It doesn't. You will have to use the TM on it.
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It doesn't learn Earthquake through level up, but it can learn it through the use of the Earthquake TM.
not, you will need to use tm80, rock slide, on onix.
Aggron can learn earthquake by using the TM on it.
Grotle is not able to learn the move Earthquake by leveling or by TMs. It's evolved form Torterra is able to learn Earthquake at level 32.
Level 54 :D