Answer:
Tsunamis form when there is an earthquake, an eruption or something falls into the ocean. The large impact on the water causes the water to back up. That is why the shoreline before the tsunami hits goes all the way back. The water gathers up into a wave and hits the city causing catastrophic damage.
Explanation:
https://youtu.be/F_kxubV9FLI This is a video of the Water receding. I would do more research though because I didn't list all the details
