SEATTLE — Christian McCaffrey rushed for 114 yards and two touchdowns in the first half, and the San Francisco 49ers built a 21-point lead by halftime, rolling to a 31-13 win over the Seattle Seahawks on Thursday night.

The 49ers (8-3) won their fourth straight against their division rivals and have a two-game lead over Seattle in the NFC West. San Francisco won its third straight following a three-game losing streak and picked up its 10th straight win against an NFC West opponent.

McCaffrey was terrific, running through arm tackles and breaking off a handful of big plays. McCaffrey rushed for touchdowns of 8 and 1 yards as the 49ers built a 24-3 halftime advantage.

Deebo Samuel also ran for a 2-yard touchdown on San Francisco’s opening possession and Brock Purdy was nearly flawless in the first half, completing 15 of 20 passes.

Purdy threw an interception that was returned for a touchdown early in the third quarter, but avoided any other miscues and put a capper on the victory with a 28-yard scoring pass to Brandon Aiyuk with 7:51 left that sent some Seahawks fans headed for the exits.

Purdy finished 21 of 30 for 209 yards, and while he wasn’t perfect like last week against Tampa Bay, he outplayed the quarterback on the other side.

Seattle’s Geno Smith was playing banged up after leaving last Sunday’s loss to the Los Angeles Rams in the second half with an injury in his right arm, near his elbow. He’ll have more bumps and bruises after the 49ers finished with him.

Smith was 18 of 27 for 180 yards. He was sacked six times, threw an interception in the first half and nearly had a few more throws picked off. The Seahawks’ first five possessions gained 15 total yards, and when Seattle tried to rally in the second half, Smith took a couple of untimely sacks.

Smith also ran to argue with an official after there was no flag for defensive holding or pass interference on a throw intended for Jaxson Smith-Njigba in the end zone that would have given Seattle first-and-goal late in the third quarter. Seattle settled for a field goal and never threatened again.

Seattle’s last chance at a rally ended when Zach Charbonnet was stopped on fourth-and-1 at the San Francisco 21 with 4:14 remaining. The Seahawks (6-5) lost for the second time in five days and have dropped three of the last four.

Seattle has scored just three offensive touchdowns in the past four games and has just one in its last eight quarters.

SACK ATTACK

San Francisco’s six sacks were spread around. Nick Bosa had two sacks to give him 7 1/2 for the season. Javon Hargrave had 1 1/2 sacks, while Tashaun Gipson and Arik Armstead each had one. Kevin Givens was also credited with a half-sack. The 49ers have 15 sacks in the past three games.

BENCHED

Seattle appeared to bench starting cornerback Riq Woolen in the first half, replacing him with Michael Jackson. Woolen was not announced as having an injury and remained in on special teams. Woolen missed on a couple of tackles early in the game

INJURIES

Seattle right tackle Jason Peters left late in the first half with a shoulder stinger and was replaced by Stone Forsythe. Peters did not return.

San Francisco backup safety George Odum suffered a biceps injury and did not return.

UP NEXT

49ers: At Philadelphia on Sunday, Dec. 3.

Seahawks: At Dallas next Thursday.