The Mariners have defeated the Athletics in seven straight meetings.
Oakland tied it in the seventh on Matt Holliday's homer and it remained a 7-7 game until Russ Springer (0-1) gave up the winning run in the ninth.
Chavez reached on a one-out single off Springer, Mike Sweeney walked and, with two away, Branyan was intentionally walked to load the bases.
Lopez worked the count to 2-2 and then fouled off pitch after pitch before sending a ball to right field for the game-winning hit.
Shawn Kelley (1-1) recorded the last five outs to get the victory.