(Adds no reports of damage or injuries, other details)
Sept 22 (Reuters) - A magnitude 5.7 earthquake struck on Friday off the coast of Northern California in the Pacific Ocean, the U.S. Geological Survey (USGS) said, but there were no reports of damage or injury.
It was quickly followed by a second quake of 5.6 closer to shore, the USGS said. The larger temblor was initially reported by the agency as magnitude 5.8.
Both quakes struck west of the California town of Petrolia and were very shallow, amplifying their effects, but a USGS map showed that they were not widely felt along the coast.
The Office of Emergency Services for Humboldt County, California said there were no initial reports of injuries or damage from the quake.
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. (Reporting by Sandra Maler in Washington, D.C. and Dan Whitcomb in Los Angeles; Editing by Alistair Bell and Cynthia Osterman)