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.