The Seahawks rank 17th in total defense, allowing 332.7 yards per game. Big disappointment, right? Well, the Seahawks ranked tied for 16th on defense last season, allowing 316.8 yards per game. They went to the Super Bowl as the NFL's highest-scoring team. This would support the proposition that Seattle's offensive dropoff, from second last season (369.7) to 22nd (308.5), is primarily responsible for the team slipping from 13-3 to 8-6. I think there's some truth in there (download full rankings here).
And yet it would be misleading to suggest that the overall defensive rankings are pretty much the same. One huge difference is that Seattle is allowing 4.5 yards per rush, 25th in the league, compared to 3.6 (fifth) last season. The Seahawks have also gone from second to 26th in red-zone touchdown percentage, an important indicator. What we cannot say, however, is whether those numbers have fallen largely because the offense is not doing its part. A high-scoring offense can mask minor defensive shortcomings.