As /u/apis_cerana wrote in What did people think was going on during earthquakes, when they were not even defined as ‘earthquakes’ or as a scientifically phenomena?, popular superstition in pre-Meiji period Japan (prior to 1868) was that the giant catfish living underground cause the earthquake by trying to free from the yoke of the seal stone imposed by the gods (see this woodcut and another).

Alternatively, more sophisticated reasoning were either of the two I mentioned in How were earthquakes perceived by ancient cultures?: Celestial message/ warning to the ruler or doings of the evil aspects of the god (or recently deceased notable who had allegedly bore bitter malice towards the ruler or the whole society).

Instead of the dead link in my previous post, you can see some popular woodcuts including those of catfish (Namazu-E, literally translated as 'catfish illustration', published after the Ansei earthquake in 1854, in this link (sorry for only annotated in Japanese).