SEATTLE — There is always an unease when the 49ers travel to Seattle. Maybe it’s that brisk air that comes in off Puget Sound, or the ghost of Russell Wilson (who some claim is still out there, somewhere).

The 49ers’ NFC West-winning performance on Thursday night had the slightest tinge of unease late in the game, but make no mistake; San Francisco outplayed Seattle on Thursday, in 21-13 victory that evidenced the 49ers as operating in a tier, or several, above the Seahawks.

The defense just about silenced an offense boasting a resurgent Geno Smith, an Offensive Rookie of the Year candidate in Kenneth Walker III, and two of the league’s best receivers in Tyler Lockett and D.K. Metcalf.

The offense made Seattle look foolish on defense on multiple occasions, with George Kittle getting the best of Quandre Diggs on a couple of touchdowns. Kyle Shanahan clearly got the better of Seattle defensive coordinator Clint Hurtt, scheming players into acres of space on multiple occasions.

It began with a eye-catching touchdown to Kittle. After both teams opened with punts, the 49ers worked their way down the field, with a heavy dose of Christian McCaffrey.

Then Shanahan dialed up the ole double fake screen.

Brock Purdy pumped left to Ray-Ray McCloud, right to McCaffrey, and the Seattle linebacking corps, along with most of the secondary, froze. When they turned back to Purdy, he was already delivering a ball to a late-departing George Kittle.

Kittle had only Quandre Diggs to beat, and did so, with ease, galloping in to offer an early touchdown lead.

After getting a first down on their opening drive, Seattle had three-straight three-and-outs. Their offense looked incapable of moving the ball until early in the second quarter.

For the first time this season, the Seahawks offense put points on the board, kicking a field goal at the end of a 12 play, 67-yard drive.

There was a moment there at the end of the first half, when the Seahawks had a real chance to draw the game closer.

With 1:48 left, trailing 7-3, Seattle got a first down, then went no huddle.

But it all came undone in a signature moment for the defense, by a man who had maybe the most signature defensive moment in 49ers history — at least against the Seahawks.

Smith checked the ball down to Travis Homer, who scampered upfield. Jimmie Ward got to him first, turning Homer right into the path of a heat-seeking Dre Greenlaw, who got off a fumble-forcing shot on Homer.

Charvarius Ward, who just missed out on a fumble return against the Buccaneers, took his chance this time, returning it 40 yards to the Seattle 5-yard line.

Two plays later, McCaffrey — who touched the ball approximately 70 times on Thursday night — ran it in for a touchdown and a 14-3 lead for the 49ers at the half.

San Francisco received to start the second half, and it got off to a fairly similar start as the opening quarter.

Again, it was Kittle. Again, he was wide open. Again, he beat Diggs in the open field.

This one was far more embarrassing for the Seahawks. Kittle was completely uncovered, but once he got the ball, Diggs had an advantageous angle on him. Kittle gave him a juke worthy of a Chris Berman “Whop!” and then offered another, in more disrespectful fashion, to Cody Barton.

It put the 49ers up 21-3.

And it probably should’ve been 28-3.

On a third-and-8, Geno Smith dropped back to pass and was hit as he threw by an oncoming Nick Bosa. The ball fluttered, and Deommodore Lenoir plucked it for what, for the moment, looked like a result-sealing pick-six.

Instead, a flag was thrown, penalizing Bosa in questionable fashion for landing on Smith with his body weight. It was a flag-heavy game in general, with 15 penalties and 139 yards of penalty yardage. Seattle had eight penalties for 79 yards to the 49ers’ seven penalties for 60 yards.

That was, at least, a momentum-halter.

Seattle went on to score a field goal on that drive, and as the noise increased, the 49ers offense struggled to find the same sort of groove.

The defense made those struggles moot, for the most part. Geno Smith was under constant pressure in the second half from just about everyone, but especially Bosa, Arik Armstead and Samson Ebukam.

And the 49ers could have put the game away about halfway through the fourth quarter. They went 53 yards on eight plays. McCaffrey had a run that could have gone for much more than the 11 yards it did, and a completion to Tyler Kroft for 28 yards could’ve gone further, too.

There was even a 3rd-and-6 drop from Brandon Aiyuk that could’ve kept the offense on the field.

But the backbreaker was a Robbie Gould 43-yard field goal miss. He’d looked less than superb kicking from that side of the field in warmups.

If he’d hit it, the 49ers would have gone up 24-6 with five minutes left; almost certainly a game-icing kick. Instead, he pushed it right.

Seattle, for their part, came back desperate. Despite the pressure Smith was under, he found a release valve in Walker III, who picked up a 33-yard gain on a dump pass. The next play, he found Noah Fant wide open for a touchdown to make it 21-13.

With 3:35 on the clock, they made sure of the win.

But it took some Brock Purdy heroics. On a third-and-1, with the game very much in the balance, Shanahan dialed up a play-action pass. Seattle read it. But Purdy, as he’s shown himself very capable of, scrambled for the yard, getting it in spite of a desperate Pete Carroll challenge.

A 56-yard Jordan Mason Run on third-and-1, down to the 1-yard line, was the icing on the cake.

San Francisco, having won seven-straight games, at 10-4, has won the NFC West at the earliest point since 2011. They are the first team in the NFL to win a division title and will look now to competing for the 2nd seed with the Minnesota Vikings.