Seattle doubled their lead just before the break when they were awarded a penalty as Martins was brought down by Kwarasey. Brad Evans took the spot kick and fired his strike into the net.
Portland were back in it just before the hour mark as Valeri's corner was bundled over the line from close range by Jewsbury to make it 2-1.
The hosts had to withstand late pressure to secure the win, with Portland coming closest to an equaliser in the final minute when Dairon Asprilla let fly from outside the box, his effort missing to the right by inches.
Four minutes of nervy stoppage time followed, but Portland held on for maximum points.