The 49ers haven't seem to right themselves from their wobbly effort Sunday against the Giants and trail the Seahawks 6-3 at the half.
49ers lapses, and continued shoddy play from quarterback Alex Smith, marked the first half.
Cornerback Perrish Cox, on the field in the "dime" defense, lost track of Seattle receiver Ben Obomanu who caught a short pass for 36 yards.
Aldon Smith missed a tackle on a 15-yard run by Marshawn Lynch while Tarell Brown lost sight of the ball on a 27-yard catch and run by Sidney Rice.
Penalties, which were a problem in Sunday's loss to the Giants, also tripped up the 49ers and aided Seattle's drives. Following a play in which he was steamrolled by Lynch, safety Dashon Goldson drove his shoulder into the prone running back and then stood over and appeared to taunt Lynch, drawing a 15-yard unsportsmanlike penalty.
The 49ers have ben penalized five times for 25 yards. Another unsportsmanlike call on linebacker Larry Grant was offset by a penalty against the Seahawks.
The run defense, which was bludgeoned in the fourth quarter Sunday, also has struggled. Lynch has nine carries for 55 yards, and several 49ers -- including Patrick Willis and Aldon Smith -- have missed tackles.
The Giants Ahmad Bradshaw, who had 116 at game's end Sunday, had 23 yards at the half.
Smith, meanwhile, has completed fewer than half of his pass attempts (5-17) for 59 yards and has a passer rating of 57.4. He had a 43.1 rating in the loss to the Giants.
-- Matt Barrows