Seahawks have the best secondary in the league. They not only cover but they come after your ass too.
A big reason why Seattle got so many sacks against GB was the coverage downfield. They completely took away all deep routes. That D line just feasted and dominated until GB finally adjusted.
The entire defense is amazing. I was reading that they run a hybrid defense because they have the personnel to do it. Changing from 4-3 to 3-4 all the time + air-tight press coverage and complete safeties = elite defense. The Seahawks are a Quarterback away from being a serious contendor.