A 6.6-magnitude earthquake struck off the coast of Northern California (search), but there were no immediate reports of damage or injuries, authorities said.

The quake late Thursday came just days after a larger temblor in the region generated a tsunami warning that sent residents scrambling.

Thursday's quake, which hit at 11:22 p.m. about 125 miles off the coastal city of Eureka (search), was likely an aftershock to the magnitude-7.2 temblor that hit off the coast of Crescent City late Tuesday, said Stephanie Hanna, spokeswoman for the U.S. Geological Survey. It was originally listed as a 6.4-magnitude temblor, but was upgraded after a seismologist reviewed the data.

Hanna said there was little risk of a tsunami because the quake was below magnitude-7.0.

"Often earthquakes below 7.0 do not generate a tsunami," she said.

A spokesman for the Humboldt County Sheriff's Department said there were no reports of damage or injury.

California has been rocked by four moderate to large earthquakes since Sunday.

A magnitude-5.2 quake shook the Anza area of Riverside County in Southern California on Sunday. A temblor of magnitude 4.9 hit earlier Thursday near Yucaipa in San Bernardino County in Southern California.

Eureka is about 85 miles south of Crescent City and 270 miles northwest of San Francisco.