Boston clinched home-field advantage throughout the postseason with Oakland's loss. The AL West champion A's will play their playoff opener at home next Friday against AL Central champion Detroit, and AL East winner Boston will start at Fenway Park against the team emerging from the wild-card playoff: Cleveland Tampa Bay or Texas.
Brandon Maurer (5-8) gave up two runs and six hits in 5 1-3 innings. He had been 0-1 in three starts and three relief appearances since beating the A's on Aug. 20.
Danny Farquhar pitched a perfect ninth for his 16th save in 20 chances.
Jarrod Parker (12-8) allowed seven runs and seven hits - including three homers - in 4 1-3 innings. It was the second time in three starts he allowed seven or more runs without making it through the fifth inning.
Justin Smoak's career-best 20th homer, a two-run drive. put Seattle ahead in the second. Coco Crisp hit a sacrifice fly in the third, and Miller's homer made it 3-1 in the bottom half. Miller's slam in the fifth was Seattle's seventh this year, second in the majors behind Boston's nine.
Oakland's Brandon Moss and Alberto Callaspo hit back-to-back homers in the seventh inning off Chance Ruffin. Moss's two-run drive was his team-leading 30th.
The A's failed to score after loading the bases with two outs in the sixth and eighth innings.
NOTES: Seattle manager Eric Wedge said Saturday his uncertain contract status wasn't the reason he decided not to return next season. "If they'd offered me a 5-year contract, I'm not coming back here," he said. Instead, Wedge said the reason behind his departure was a difference of opinion between him and the Mariners' front office: CEO Howard Lincoln, president Chuck Armstrong and GM Jack Zduriencik. "My vision of the future and theirs are just different," Wedge said. ... Oakland OF Yoenis Cespedes was out of the lineup a day after leaving in the fourth inning with shoulder stiffness. Manager Bob Melvin said Cespedes was day to day.