http://www.smogon.com/dp/moves/seismic_toss
Seismic Toss doesn't list any of the Pokemon that can learn it through 3rd gen tutors. I don't have time to look through all of them, but at least Blissey and Clefable don't have it listed as a move either.
http://www.smogon.com/dp/pokemon/blissey/moves
http://www.smogon.com/dp/pokemon/clefable/moves