One of the largest earthquakes to hit California in decades rattled the North Coast on Sunday night, but its depth and distance from shore reduced the impact on land, where there were no reports of injuries or damage, scientists and authorities said on Monday.
The magnitude-6.8 quake struck at 10:18 p.m. and was centered 50 miles west of Eureka and about 10 miles beneath the Pacific seabed, according to the U.S. Geological Survey. It was initially reported as a magnitude-6.9, but later downgraded.
Within a few short minutes of the quake, the Del Norte County Sheriff's Office was fielding dozens of calls from people wondering if a tsunami was close behind, but the West Coast and Alaska Tsunami Warning Center quickly reported that there was no tsunami danger due to the depth and location of the quake.
Most commenters on the Triplicate's Facebook page reported a long, rolling shake lasting 10andndash;30 seconds that made houses tremble. One person compared the feeling to a coin-operated vibrating bed, while another in Granite Falls, Wash., said heeven felt it through his floorboards there, more than 600 miles from Eureka.
By late Monday morning, it had already produced 20 aftershocks of magnitude-3.5 or larger, and more were expected over the coming days, said Keith Knudsen, deputy director of the USGS's Earthquake Science Center in Menlo Park, Calif.
Knudsen said there was also a 5 to 10 percent chance of a larger quake in the area in the next week.
Sunday's quake was felt widely across the region, butfire and sheriff's officials in Del Norte and Humboldt County said they had no reports of damage or injuries.
"Everybody felt it region-wide to the point that there was concern for damage," said Humboldt County sheriff's Lt. Steve Knight. But other than triggering some home alarms and knocking down shelved items, the county escaped unharmed, he said. "We're very grateful."
In 2010, a magnitude-6.5 earthquake caused more than $20 million in damage, but that quake was less than 20 miles from Eureka and directed the most destructive energy waves toward the city.
Sunday's quake directed most of its energy to the southwest, away from the coast, according to Lori Dengler, a geology professor at Humboldt State University who studies earthquakes and tsunamis.
"It also ruptured slowly.When it breaks slowly, it tends to be gentler," Dengler said.
More than 4,000 people reported on the USGS website that they felt the quake within hours of it striking, including some across the border in Oregon and Nevada.
"It was a big bump, and then it rolled for about 30 seconds," said Diana Harralson, 64, who lives in an apartment in Rio Dell, about 55 miles southeast of the earthquake's epicenter. "It was a real good shaker."
Harralson said some knickknacks fell off her wall, but there was no damage. A California native who has experienced other earthquakes, she said she and her cat slept comfortably through the night.
Amandip Heer, a manager at a gas station and convenience store in Eureka, described the quake as a "vibration," but said nothing fell off the shelves at the store, and there was no other damage.
Earthquakes are common in Eureka. Since 1980, there have been 10 earthquakes greater than magnitude-6 in the area, Knudsen said. California has experienced at least seven earthquakes of magnitude-6.9 over the same period, according to the USGS.
Sunday's quake was far enough from shore to allow much of its energy to dissipate, Knudsen said.
"We're fortunate it didn't strike closer to a populated area," he said.
An offshore earthquake of magnitude-7.2 about 30 miles southwest of Eureka in 1992 left 95 people injured and caused tens of millions of dollars in damage, according to the USGS. The earthquake was felt as far south as San Francisco.
It was followed by a magnitude-6.5 earthquake about 12 hours later and a magnitude-6.7 earthquake a few hours after that, both of which caused additional damage.
Sunday's quake preceded Tsunami Safe Week in Del Norte County, starting March 19 and culminating with a full evacuation drill March 26. Events are planned in commemoration of the 1964 tsunami.
For more information, visit Preparedelnorte.com.
Triplicate staff writer Adam Spencer contributed to this report.
From staff and wire reports.