What's up with the run defense? Closer study from multiple angles suggests several minor things.

First, the good news. Run defense holds the weakest correlation to winning. Weaker than penalty rates, even. Pass defense holds a distant second, in strength of correlation to winning, to passing offense, and even from Football Outsiders you see there isn't a single top defense that is weak against the pass, but you have a handful of top defenses that may not be entirely inspiring when it comes to run defense.

As Seattle's passing game has risen the past two months, the team has gotten better. They've become a more formidable matchup for anyone, and have a shot to make a run in the playoffs. That's all good, and much more desirable than maintaining a great or elite run defense.

But as the run defense has slipped -- now "just" -8.1% DVOA, or 16th in the league (read: below average), explanations have not been readily available.

So I've finally taken some time to explore this, and I think I have a handle on why. And that leads to the bad news: most of the reasons aren't particularly fixable; of those that are, the apparent solutions aren't likely to be taken by Pete Carroll & his staff.

But the implications of the bad news should lead you back to the good news: run defense win correlation is weak! So hurray. Now, the contributing factors:

• The notion that this was a top-notch run defense early might have been a bit of a mirage to begin with. Shocking, I know, and I'm not being facetious. I'll explore why.
• Over-reliance on the starters.
• The rotational linemen aren't so good at run defense.
• The young LBs aren't quite as good as we thought.
• Teams have adjusted, primarily by finding ways to add more bodies to the play side.
• Seattle's philosophical approach to game strategy is conducive to running the ball remaining a viable tactic for a longer duration.

Let's take this on one by one. How could this run defense not be good? How could it be a mirage? They've not not been one of the better run-stuffing units for quite some time. In 2010 they finished as an uninspiring unit, but that was a fade from a hot start as the opponents got stronger and Red Bryant went out with an injury. 2009 was a year of good run defense, the sole bright spot on the team, even though Red suffered a similar fate. 2008 was an all-around poor year. 2006 was the other year of poor run defense, 2005 & 2007 being pretty good. So the reputation that preceded the 2012 squad was consistently reliable. The front four were all returning starters from a good 2011 unit.

I say "reputation that preceded them" because as I looked back through the performances this year, it looks like they weren't substantially challenged by opponents early on, though some cracks were beginning to show.

This graph doesn't measure the quality of the results, intentionally. Several caveats, the ones you'd expect. Pico-scale sample size. Game situation influences the number of rush attempts a team may perform. Strength and style of the opponent, the matchup and their subsequent game plan, also influence. It's common for teams ahead to run more in the 2nd half. The graph suggests a subtle but pervasive trend, here. I'll get more into that in a minute. Let's review some of the individual games. Again, not the quality of the results or schematic breakdowns; we've been there. We know we won the difficult Carolina matchup and convincingly lost the SF matchup, with regards to run defense. We know Miami got the better of us. But I want to highlight how some cracks in the run defense were beginning to show, and then loop back around to my bullet points above.

Green Bay found success running the ball in the 2nd half. Prior to being coerced by an inability to pass protect, Green Bay had three rush attempts in the first half. Carroll attributed the success of the anticipated shift to running in the second half, to the linemen's inability to pull themselves back and stop penetrating their gaps so quickly.

St. Louis didn't find success, overall, but we saw the beginning of the adjustments teams would make against the Seahawks: continual presnap shifts of personnel to the play side. Though most of the runs were not successful, Bobby Wagner played a great game and covered up the fact that many of the defeated plays still had plenty of playside open space to work with. Better backs, with quicker change of direction ability, would be able to capitalize on these creases around larger men in space.

New England's running game was contained, and it obviously works in substantial part by playing off the threat of their passing game and their versatile tight ends. But while contained, the Pats' runs were not often stuffed, and it was one of the weirder run game matchups I've witnessed, with what seemed like a chaotic, sloppy mess of blocking assignments, their linemen staying too tall to get leverage yet they stayed balanced and in the way and their backs ran with extreme patience and discipline. I noticed the Patriots' really effective running against the Broncos the week before look similar. The concepts are power, but they didn't seem to seek to move defenders out of the way to open creases, but rather something like a "counterpuncher style" in boxing that lures you into mistakes and capitalizes on them. That's not an approach we'll see in the remainder of the opponents. But I found it very curious and wanted to mention it.

San Francisco would be the game where the problems became much more apparent. The rest, for the most part, is history, save for Miami's striking and unexpected domination on the ground.

Brandon Mebane had been playing at an all-pro level, the first couple months. He's still playing very well, winning most matchups and enabling his teammates. We're just not seeing as many individual play defeats. Alan Branch and Red Bryant are large gentlemen. They definitely have good run defensive skills. And as a unit they'd proven themselves formidable in the past.

The first factor that I think is in play here, is a shifting of respect from the front seven toward the secondary. The run defense's stoutness had been established, and they weren't tested early on. The secondary, tested and found to pass with flying colors, was the unit that became more avoided.

But it's not just that the secondary is clearly better; it's that the run defense is not the unmovable rock that they seemed to be. Looking through all the video retrospectives (Field Gulls 2012 MVP: Nate Dogg), I don't see gap discipline issues, mistakes or over-aggression. San Francisco definitely blew the Seahawks off the line, but that's not surprising or indicative of a weakness or problem. But I don't see dominance that I'd seen before. I do see opponents sending extra bodies routinely to the play side of runs; an increase in pitches and end-arounds and scheming to create open space, even behind the line of scrimmage, for more agile runners to have a shot at beating a man packing an additional 80-120 pounds.

OK, that's my 5th bullet point, and there's not much else to say about it. We saw Detroit, Minnesota, Miami and Chicago all employ this tactic against this front. That it didn't work more is somewhat a testament to the soundness of Seattle's front. But the more agile the runner, the more potency it has held. Seattle held up against Detroit well, considering the abilities of the runners and how much the passing game had opened up. But against Miami, the matchup was a sharp, stinging wakeup call.

Arizona schemes the run well. They just have unusually poor talent to execute it. Expect Arizona to attempt what everyone else has. They may surprise us all by approaching competence on the ground, pending what room game situation leaves them to try. Color me worried about the final road matchup against CJ Spiller.

Next factor: over-reliance on the starters. After compiling the defensive line snap counts (including special teams snaps) for Seattle and ten other comparable 4-3 teams (for instance, I did not compare to New Orleans because they are bad and thus have very high snap counts), these are not quite the results I was expecting or looking for. But I'm not into publication bias, so I'll publish them anyway:

Seattle's definitely not the worst in over-relying on starters (Minnesota, good lord! 7 DL?). But they definitely lean to that side. A below-average number of rotational linemen, and a league-comparable number of snaps for the starters despite being amongst the lowest total defensive snaps in the league (fewer snaps here than anyone else in the graph). A good defense will result in fewer snaps.

But this would all look worse if we split the figures between tackles, ends, and roles. Among the four starters, the fewest snaps belongs to Branch at 512, but the next-most among Seattle linemen is Bruce Irvin, at 353. A gap of over 150 is unusual: compare to Chicago's 5th starter Paea being but 8 snaps behind 4th starter Wootton, or Atlanta 5th starter Jerry 54 snaps behind 4th starter Walker.

To exacerbate the issue of snap distribution, the rotational linemen's value and role has primarily been to provide pass rush: Irvin leads with 353 snaps, followed by Jason Jones (238) and Clinton McDonald (189). Even Greg Scruggs is more of a pass rusher at this point (110), and Jaye Howard's contribution barely makes a dent (14 (and only 7 on the line).

Jones and McDonald line up as tackles, but mostly on passing downs versus passing personnel packages. There is virtually no one to sub in for Branch and Mebane to take some of the punishment from the pounding on the ground away from them. Bryant has subbed in on the interior more often this year than last year, at least. But factor number three doesn't only lead to the over-reliance on the starting tackles, it also partly hurts the run defense because when these guys sub in, they're just not as stout.

The best linemen in the league routinely get a high number of snaps. It doesn't intrinsically or necessarily lead to a loss of effectiveness. And no one has had an unusual number of snaps, comparable to the rest of the league. But Seattle has strictly subbed by situation this year, whereas in previous years we'd see a more distinct impact of substitution when rotational guys would come in for an entire series.

Combine this with the 5th bullet point -- Seattle's approach to offense and defense keeps game situations close enough that rushing snaps have not been getting eradicated in the 2nd half of games -- and you can see how our keystone run defense personnel could be getting worn out even if their bottom line snap counts aren't high. They're getting less rest, throughout each game, and opponents have begun to raise the rushing snap counts up in the 1st half, while having no particular need to abandon the run in the 2nd half.

It all adds up to a qualitative result as a mediocre run defensive performance for the year. The talent is still there, and the execution is not exactly lacking, although Leroy Hill's swan song was last season, it turns out, and the LBs aren't playing cleanup as well as they ought. They keep their gap responsibilities well but don't have well-developed play recognition, yet, especially post-snap.

I don't think nagging injuries are a factor, but it's possible that some lingering things are going unreported because Mebane & Branch are simply needed and they aren't too injured to play and by golly they're going out there against 22 personnel, dammit. It's a long season, and 7 of 12 were on the road.

I would call it a scheme issue. I would not avoid calling it that. I don't think the highly-specialized, unbalanced line is schematically unsound. We've worried about this before, but it's proven itself well. But, teams have adjusted, it's worked, and we have been unable to adjust. In Derek Stephens' recent article, we mused about the lack of interior rush, the level of need for another interior rushing tackle, and the possible redundancy of Bryant and Branch, and the outlook on contract extensions and looming free agency.

Yet it may be lost on us how the more glaring need for the front seven, in terms of the impact of absence, is one additional lineman who can plug the run well, whether from the interior or strong-side end. Better ability and use of run plugging in rotation could potentially be a key to not just return the run defense to prominence, but also improve pass rush and 3rd down effectiveness, by creating more 3rd & long situations as the defense is designed to do.

Oh, but Seattle's been embarrassing at stopping 3rd & long, you say? Yes, indeed, and part of that has come from the need to be more balanced: runs have remained a more viable threat throughout game and situation, and slot & seam weapons from rushing personnel have pulled the LB responsibilities back to include more zone drops because our nickel & slot defenders have not been up to snuff. Even mediocre 3rd down defense wins almost twice as often as loses, and stuffing a few more runs would surely lead to a few more sacks.