On July 16, a magnitude-6.8 earthquake struck Niigata Prefecture, causing widespread damage and an emergency shutdown of four of the seven reactors at the Kashiwazaki-Kariwa nuclear power plant. (The remaining three reactors were undergoing regular checks.) The Chuetsu-oki Earthquake touched off a fire and caused considerable damage to equipment at the plant.
Since they require a large amount of cooling water, nuclear power plants must be located on a seacoast. As a result, they are exposed to risks of earthquakes originating under the seafloor.
When seeking official approval of plans to build nuclear power plants, utility companies are required to make sure that the plants are sufficiently quake-resistant. The fact that the Kashiwazaki-Kariwa power plant, one of the world's largest and most advanced of its kind, suffered such damage demonstrates that the scale of the quake exceeded designers' expectations.