A magnitude 7.3 earthquake struck off the coast of northern California at 10:44 a.m. on Wednesday, rattling cities as far south as Oakland, according to a preliminary estimate from the U.S. Geological Survey.
The epicenter was under the Pacific Ocean, about 65 miles off the coast of Eureka in Humboldt County, originating about 0.6 miles below ground. The strongest shaking occurred in the small town of Petrolia.
There were no immediate reports of injuries or damage.
A tsunami warning has been issued for the California coast from the Santa Cruz County village of Davenport to the Oregon coast.
The tsunami is expected about about 12 noon, with the possibility of widespread, dangerous coastal flooding. People are advised to evacuate to higher ground or a tall building immediately. In 2011, a tsunami also sent a tidal surge several feet high across the Northern California coast. It flipped boats and wrecked wooden docks at Santa Cruz Harbor, causing $25 million damage. It also killed a man in Crescent City who was swept out to sea.
Wednesday’s quake occurred along a restless region where three tectonic plates converge, called the Mendocino Triple Junction, where the Pacific, Gorda, and North American tectonic plates meet. It is considered part of the Cascadia subduction zone which extends further north along the coast; this area is known for frequent earthquake activity due to the complex plate interactions there.
It is unrelated to the Bay Area’s three major faults. The San Andreas Fault runs roughly 750 miles through California between Cape Mendocino to San Diego, including the San Francisco Peninsula. The Hayward Fault runs roughly 74 miles along the western base of the densely populated East Bay Hills.The Calaveras Fault runs roughly 76 miles from Hollister to Danville.
For northern Californians, the most likely source of a dangerous quake is the Hayward-Rodgers Creek Fault, with a 31 percent chance in the next 30 years. This fault runs through the heart of East Bay cities. The Hayward section of the fault runs from near Mount Misery, east of San Jose, north to San Pablo Bay. The Rodgers Creek portion picks up from there and runs north to Santa Rosa.
The last major earthquake in the Bay Area was the 1989 Loma Prieta earthquake — a magnitude 6.9 quake that struck on Oct. 17, 1989, in the Santa Cruz Mountains. It killed 62 people and caused $6 billion in damages. The largest earthquake since Loma Prieta shook Napa in August 2014 with a magnitude of 6.0. No one was killed in the quake but hundreds of buildings were damaged and dozens were red-tagged. Before that, a magnitude 5.4 temblor hit near Alum Rock in San Jose in 2007.
In 2014, a panel of experts estimated that there is a 72 percent chance that in the next 30 years the San Francisco Bay Area will experience an earthquake at least as powerful as the magnitude 6.7 Northridge earthquake that rocked Southern California in 1994. There is a far greater chance — 99 percent — that a smaller but still substantial magnitude 6.0 earthquake will strike during that time.
The scale of earthquake measurement is logarithmic: a recording of 7, for example, signifies a disturbance with ground motion 10 times as large as a recording of 6. An earthquake of magnitude 2 is the smallest size normally felt by humans. Earthquakes measuring 5 or higher are potentially damaging.
The San Francisco earthquake of 1906 has been estimated at a magnitude 8.3. The earthquake that leveled the capital of Haiti measured 7.
As with any large earthquake, there is a possibility of damaging aftershocks. If an aftershock occurs, the USGS recommends that people who are indoors stay there, taking shelter under a piece of furniture, in a hallway or against an inside wall, away from windows, fireplaces and heavy objects.
If you are outdoors, get into the open away from buildings, power lines and other things that could fall. If driving, stop carefully and move out of traffic. Avoid bridges, trees and other falling objects. Stay in your car until the shaking stops.
Check back for more on this breaking news story as it develops.