(This article has been revised to accurately reflect the fault location associated with the earthquake.)

SAN FRANCISCO ‒ A significant earthquake with a magnitude of 7.0 hit near Petrolia at 10:44 a.m. local time on Thursday, triggering a tsunami warning that extended from southern Oregon to San Francisco, which has since been lifted.

The earthquake took place in a region along California’s northern coast, an area known for its seismic activity called the Mendocino Triple Junction, where three tectonic plates converge, including the Cascadia subduction zone and the San Andreas Fault. The quake was centered on the Mendocino fault zone.

In Arcata, around 60 miles from Petrolia, 25-year-old Jared Mitchell experienced the tremor while having lunch at home. He recounted, “There wasn’t a huge violent shake, but my apartment started swaying back and forth. It felt like I was on a raft in the ocean, which was pretty frightening.”

By 12:30 p.m., just under two hours following the main quake, the region had experienced 13 aftershocks ranging from magnitudes 5.1 down to 3.1, as reported by the U.S. Geological Survey (USGS). By 2:30 p.m., the USGS noted at least 39 aftershocks, each measuring at least 2.5 in magnitude in areas including Petrolia and Ferndale.

The National Tsunami Warning Center terminated its tsunami alert at 11:54 a.m., stating that no tsunami activity had been detected. They emphasized that no destructive tsunami had been observed but cautioned individuals against returning to hazard zones until authorities declared it safe.

How many experienced the earthquake?

The USGS initially predicted that over 150,000 individuals could have felt the earthquake’s tremors from just north of Fort Bragg to Eureka. One YSL News reporter located in San Francisco did not sense the quake’s impact, while another person in the San Francisco Bay area described it as the longest earthquake she’d ever experienced.

Traci Grant, a 53-year-old public relations expert, mentioned that she experienced her San Francisco apartment swaying while chatting on the phone.

“The entire apartment felt like it was moving in slow motion,” explained Grant, whose older building in the Marina District is retrofitted. “For a second, I thought I should grab my purse and my cat and head out.”

Grant reported that the rocking sensation lasted nearly 30 seconds, which felt prolonged to her, but fortunately, nothing fell in her apartment.

“It just kept going,” she added. “It was both frightening and exciting. It wasn’t a quick shake; more of a rolling motion.”

Locate the epicenter Earthquake strikes in a complicated tectonic zone

The Earth’s crust comprises numerous tectonic plates that shift slowly. Thursday’s earthquake hit a notably complex seismic area, highlighted by Stephen DeLong, a supervisory research geologist with the USGS.

This location is near the Mendocino Triple Junction, where the Pacific, North American, and Juan de Fuca plates converge. This junction is also where the northern Cascadia subduction zone meets the southern San Andreas Fault system.

Thursday’s seismic activity is associated with the Mendocino fracture zone, according to geophysicist Justin Rubinstein from the USGS in Menlo Park.

Notably, significant earthquakes have previously occurred near this junction in December, including a 6.1 and a 6.0 near Petrolia in 2021 and a magnitude 6.4 near Ferndale in 2022.

More aftershocks on the horizon

The USGS provided an aftershock prediction on Thursday.

There is a greater than 99% likelihood of experiencing one or more earthquakes of magnitude 3.0 or higher in the coming day, a 74% chance of a magnitude 4.0 or greater, 22% chance of a magnitude 5.0 or more, and a 3% chance of reaching magnitude 6.0 or above.

Approximately 10,000 customers of Pacific Gas and Electric (PG&E) in Humboldt County were reported without power, as the utility began evaluating its gas and electric systems. “PG&E will commence inspections of its electrical and gas infrastructure by ground and air once it’s deemed safe,” stated spokesperson Megan McFarland.

How often are tsunami warnings issued in the Bay Area?

Research conducted between 1850 and 2004 highlighted that 51 credible tsunamis were documented in San Francisco Bay. However, only two caused any damage: one from the 1960 Chile earthquake and another from the 1964 Alaska earthquake, which resulted in waves nearly 4 feet high at the Presidio, per findings from Lori Dengler, a professor emeritus at Cal Poly Humboldt.

Previous tsunami warning rescinded

A tsunami warning had previously suggested that a tsunami could reach the coastal areas starting from Fort Bragg at 11:10 a.m. local time, with projections for it traveling down Northern California’s coastline and arriving in San Francisco by 12:10. Residents in the impacted areas, stretching from Davenport, California, near Santa Cruz, to the Douglas County line in Oregon, were advised to evacuate inland and away from tsunami hazard zones.

“Leave the water, move off the beach, and distance yourself from harbors, marinas, breakwaters, bays, and inlets,” advised the tsunami warning center, noting that repeated coastal flooding might occur as waves come ashore, push inland, and recede.

What does a 7.0 magnitude earthquake mean?

Magnitude measures how powerful an earthquake is. It is officially defined on the moment magnitude scale. This scale works logarithmically, indicating that each whole number represents an earthquake ten times as powerful as the one before it. For instance, a 5.0 quake is ten times more potent than a 4.0.

According to Michigan Technological University, the magnitude and impact of an earthquake are categorized as follows:

• Less than 2.5: Usually not felt.
• 2.5 to 5.4: Minor damage or none.
• 5.5 to 6.0: Light damage to structures.
• 6.1 to 6.9: Major damage.
• 7.0 to 7.9: Significant earthquake causing serious damage.
• 8.0 and above: Catastrophic damage, capable of obliterating communities.

Initial reports from the U.S. Geological Survey suggest there is a 32% likelihood of economic damages ranging from $1 million to $10 million, as well as a 32% chance of losses between $10 million and $100 million. The odds of incurring losses exceeding that amount are estimated at 15%.