7.0 Earthquake off Northern California Coast

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An earthquake ruptured off California on Thursday morning, briefly triggering a tsunami warning for the coast of Northern California and southern Oregon.

The earthquake, a magnitude-7.0 temblor, rumbled at around 10:44 a.m. about 62 miles west of Ferndale, California, according to the U.S. Geological Survey.

A tsunami warning was issued for the coast from Davenport, California, to the border between Douglas and Lane counties in Oregon. The Tsunami Warning Center canceled the warning at about 11:55 a.m.

Humboldt County, the community on land closest to the epicenter, reported no injuries or deaths, Humboldt County District 2 Supervisor Michelle Bushnell told NBC News. But she said she had heard of broken water mains, broken windows and houses that had been dislodged from their foundations.

About 10,000 people were without power in Humboldt County, state Sen. Mike McGuire said.

The 7.0-magnitude quake is one of at least 17 events that rattled Northern California on Thursday, with most of the seismic activity concentrated around Humboldt County.

At least four events were recorded in the Ferndale area, with high activity also observed around the town of Petrolia, where at least 10 quakes ranging from 3.1 to 4.2 magnitude broke out.

Christine Goulet, the director of the USGS Earthquake Science Center, based in Los Angeles, said the 7.0 earthquake initiated less than a mile beneath the Earth’s surface, making it uncommonly shallow. More than a dozen aftershocks were reported.

Goulet said the USGS analysis suggests there is about a 5% chance that an earthquake greater than magnitude 6.0 could rupture in the area within the next week.

“Following a 7, there could be fairly large aftershocks,” Goulet said. “As time goes by, they’re going to be less frequent and smaller.”

Earthquakes can trigger tsunamis when they displace the seafloor, causing waves that can rush toward shore. A tsunami warning indicates that significant inundation and coastal flooding are expected, based on preliminary information about a quake’s position.

Harold Tobin, the director of the Pacific Northwest Seismic Network, said the temblor was a strike-slip earthquake, which means that two plates slid past each other, causing shaking.

The quake ruptured in the Mendocino fault zone, at the intersection of three tectonic plates: the Pacific, North American and Juan de Fuca plates.

“This is the exact point where the Cascadia Subduction Zone ends to the south and the San Andreas Fault begins,” Tobin said. “It’s the most seismically active place in California, overall, over the past decades. It’s not a surprise to get an earthquake of this magnitude.”

Tobin said it was the highest-magnitude earthquake produced in the area since the 1990s. He added that it did not take place on the Cascadia Subduction Zone and that it was not likely to increase the risk of an earthquake there.

The Cascadia Subduction Zone is one of the biggest hazards in the U.S. The fault runs offshore along the West Coast from Northern California to northern Vancouver Island in Canada. It is capable of producing magnitude-9.0 earthquakes and tsunami waves about.

The earthquake triggered USGS’s ShakeAlert Earthquake Early Warning System, which automatically sends warnings about big earthquakes. Goulet said some communities near the epicenter, from Eureka to Fort Bragg, most likely received notices on their phones a few seconds before shaking occurred.

“It looks like the people close got several seconds of advance notice,” she said.

Dengler said the event should prompt changes to the tsunami warning system. A tsunami warning remained in effect for more than hour and sent people rushing to higher ground, but a damaging wave never materialized.

"We need to work hard at reducing unnecessary alerts," Dengler said. "We need to know within 5 to 10 minutes if a tsunami was produced, and at present, it was kind of crazy having a tsunami warning in place for so long."

Given the proximity of the quake, communities near Cape Mendocino would have seen the first surges "on the order of minutes," Dengler said.

The National Oceanic and Atmospheric Administration has a network of buoys, called DART, designed to detect tsunamis by sensing pressure changes along the sea floor. But Dengler said more scientific instruments need to be added to the system to produce quicker and more reliable tsunami forecasts.

"The closest DART systems are hundreds of miles away" from the epicenter Thursday, Dengler said. "You really need more ocean-bottom instruments to detect what’s happening near the source quickly."

So how much sooner when we expect "the big one" to happen? As in the earthquake that will rip California and maybe British Columbia away from the rest of North America and cause intensely catastrophic destruction?