The closure is affecting northbound lanes No. 3 and 4 near High Street.
Caltrans spokesman Gregory Townsend said the aging stretch of highway is deteriorating, and a hole appeared in the lower portion of the freeway, below the roadway.
He said traffic was not affected by the problem but that the lane closure is necessary so repairs can be made. Motorists are advised to expect delays.
The repairs are expected to be completed by 4 p.m.