Seattle started brightly, with former Newcastle United man Martins drilling just wide of Tyler Deric's goal on the 17-minute mark in the only clear-cut chance of a cagey half.
The visitors almost took a big step into the playoffs after the break when Deric's smart save kept out ex-Tottenham Hotspur forward Clint Dempsey on 48 minutes.
Against the run of play, Owen Coyle's troops preserved their own slim chances of reaching the post-season when Clark drilled home from outside the box.
However, Seattle hit back with eight minutes of normal time left through Martins's header from point-blank range, but a point would not be enough to seal their progression on the night.
A home win over Real Salt Lake in next weekend's final round of regular fixtures would see the Sounders advance.