Namazu is a giant catfish who causes earthquakes. He lives in the mud under the islands of Japan, and is guarded by the god Kashima who restrains the catfish with a stone. When Kashima lets his guard fall, Namazu thrashes about, causing violent earthquakes.
Following an earthquake near Edo (modern day Tokyo) in 1855 (one of the Ansei Great Quakes), the Namazu became worshiped as a yonaoshi daimyojin (god of world rectification). Namazu-e (catfish prints) are a minor genre of ukiyo-e. They are usually unsigned and encompass a large variety of scenes such as a namazu forcing the wealthy to excrete coins for the poor, and a namazu atoning for the earthquake he caused.
It is believed by some that the origin of the story is the notion that catfish can sense the small tremors that happen before many earthquakes, and are more active at such times. Supposedly, the sudden activity was observed in ancient times and people believed the quakes to be the result of a giant catfish.
- Namazu on Wikipedia