Trailing 2-1 on aggregate against Santa Tecla, the Sounders got the job done at home, crushing their opponents 4-0 at CenturyLink Field.
Seattle needed until the second half to claim the win and set up a meeting with Chivas, with Will Bruin making it 1-0 when he tapped in a Nicolas Lodeiro cross in the 47th minute.
Lodeiro went from provider to goal-scorer with 21 minutes remaining, heading in a Clint Dempsey cross from the left.
The Sounders kept pushing and struck twice more, Chad Marshall heading in a Lodeiro corner before Magnus Eikrem scored after combining with Dempsey.
In New Jersey, the Red Bulls recorded a 2-0 win over Olimpia to progress 3-1 on aggregate.
Bradley Wright-Phillips opened the scoring in the 54th minute with a brilliant turn and finish from 25 yards.
Sean Davis sealed the win 10 minutes later, his deflected strike from distance making it 2-0.
The Red Bulls will face Tijuana in the last eight.