LOS ANGELES, June 17 - Just hours after a moderate earthquake shook most of southern California, a strong quake struck off the northern coast, the fourth significant quake to jolt California this week.
Neither of the quakes, on Thursday, caused serious damage. One person was reported injured.
The latest quake, at 11:30 p.m., had a magnitude of 6.4. Its epicenter was about 125 miles off the coast of Eureka. The earlier earthquake, with a magnitude of 4.9, rumbled southern California, shaking the ground from its epicenter near the town of Yucaipa, about 70 miles east of Los Angeles, as far as the coastal town of Laguna Beach.
The Thursday morning earthquake was the third to shake the state since early Sunday morning, when a quake centered about 20 miles south of Palm Springs hit. The second quake, which hit Tuesday, began several miles below the ocean floor off the coast of northern California near the Oregon border; that undersea quake caused fears of a tsunami along the coast.
But seismologists said that the quakes did not necessarily portend a worse quake in the coming days.
"These events are probably unrelated, that would be the wisdom on this one," said Tom Jordan, director of the Southern California Earthquake Center, in a telephone interview. "Typically earthquakes don't interact on that scale."
In Yucaipa, in the foothills of the San Bernardino Mountains, the quake shook tiles from ceilings in a supermarket, knocked books off shelves and rattled nerves all over town, though no injuries were reported.
At Rosie's, a Mexican restaurant on Yucaipa Boulevard, a 15-year-old boy who identified himself only as Alex said he had been playing video games at a friend's house when the quake hit.
"I saw the window shaking and the picture frames started falling off the walls," Alex said. "My friend's brother - he's 11 - started panicking. He didn't know what to do, so my friend told all of us to get under a doorframe. So we did, the three of us."
A firefighter who asked not to be identified because he was not authorized to speak on the record said he and his colleagues had inspected about 40 buildings in town after the quake and found damage in only a few, like the cracks in the walls of the Baptist and Seventh-day Adventist churches. At the Stater Brothers supermarket, employees swept up debris from fallen ceiling tiles.
"The bigger the building, the less give they had when the earthquake rode through," said the firefighter, a 30-year veteran of the California Department of Forestry, which helps provide fire protection to the city.
But, he added, the quake did not cause a major ruckus.
"Earthquakes are no big surprise for us in this place," he said.
Another firefighter, Patti West, an engineer and paramedic at Fire Station No. 1, said she and her colleagues moved four fire trucks out of the station immediately after the quake in case it got stronger and knocked the building down. They then answered a call from a woman who thought she smelled leaking gas in her home. None was found.
Tami Johnston, 46, a homemaker in Yucaipa, described the quake as "a big jolt at first." She added, "When we thought it had stopped, it started shaking again. We felt several of the aftershocks."
Ms. Johnston, who lives about half a mile from the epicenter, said she has a baby Chihuahua who is a pretty good indicator of aftershocks.
"She starts to whine, and then we look on the Internet, and we've had another one," Ms. Johnston said.
As of Thursday afternoon, about 25 aftershocks had hit southern California, said seismologists at the California Institute of Technology. The Thursday night quake was considered likely to have been an aftershock from the one on Tuesday.
Kate Hutton of CalTech said the clustering of earthquakes in the same week is not necessarily significant.
"If you look at history, there are a lot of occasions where quakes have clumped in time even though they have been at slightly different locations," Dr. Hutton said. "It doesn't alarm us to see that happen, but on the other hand we don't have an explanation for it."