UPDATE: U.S. Geological Survey says there was only one earthquake, not two as initially thought.

A magnitude 5.7 earthquake struck on Friday off the coast of Northern California in the Pacific Ocean, the U.S. Geological Survey said, but there were no reports of damage or injury.

The USGS initially reported that it was followed by a second quake of magnitude 5.6 closer to shore, but later rescinded that. The agency said discrepancies in seismic readings from different sets of scientific equipment had temporarily led to the belief there had been two quakes.

The earthquake, which struck west of the California town of Petrolia about four kilometres under the Pacific, was initially reported by the USGS as magnitude 5.8. Normally the effects of a shallow earthquake can be amplified, but a USGS map showed the tremor was not widely felt along the northern California coast.

The Office of Emergency Services for Humboldt County, Calif., said there were no initial reports of injuries or damage.

Magnitude 5 quakes and above are not uncommon in seismically active California. They are considered moderate and capable of causing considerable damage if they hit a heavily populated area directly. But rarely do they cause problems when they strike offshore from the sparsely populated Mendocino coast.