“You could hear it as well as feel it,” said Sue Nickols, a vice principal at an elementary school in Eureka, a city of about 26,000 roughly 50 miles northeast of the epicenter, who was home at the time of the quake. She said she hurried to the school campus, where frightened students had been evacuated to a field. But children were back in class by lunchtime and the school appeared to be unscathed, she said.

The National Weather Service briefly issued a tsunami warning in coastal counties in Northern California and Southern Oregon, telling residents to head inland or for high ground, But the service canceled the alert about an hour after the quake occurred.

More than a dozen aftershocks were reported in less than two hours after the initial quake, mostly off the coast, including one with a magnitude of 4.3.

Here’s what to know:

About the quake: Stephen DeLong, a supervisory research geologist at the U.S.G.S. Earthquake Science Center in Menlo Park, Calif., called the quake a “large but somewhat typical event.” It struck on the Mendocino Fault, Dr. DeLong said, near an area known as the Mendocino Triple Junction, which he called the most seismically active area in California. “It is a complex junction of tectonic plates moving in different directions,” he said.

Service disruptions: More than 10,000 customers do not have power in Humboldt County in Northern California, according to poweroutage.us, which tracks data from utility companies. Bay Area Rapid Transit, which serves San Francisco, Oakland and the surrounding communities, was working to resume normal train service after the tsunami warning was lifted, but major delays were expected.

More seismic activity: 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.

The last major quakes: 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.

The moment magnitude scale can be hard for nonexperts to decipher. It is logarithmic, meaning that each whole number of magnitude represents about a 32-fold increase in the amount of energy released during a rupture.

So, for instance, an earthquake with a magnitude of 2.0 is not twice as strong as a quake with a magnitude of 1.0. Instead, the amplitude of shaking would be 10 times as great, and it would release 32 times as much energy.

The strongest earthquake ever recorded, a 9.5-magnitude one that occurred in Chile in 1960, was 30,000 times more powerful than a 6.5-magnitude quake, which itself can be very destructive.

Magnitude is far from a complete measure of a specific quake’s destructiveness, however. Other geological factors, like the location and depth of the fault and the type of rocks and soil it occurs in, can affect the amount of shaking and destruction that an earthquake can cause.

Construction methods and building standards can also play a major role in the amount of damage and the number of casualties. The timing of the event and whether people are at home, at work or out and about are also factors.

Scientists calculate a quake’s moment magnitude by using data from a network of instruments, called seismometers, spread across the world. Those instruments record the waves of shaking from the quake’s point of origin moving through the earth.

Worldwide, the frequency of earthquakes has remained largely unchanged over many decades of study. On average, there are about 1,500 quakes of a magnitude of 5.0 or higher every year. Of these, about 15 have a magnitude of 7.0 or higher.

People nearby might not feel a tremor of less than 4.0 magnitude. A 5.0 event might rattle nerves but do only light damage. At 6.0, moderate damage can be expected, especially to older and less resilient structures.

Strong quakes of 7.0 and up can do major damage over a wide area, and a quake of 8.0 or greater anywhere near a population center would probably cause catastrophic damage and loss of life.

Theoretically, a quake of magnitude 10 is possible, but the fault would have to be about 8,000 miles long, or about one-third of Earth’s circumference.

Kenneth Chang contributed reporting.