earthquake is usually played on the last activation, when you just have to wait for your opponent to pass, then you earthquake him away from the 3 objectives he took all turn to grab.
It's a generic card, and it's too easy to mess up an objective based deck, as it does not prevent you to go along with your bashing around plan, and just ruins 1 complet turn for the opponent.
hate that card, but I play skaven and the guard, go figure ^^