In a meeting of the conference's top two clubs, Seattle won its fifth in a row and its eighth out of nine.
The Sounders (15-8-4) jumped past Salt Lake into a one-point lead and also has two games in hand.
Martins gave Seattle a quick lead when he sent a pass from the top of the penalty area to Andy Rose on the left wing. Rose quickly sent the ball back to Martins, who was unmarked in front of the goal and easily beat Nick Rimando.
Lamar Neagle added a goal for the Sounders in the 38th minute.
'SNL': 'Anchorman 2' cast, One Direction sing 'Afternoon Delight' [VIDEO]
Selena Gomez drops F-bomb, walks off stage during Jingle Ball performance