"I think everybody is going to feel the same way that this wasn't good man," Harbaugh said. "Can't feel like you coached well, can't feel like you played well after this one."
Russell Wilson threw a career-high four touchdown passes to move into second place for most TD passes by a rookie. Marshawn Lynch scored two first-quarter TDs, and the Seahawks blew out the 49ers 42-13 in a NFC West showdown that turned into a blowout at CenturyLink Field.
Richard Sherman returned a blocked field goal 90 yards for another touchdown as the Seahawks (10-5) jumped to a 21-0 lead. That only added to an already hyped crowd on a typically cold and rainy December night, with noise echoing off the walls and overhanging roof that might have been heard all the way across Puget Sound.
No one cared about the cold rain, not with the performance they were seeing on the field, and not with a ticket to the postseason guaranteed thanks to Seattle's first 10-win season since 2007.
"When we're on our stuff, this is the outcome you get," Seattle defensive end Red Bryant said. "I'm not surprised. We knew going into it we had a great opportunity."
Seattle's win delayed the victory celebration for the 49ers (10-4-1), although it could still come next week at home.
San Francisco needs just a win over lowly Arizona to clinch the NFC West title, but it no longer controls its route to the No. 2 seed in the NFC.
Green Bay is a half-game ahead, and the 49ers – who came into Sunday night banged up at key spots – might not get that extra week of rest even with a win in the finale.
Seattle will likely be the No. 5 seed in the NFC. There remains a slight chance of winning the NFC West if the Seahawks beat St. Louis in the season finale and Arizona can top the 49ers.
The Niners will need a victory over the Cardinals combined with a Green Bay loss at Minnesota next Sunday to secure the No. 2 seed in the NFC playoff derby for the second consecutive season. The Packers beat the Vikings 23-14 in Green Bay on Dec. 2.
At the very least, the Seahawks relished in making Harbaugh's 49th birthday a wet and miserable night.
"It's not our first loss, and we'll wake up (Monday) and we'll have a half-game lead on the division, and our hand will be on the dial to take care of business next week," Harbaugh said. "I know how I can expect our team to handle that."
Seattle was the first team since 1950 to score at least 50 points in consecutive weeks thanks to its 58-0 win over Arizona and 50-17 victory against Buffalo. It seemed inconceivable the scoring binge could continue against San Francisco, the best scoring defense in the NFL.
But it did.
Seattle has outscored its last three opponents 150-30.
The 42 points were the most allowed since Harbaugh took over the 49ers, and the most San Francisco yielded since giving up 45 to Atlanta in 2009.
It was the perfect way for Seattle coach Pete Carroll to snap a three-game losing streak in a rivalry with Harbaugh that goes back to their college coaching days when Carroll was at USC and Harbaugh at Stanford.
"It was a lot of points again, and we're just thrilled about it," Carroll said. "Things have just changed. We have changed on offense, and Russell has been a huge part of it and the coaches allowing it to happen. We don't hold ourselves to points because the standard isn't out there for us. We just try to play really good football and see what happens at the end."
Colin Kaepernick, San Francisco's young quarterback in his sixth NFL start, had already proven himself capable of winning on the road with victories in New Orleans and last week in New England. But Seattle is a different beast, widely regarded by players as the loudest venue in the NFL.
His inexperience playing in such an environment showed. He was flustered and disorganized at the line of scrimmage, letting the noise from Seattle's fans affect him.
Kaepernick's forgettable night was capped when Sherman stepped in front of his pass for Randy Moss at the back of the end zone on the first play of the fourth quarter for Sherman's seventh interception of the season – the third pick Kaepernick has thrown as San Francisco's starter.
Kaepernick was 19 of 36 for 244 yards with an 18-yard TD pass to Delanie Walker with 1:40 left. Frank Gore had just 28 yards on six carries after rushing for a season-high 131 when the teams met in Week 7.
"Every time you are on the field you are learning something," Kaepernick said. "We just have to take what we can from this game and move on to next week."
San Francisco played without defensive tackle Justin Smith due to an elbow injury that ended a streak of 185 starts. The 49ers lost tight end Vernon Davis in the first quarter with a concussion sustained when he was knocked off his feet on a huge hit along the sideline from Seattle safety Kam Chancellor that looked legal but drew a penalty for hitting a defenseless receiver.
San Francisco wide receiver Mario Manningham went down with a left leg injury early in the third quarter when he was tackled low by Leroy Hill and fumbled.
The loss of Smith affected the entire defense. Aldon Smith was left stuck on 19½ sacks for the second consecutive week after being locked up by Seattle offensive tackle Russell Okung.
Smith, the previous NFL leader in sacks for several weeks, lost that lead to Houston's J.J. Watt, who had one sack in the Texans' 23-6 loss to Minnesota, giving Watt a league-high 20½ for the season.
"I think not having Justin is a huge deal," 49ers safety Donte Whitner said. "Just a year ago he was up for defensive player of the year. Not only for what he does against the run, but for what he does against the pass. How he helps all the men out over there also. Eating up two guys, locking to go one-on-one. He's a huge part of this defense, we all know it, and he's not there right now."
NOTES: Lynch finished with 111 yards on 26 carries, his third straight 100-yard game vs. San Francisco. ... Seattle WR Doug Baldwin had two TD catches, his first touchdowns since Week 6. ... Seattle finished 11 of 13 on third-down conversions. ... The 49ers' previous worst defeat under Harbaugh was a 26-3 loss to the New York Giants earlier this season.
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