limit to what can be accomplished on a local scale.
“I know (the prefectural government) has been building infrastructure to prevent disasters,” he said. “But those preventive measures haven’t caught up with the disasters, which have occurred in unexpected locations” due to heavy rain.
Yasuda also noted that the residential areas were built on cleared mountain land.
“There was not much flatland available for building in Hiroshima in the first place, which limited the city’s options to clearing mountains or using landfill,” Yasuda said.
He warned that as rain patterns shift over the years, similar disasters could strike anywhere in Japan. He stressed that people need to think about where they live, given the limits building technology.
“One idea might be to not build residential areas near (mountain) slopes in anticipation of possible landslides,” Yasuda said.
As of Thursday evening, the death toll stood at 39 and the number of missing rose to 51. Keiji Furuya, minister in charge of anti-disaster measures, inspected the site of the landslide Thursday and said that evacuation orders had been late.
Information from Kyodo added