Seahawks' defense ready to contain 49ers?

SAN FRANCISCO -- Trailing 17-16 deep into the fourth quarter at Seattle last season, the San Francisco 49ers faced an increasingly desperate set of circumstances.

2011 Alex Smith vs. Seahawks

They had just surrendered a blocked punt deep in their territory. The Seahawks' Marshawn Lynch had scored the go-ahead touchdown on a 4-yard run as a result. A penalty against receiver Braylon Edwards -- then with San Francisco, now with Seattle -- had forced the 49ers into a second-and-18 situation.

Less than five minutes remained. Alex Smith, the 49ers' quarterback, had completed only one pass longer than 17 yards all game.

What happened next -- Smith's 41-yard strike to Michael Crabtree along the left sideline -- had significant ramifications.

The play moved the 49ers into position for the winning field goal, improving their chances to secure the No. 2 seed in the NFC playoffs. The loss would extinguish the Seahawks' flickering playoff hopes.

The late-game defensive lapse also demonstrated, again, just how much the Seahawks needed to improve their pass rush in critical situations. They had given up a go-ahead, 50-yard touchdown pass to Washington during a 23-17 defeat, also at home.

Seattle wound up using its 2012 first-round draft choice for pass-rusher Bruce Irvin. A second-round pick secured linebacker Bobby Wagner, whose speed would make him a blitzing threat. In free agency, Seattle added Jason Jones, a pass-rushing defensive tackle to help with the nickel rush.

Will any of it matter when the Seahawks visit the 49ers at Candlestick Park on Thursday night? Yes and no:

  • Why it matters: Wagner's speed gives the Seahawks a better chance to defend the 49ers in all situations, not just against the pass. Irvin definitely upgrades the defense in obvious passing situations. His sack and forced fumble against Carolina's Cam Newton comes to mind. The Seahawks' improved speed on defense also lets them match up better with Colin Kaepernick if the 49ers go to their Wildcat package. Wagner chased down Newton for a 4-yard loss.

  • Why it might not matter enough: The Seahawks already were pretty good against the 49ers on third down. They need to be better against them on early downs. The chart shows Smith's efficiency dropping off significantly on third down. That has often been the case for San Francisco regardless of the opponent. It was the case against Seattle in 2011.

Smith's first-down stat line against Seattle last season featured 12 completions in 15 attempts (80 percent) for 130 yards with no sacks or interceptions.

One on of those first-down plays, Smith found Crabtree for a 27-yard gain on a play-action throw to the left perimeter with both teams' base personnel on the field. There was a shorter play-action pass to tight end Vernon Davis over the middle for a 17-yard gain, also with base against base. Another time, Smith completed a 19-yard pass to Davis, this one also to the left perimeter, from three-receiver personnel against Seattle's nickel defense.

Those were all first-down plays hurting Seattle last season.

The 49ers, prior to their struggles during a 26-3 loss to the New York Giants in Week 7, had been a dominant team on first down this season. That was one reason Smith vaulted into the NFL lead for Total QBR, as Albert Larcada of ESPN's analytics team explained last week. Smith had been particularly effective on first-down throws when opponents showed at least some respect for the running game.

"A lot of his success on these plays has come by throwing the ball over the top of aggressive defenses," Larcada wrote before the Giants game. "On throws at least 15 yards downfield in these situations, Smith is 10-for-16 with two touchdowns and no interceptions and is one of three QBs with a 100.0 QBR."

Frank Gore was averaging 6.1 yards per rush and gaining first downs 30 percent of the time in those first-down situations. That was up from 4.3 yards per rush and 18 percent first downs from 2009 to 2011, Larcada said.

I'm not expecting Seattle's passing game to fare particularly well against the 49ers' defense, which has limited downfield throws, particularly for touchdowns. The Seahawks' defense has shown it can keep the team in games anyway. That can happen Thursday night and third down will be key, as it tends to be. But those early-down plays are where the 49ers figure to pose the greatest threat.