The Sounders (13-13-3) moved into fourth place in the Western Conference.
San Jose goalkeeper David Bingham initially made the diving save on Zach Scott's header, but it caromed in front of Martins, who converted the left-footed shot with his back to the net. Martins has five goals in his last five games and 12 this season.
Fatai Alashe put the Earthquakes (11-11-6) in the lead in the 70th minute, scoring his second career goal on his own bobblehead night on a header in the left corner of the goal. Matias Perez Garcia, who assisted on the goal, entered as a substitute in the break immediately prior to the free kick.
Seattle's Roman Torres suffered an apparent ankle injury shortly before halftime.