By Steve Gorman
LOS ANGELES, March 10 (Reuters) - A series of small aftershocks continued to rattle the extreme northern coast of California on Monday, hours after a magnitude 6.8 earthquake shook the town of Eureka and an area extending into Oregon and Nevada with no reports of damage.
The main tremor, which struck at about 10:30 p.m. on Sunday(0530 GMT Monday), was centered in the Pacific about 50 miles (80 km) west of Eureka and 10 miles (17 km) beneath the ocean floor, according to the U.S. Geological Survey.
While earthquakes of that size can unleash widespread destruction if they score a direct hit on an urban area, as the 1994 Northridge quake did in Los Angeles, the impact of Sunday's tremor was largely buffered by its location out at sea, seismologists said.
Shaking from the quake was felt most strongly by residents in Eureka, a coastal community about 270 miles (430 km) north of San Francisco, and the nearby town of Arcata, site of Humboldt State University, the USGS reported.
But no tsunami warnings were issued, and the morning watch commander for the Eureka police department, Sergeant Steve Watson, said there were no reports of injuries or damage to property in town.
"It lasted around 30 seconds," Watson said, describing the shock as a side-to-side, rolling motion that lacked the sharp jolts of tremors he had felt in the past.
"Nothing fell off our shelves or anything," he said.
Lighter rumbling from the quake extended north into coastal Oregon and eastward as far as Carson City, Nevada, with a handful of reports of light shaking felt as far south as San Francisco.
USGS monitoring showed that the main tremor, initially measured at a 6.9 magnitude but later downgraded, was followed by about 20 aftershocks that continued into the morning hours, said David Oppenheimer, a USGS seismologist.
While earthquakes producing noticeable shaking are routine in California, 6.8 magnitude tremors are less commonplace, and potentially dangerous. Quakes of that magnitude are capable of causing substantial damage to some buildings, such as a partial collapse or the shifting of structures off their foundations.
Several dozen people were killed and thousands more injured in the Northridge quake, which unlike Sunday's tremor occurred directly under a densely populated city.
Oppenheimer said the quake's location at sea made all the difference, accounting for why there was so little consequence.
"That's the primary reason," he said. "The real explanation is that the earthquake was so far offshore." (Reporting by Steve Gorman; Editing by James Dalgleish)