Traber outpitched Seattle's Freddy Garcia, shutting the Mariners down over his seven innings of work en route to a 3-0 Indians win.
He evened his record at 6-6 on the season, striking out five batters along the way.
Garcia (9-12) pitched better than he had over the past month, but still came out on the losing end. He struck out seven and walked none in 6.1 innings of work.
Danys Baez pitched a scoreless ninth for his 24 save of the year.
The game was tied through three innings of play before Cleveland's Jody Gerut belted his 14th homer of the year off of Garcia in the fourth. Travis Hafner, who had a three-run homerun Wednesday, went deep again Thursday with a solo shot off of Garcia in the fifth.
Cleveland improved to 47-67 on the year.
Despite the loss, Seattle (69-45) kept its three-game lead over Oakland as the Athletics lost earlier in the day to Detroit.
The Mariners managed seven hits on the day, led by the 2-3 day by newcomer Rey Sanchez. The veteran shortstop is batting .375 since joining Seattle.
Bradley and Hafner each had two for the Tribe.