A shallow magnitude 5.8 earthquake struck about 100 miles off Northern California's Lost Coast on Thursday morning, according to the U.S. Geological Survey.
The temblor was reported at 8:39 a.m. in the Pacific Ocean west of Capetown at a depth of three miles.
Paul Caruso, a geophysicist at the National Earthquake Information Center in Golden, Colo., said the USGS recorded a 5.1-magnitude aftershock in the same area just before 9:30 a.m.
The aftershock was unintentionally posted twice on the USGS website as separate 5.1 and 5.3 quakes, he said. What likely happened, he said, is that a tsunami warning agency or some other entity also recorded the temblor and a duplicate posting was made.
"The computers hadn't sorted it out," he said. "It happens. It's more common than we would like it to be."
The USGS had received a few reports of the 5.8 quake being felt, but said the intensity was weak because it was so far off the coast.
"People may feel a little shaking, the chandeliers moving in the house," he said. "We wouldn't expect any severe damage."
Donna Johnson, a finance clerk for the city of Ferndale, said she didn't feel the earthquake but that a colleague said she felt "a little rumble" and saw "a slight swaying in the light fixtures."
"If I hadn't read it in the newspaper, I wouldn't have even know it happened," Johnson said.
No one has called the city reporting damage, she said.
Caruso said that earthquakes in the ocean are monitored by tsunami warning centers but that the 5.8 quake is "not even close" to strong enough to trigger a tsunami. Usually, that takes at least a magnitude 7.0 or greater earthquake, he said.
In the last 10 days, there have been no earthquakes of magnitude 3.0 or greater centered nearby.