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.