The sinkhole, apparently caused by a collapsed drain, opened on the shoulder of the eastbound 10 near 6th Street.
Two lanes on that side of the 10 were closed as emergency crews respond.
But officials caution a full closure of the freeway is possible if the surface continues to fall away.
As the sinkhole opened, it apparently led to a mudflow beneath the freeway overpass.
The mud trapped a car and bent a power pole, but no serious injuries were reported.
Later Thursday night, Caltrans reported a contractor was on scene to perform emergency repairs. The two lanes and the on-ramp at 6th Street were expected to remain closed for several days.