Question

Is there a earthquake TM for platinum before the E4?

Answer 1

Yes it can be found in wayward cave.
Just follow the video: http://www.youtube.com/watch?v=hQ0zApftrz0
Also they can be found using pickup or being transfered from Pokemon battle revolution.
Source: http://bulbapedia.bulbagarden.net/wiki/TM_26

Answer 2

TM26 (Earthquake) is found in Wayward Cave in DPPt, requiring Strength. Directly quoting Smogon,

In the hidden area of Wayward Cave that's under the Cycling Road, go through the puzzle to get to the hidden room. In that hidden room, you'll find the TM for Earthquake

TM26 can also be found on Pickup Pokémon that are from levels 91-100. It can also be purchased from the Battle Tower/Frontier for 80 BP.