To unlock Earthquake you must kill 250enemies with the earth charm equipped. The earth charm is located in the prologue.
You cycle through your charms by using LB and RB. Select earth.
Now kill 250 enemies with either your sword or your bow. The enemy does not have to be weak to earth, although that helps speed up the killing process.
If you are looking for a grinding spot, for earth kills the first level or the final level are as good as any.