Keane set up Sean Franklin's tally from close range in the 12th minute and the Galaxy (6-4-2) never looked back in rebounding from a 1-0 loss to New York last weekend.
Keane made it 2-0 with a right-footed shot from the center of the box in the 24th and added penalty-kick goals in the 34th and 44th minutes, becoming the 11th Galaxy player to record a hat trick.
Robbie Rogers, the first openly gay active player in American professional sports, entered the game as a second-half substitution in his Galaxy debut.
The Sounders (4-4-3) saw their streak of six games snapped with the one-sided loss.
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