The numbers behind Seattle's string of defensive dominance

Richard Sherman and the Seahawks' defense have put up some unparalleled stats since the beginning of the 2012 season. Mark J. Rebilas/USA TODAY Sports

The Seattle Seahawks are playing lights-out defense this season. Just as they did in 2015. And 2014 and, in fact, all the way back to 2012.

What some would call the Russell Wilson era in Seahawks history is more aptly termed their Legion of Boom years (or some such nickname). What the Seahawks have done during the past four and a half seasons is rarer than producing consistently good quarterback play. They've had the most dominant defense in football over that span, ranking No. 1 or 2 in Football Outsiders' DVOA metric each of the past four seasons, and No. 3 so far this season. (For traditionalists, the Seahawks also are tied with the Minnesota Vikings for the lowest average points allowed this season, 14.0 per game)

What exactly have the Seahawks accomplished? And how have they done it? Let's take a closer look.

First, the basics.

The Seahawks have allowed by far the fewest points in the NFL since the start of the 2012 season. They're 21 percent better than the next-best teams and 32 percent below the NFL average.

Points per game allowed, 2012-16

Points are the most important measure of defensive success, but sometimes they can mask other deficiencies. So for good measure, the Seahawks also have allowed the fewest yards and first downs since the start of 2012. Opposing quarterbacks have an NFL-low 46.6 Total Quarterback Rating against them, nearly three points lower than the next-best team.

The Seahawks' 123 takeaways, combined with their 72 giveaways, give the team a plus-51 turnover margin, the NFL's second best since the start of the 2012 season. The New England Patriots lead with a plus-56 margin, but they join the Seahawks as high-achieving outliers. The next-best team is the Carolina Panthers at plus-27.

How have the Seahawks pulled off such consistent achievement? The most obvious answer: They have a bunch of great players.

Single seasons of players with plus-14 approximate value, 2012-16

Check out the graphic above. Pro Football Reference measures every NFL player with an approximate value (AV) metric that uniformly attaches a number to each player's season performance for the purposes of comparison. (Here is a more detailed explanation.)

Let's put a strong AV season in the range of plus-14 or higher. Using that cutoff, you can see that the Seahawks have had more defensive players performing at least at that level during a single season than any other NFL team. Cornerback Richard Sherman has reached that level four times, while safety Earl Thomas and linebacker Bobby Wagner have done it twice apiece.

A total of eight such performances might not seem high, until you realize that 14 teams have not had a single defensive player at that level -- zero, nada -- over this span. So the median across the league is one, and the average is 1.6.

And if you look at the 2012-15 seasons as a whole, you'll find that five of the top 50 players in combined AV played for the Seahawks: Sherman, Thomas, Wagner, linebacker K.J. Wright and safety Kam Chancellor. And that doesn't even count defensive linemen Michael Bennett and Cliff Avril, who were mid-dynasty, free-agent acquisitions; they have been among the NFL's top 51 defensive players in AV since the start of the 2014 season.

The Seahawks' defense has done everything well since 2012. In addition to leading the league in points, yards and first downs, the Seahawks are also No. 1 in passing yards allowed, No. 2 in quarterback pressure percentage, No. 4 in rushing yards per carry and No. 6 in takeaways.

But what has made them special? The Seahawks have a relatively rare but lethal weapon: the blitz.

Since 2012, the Seahawks have blitzed -- a pass rush of five or more players -- on only 25 percent of opposing dropbacks. That's the sixth-lowest percentage in the league.

When they do send extra rushers, however, they get home at the best rate in football.

Highest QB pressure percentage by defense when blitzing

The pressure rate in the graphic above is based on dropbacks in which the quarterback is sacked, put under duress or hit. This can be attributed both to player skill and smart playcalling from the defensive coaching staff.

As a result, the Seahawks have allowed the NFL's lowest average yards per dropback when blitzing. Their performance is 20 percent better than the NFL average.

Fewest yards per dropback allowed versus blitz, past five seasons

Appropriately, we should finish where we started: with points. Opponents have a far more difficult time scoring against the Seahawks' blitz than they do against anyone else. Touchdowns per attempt when the Seahawks blitz is nearly 2 full percentage points lower than the next-best defense. The league average is 5.7 percent.

Lowest TD percentage allowed versus blitz, past five seasons

The Seahawks' offense has slumped this season. Through seven weeks, the Seahawks have the fifth-lowest points per game average in the league (18.5). But the Seahawks are 4-1-1 and atop the NFC West, in large part because of a defense that has picked up right where it left off.

ESPN Stats & Information senior statistics analyst Jacob Nitzberg contributed to this story.