Miller became the first Mariner ever to have three multi-home run games in a rookie season and the first Major League rookie to do so since Ryan Braun of the Brewers (three) and Chris Young of the D-backs (five) in 2007.
"I don't know, but I'll take it," Miller said of his penchant for coming through in pairs. "It felt good up there and I'm glad I could drive the ball and drive some runs in."
The five RBIs equaled Miller's season high and give him 36 for the year since taking over as the club's starting shortstop at midseason.
"He swung the bat well for us today," manager Eric Wedge said. "He's up there ready to hit and he's doing a better job of getting on top of that high fastball that he likes. You really need to get your top hand involved with that and he's done a better job of that. It was obviously big for us, especially the way they came back on us."
Maurer, 23, allowed two runs over 5 1/3 innings with six hits, one walk and five strikeouts. Maurer, who made the jump from Double-A ball to earn a spot in the season-opening rotation, finished the year 5-8 with a 6.30 ERA.
The rookie from Costa Mesa, Calif., closed out on an uptick with a 2.55 ERA over his last three starts after returning to the rotation from a long-relief stint.
"It felt good," said Maurer. "I got to put everything together here in this last one, working on mound presence and just going out there and trusting my stuff. Early on [in the year] I was going out there trying to throw the best pitch or throw a little harder. I just have to go out and trust what I've got."
Wedge has announced he won't be back next year, but the third-year manager said Maurer took big strides this year and will help the club in 2014.
"I'm so happy for him," Wedge said. "You look at his last three outings and he's been really consistent and really making strides in regard to his composure and handling his emotions out there. And his stuff has played up because of it.
"They're a tough team to play against because they've got you matched up both ways with the platoon situation with their righties and lefties. It's tough to match up against them, but he did a really nice job again today. He really has something solid to take into the offseason."
First baseman Justin Smoak started things off for Seattle with a two-run homer in the second inning, giving him a career-best 20 long balls on the season with 50 RBIs.
The three home runs increased Seattle's season total to 188, tied for second most in the Majors and most by the Mariners since 2000, when they hit 198 in their first full season at Safeco Field.
Parker gave up all three bombs and has allowed 18 home runs on the road this year, tied for third most in the AL.
"He just missed in the middle of the plate with some pitches, and when he did, they didn't miss them," A's manager Bob Melvin said. "Miller was hunting the heater. If [Parker] makes his pitch, maybe it's off the barrel. But he throws it down the middle and he gets him."
Oakland cut the margin to 7-5 in the seventh with back-to-back jacks by Brandon Moss and Alberto Callaspo off reliever Chance Ruffin. Moss's two-run line drive over the right-field fence was his 30th of the year.
Yoervis Medina loaded the bases in the top of the eighth, but he got out of that jam by striking out Jed Lowrie looking at a nasty curve before Danny Farquhar came on for his 16th save with a perfect ninth.
Mariners rookie second baseman Nick Franklin was 2-for-3 with a double, though he got thrown out trying to stretch that to a triple just before Miller let loose with his solo bomb in the third.
Parker fell to 12-8 on the season as he lasted just 4 1/3 innings for an A's team that has already clinched the American League West with a 95-66 record going into Sunday's finale. The Mariners have won four of their last six to stand at 71-90, including an 11-7 record against the A's.
Miller, who had two hits to raise his average to .262, said the Mariners are just trying to finish strong and take some momentum into next year.
"There are still games to be played. There's one more tomorrow," Miller said. "We're learning, but that's part of being a professional. Just going and getting your work done and going out there and trying to win every day. We're playing a really good team, a playoff team, and we want to set the tone."