What happened

A magnitude-6.7 earthquake struck near Atsuma, Hokkaido, in the early hours of 6 September 2018, triggering landslides and damaging the Tomato-Atsuma coal-fired power station, which normally supplied roughly half of Hokkaido's electricity through its three generating units. The plant's shutdown removed the island's largest single generation source and cascaded into a complete, island-wide blackout affecting an estimated 5.3 million people and 2.95 million households — the first time an entire Japanese prefecture had lost power simultaneously. Hokkaido's limited interconnection to Japan's main island of Honshu, capable of transferring only around 600 megawatts, could not compensate for the lost capacity. By Friday afternoon (7 September), just over half of households had power restored, and Japan's industry minister set a goal of restoring 80% of households by the end of that day; full restoration across the island took approximately one week. Post-event analysis, including Japan's Organization for Cross-regional Coordination of Transmission Operators (OCCTO) final report, identified structural grid vulnerabilities: over-reliance on a large, centralized coal plant in a paradigm with insufficient geographic distribution of generation, and limited transmission capacity between regions, meaning the loss of one facility could cause catastrophic cascading failure across an entire region despite Japan's frequent exposure to natural disasters. This was an accidental, earthquake-triggered infrastructure failure, not an attack.

When cited sources give different casualty, damage or spill figures for this event, they’re reported side by side above rather than merged into one number. See how confidence levels work.