The earthquake at 10:18 on Sunday evening had a preliminary magnitude of 6.9. Luckily, its epicenter was 50 miles west of Eureka beneath the Pacific Ocean, in the fracture and subduction zone between tectonic plates that historically produces sizable quakes. There was a 4.6 aftershock about 30 minutes later. No tsunami waves were detected and no damage reported, though the shaking lasted several seconds.
The Cape Mendocino area is quite seismically active due to the plates meeting offshore. On April 25, 1992, a 7.2 magnitude quake struck south of Eureka near Petrolia and caused considerable damage, followed in the next 24 hours by two quakes of 6.5 and 6.7.