Residents along the Northern California coast were rattled on Thursday by an earthquake centered off the coast south of Eureka. Preliminary estimates showed that the quake had a magnitude of 7.0, according to the U.S. Geological Survey.

The authorities said they were surveying for signs of damage. The quake was felt as far away as the San Francisco Bay Area, more than 200 miles to the south, and in the Sacramento Valley to the east.

A second earthquake struck north of San Francisco near Clearlake, with a preliminary magnitude of 5.8, the agency said.

The National Weather Service issued a tsunami warning in Northern California coastal counties and told residents to immediately move away from ocean waters and head for high ground.

The temblor struck during what experts say could be a period of increased seismic activity in the state, after decades of relative quiet.

Seismologists have long warned that an overdue “Big One,” the likes of which California has not experienced since 1906, could happen at any time. They have urged residents to prepare as much as possible by assembling emergency supplies and practicing “drop, cover and hold on” exercises with their children.

It has been three decades since a significant quake struck California.

The Loma Prieta earthquake, with a magnitude of 6.9, shook the Santa Cruz Mountains in 1989, leaving 63 people dead and more than 3,700 people injured.

An earthquake with a magnitude of 6.7 in the Northridge neighborhood of Los Angeles in 1994 left 60 people dead, about 7,000 injured and more than 40,000 buildings damaged. The catastrophe also revealed a major defect in some steel-frame buildings, including many high rises, which under extreme shaking could collapse.