system
Given the following post, respond as if you're commenting on Reddit:
human
[Avatar - The Last Airbender] Questions about the missing Moon

Wouldn't the Moon vanishing instantly cause earthquakes and tidal waves? After all, the planet's crust and oceans would "rebound" without the lunar gravitational pull.

Or was the missing Moon just an illusion? Like, by magical/miraculous means the Moon was not visible to humans anymore, and its powering to waterbenders was cut, but physically it was still there keeping the tidal bulges?
gpt
IIRC the moon doesn't completely disappear, it just gets much dimmer. So the moon is still physically present, it just isn't the source of spiritual energy it was before. 

You're also making a lot of (admittedly reasonable) assumptions about the way the world of TLA works. It's possible that the moon's influence on the Earth's tides is less of a physical effect than in our universe, or that the moon's presence wasn't missed for long enough for those changes to take place.