Once the earthquake was detected, the active reactors in Fukushima automatically began shutting down their fission reactors. This action caused a domino effect where the electricity failed and prompted the generators to begin working.
The earthquake also caused a massive tsunami that was reportedly 14 meters high. The waters swept over the plant’s seawall and completely flooded the lower grounds. It also caused the plant to flood and knock out the emergency generators. The amount of damage done to the reactors at the time of the disaster is not precisely known.