The Sounders went with what could be its regular starting lineup through the scoreless first half before getting second-half goals from Martinez and trialist Phillip Lund.
Martinez entered the game as a reserve just one day after his first 2013 training session with the club. His arrival in camp had been delayed by duty with the Honduras national team, which was followed by an excused absence while tending to business in Honduras.
In the 66th minute of his season debut, he swooped onto a poorly cleared ball and sent it into the far-side netting from the top of the penalty area.
Lund doubled the Sounders' lead three minutes later. That play began with RSL goalkeeper Nick Rimando blocking a shot by Seattle's Osvaldo Alonso. The rebound went to Lund, who swept it into the goal.
Salt Lake got on the board in the 87th minute, when John Stertzer sent a deflected ball past reserve Seattle goalkeeper Josh Ford.
Sounders coach Sigi Schmid had said he would play the first half with his current starting 11. That turned out to be goalkeeper Michael Gspuring, defenders Adam Johansson, Zach Scott, Jhon Kennedy Hurtado and Leo Gonzalez; midfielder Mauro Rosales, Alonso, Brad Evans and Steve Zakuani; and forwards David Estrada and Eddie Johnson.
All of those played through the scoreless first half, with the exception of Johansson, who was replaced by homegrown player DeAndre Yedlin in the 28th minute.
Schmid went mostly with reserves through the second half, resting his starters for their Friday friendly against Veracruz of Mexico.
Seattle is 2-0-0 in the Desert Diamond Cup, while Salt Lake stands at 0-1-1.
The Sounders' next match is 6 p.m. Wednesday, when they conclude the round-robin part of the tournament against the New York Red Bulls.
The competition concludes Saturday with the championship and third-place games.
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