It seems usefull, but I have no idea where it is or if you can even get it...
You can find TM 26(Earthquake) in Wayward Cave or you can purchase one at the Battle Frontier.
Here's a video where a guy shows you how to obtain it.
Deep in the wayward cave i would suggest a pokemon with flash go in to the entrance to the left or else you will be in the wrong one or the battle frontier for 80 bp
Go to Wayward Cave (look here: http://pokemondb.net/pokebase/11418/in-diamond-how-do-i-get-gabite ) and at the very end you will find a small room with a pokeball with TM 26 Earthquake!