Thursday’s magnitude 7.0 earthquake and more than a dozen other quakes measuring up to 5.8 are the latest in history to have strong earthquakes off California’s north coast in Humboldt County.

“Although not on land, this may be the largest seismic event to hit California in 30 years,” said Randy Reed, a geosciences professor at Shasta College in Redding.

The quakes were felt from southern Oregon to Sacramento and the San Francisco Bay Area, as posted on the earthquake and volcano tracking website volcanicdiscovery.com.

Although magnitude 7.0 earthquakes are rare, they — and thousands of weaker ones — occur regularly off the Humboldt Coast near Ferndale, south of Eureka, according to the U.S. Geological Survey.

According to Cal Poly Humboldt, tsunami warnings often follow along the Northern California coast, including Thursday.

This is why large earthquakes are shaking the Humboldt Coast, some of which are strong enough to upset the situation in Redding. and if a major earthquake could occur in inland California counties.

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What caused Thursday’s earthquake?

The north coast is a hotbed of earthquake activity. That’s because three tectonic plates meet about 20 miles southwest of Eureka, near the Eel River, Reed said.

Thursday’s 7.0 occurred along the Mendocino Fracture Zone, where the Gorda and Pacific plates touch, Reed said. The ridge of the zone extends into the sea in a horizontal line for more than 3,000 miles.

But the eastern end of the rupture zone, offshore near the town of Ferndale, is where the most earthquake “activity” is – and that’s where Thursday’s earthquakes occurred. This area is near where the rupture meets the Mendocino Triple Junction: the point where the Gorda, North American and Pacific plates meet, Reed said.

According to the USGS, it is also the northern end of the San Andreas Fault.

How strong is a magnitude 7.0 earthquake?

With a magnitude of 7.0, Thursday’s quake was considered a “major” earthquake, the USGS reported.

For comparison, the 1906 earthquake along the San Andreas Fault, commonly called the San Francisco earthquake, had a magnitude of 7.8.

Geologists and seismologists measure the strength of an earthquake – its strength on a logarithmic scale: a number where each number is 10 times stronger than the previous one and 100 times stronger than the second previous number. So a magnitude 7.0 earthquake is ten times stronger than a magnitude 6.0 earthquake and 100 times stronger than a magnitude 5.0 earthquake.

When quakes are stronger than 6.0, they can cause severe damage to human structures, and anything above 8.0 can be catastrophic, according to Michigan Technological University. Earthquakes with a magnitude less than 2.5 are not typically felt by people, and quakes with a magnitude less than 5.5 typically do not cause much, if any, damage.

Smaller earthquakes can precede, but also after, larger earthquakes. Cal Poly Humboldt’s Redwood Coast Tsunami Work Group reported small earthquakes measuring up to magnitude 4.4 along the Mendocino Fracture Zone on Wednesday morning.

Has the northern state felt other major earthquakes from the Mendocino Fracture Zone?

Yes. Strong earthquakes along the rupture zone can have effects as far away as Redding and Yreka.

One of the strongest winds recorded near the Humboldt Coast in the past 30 years was a magnitude of 7.2 that struck Redding, Salinas in Monterey County and Siskiyou County in 1992.

At 11:06 a.m. on April 26, a magnitude 7.2 earthquake struck Humboldt County near Ferndale. Arcata residents reported feeling the ground shaking for more than 45 seconds.

That night and early morning of April 27, the area was rocked by quakes along the ridge, including two with magnitudes of 6.7 and 6.6, according to the USGS and Cal Poly Humboldt (then Humboldt State University).

The quakes caused “hundreds of injuries, landslides and extensive damage to buildings in southern Humboldt County,” including in Ferndale, and caused “a fire that destroyed the business district of the town of Scotia.” According to Cal Poly Humboldt’s Redwood Coast Tsunami Work Group, the earthquakes also caused a “small tsunami that was recorded as far away as Hawaii.” In total, the quakes caused $66 million in damage.

Humboldt residents reported that aftershocks continued for two months after the magnitude 7.2 quake.

An almost equally powerful 7.1 earthquake near Ferndale, Eureka and Petrolia was felt as far away as Sacramento on January 22, 1923. It also triggered a small tsunami, according to the California Residential Mitigation Program.

Could there be large earthquakes here in Shasta and Siskiyou counties?

What happened Thursday morning wasn’t concentrated in Shasta or Siskiyou counties, but the area produces its own earthquakes, said Alex Hatem, a research geologist with the USGS.

Shasta and Siskiyou counties are part of the Shasta Cascade region, which is classified by the USGS as a moderate seismic hazard area. That means there aren’t frequent earthquakes strong enough for people to feel, but the area does occasionally shake, Hatem told Record Searchlight in 2021.

Siskiyou and Shasta counties lie at the nexus of these two processes, which make the Shasta Cascade region vulnerable to earthquakes.

• First, the Pacific tectonic plate is moving eastward and pushing under the North American plate. This creates volcanoes – including Mt. Shasta – along the Pacific Ring of Fire, a circle of volcanoes that winds through Shasta and Siskiyou counties.

• Second, expansion of the North American plate causes Yreka, Mount Shasta, Redding, and everything west of the Sierra Nevada to migrate further west. Everything east of the Wasatch Mountains, near Salt Lake City, is moving east. The gap between the Sierra Nevada and the Wasatch Mountains is called the Great Basin and Range.

“Think of it like taking apart a piece of Silly Putty,” Hatem said. Imagine Yreka, Mount Shasta, and Redding in a vertical line along the left edge of the putty. On the right is Utah. The middle is the Great Basin: Nevada, Utah and Eastern California. “Now pull the putty left and right and watch the Great Basin expand,” she said.

Jessica Skropanic is a features reporter for Record Searchlight/USA Today Network. She reports on science, art, social issues and news. Follow her on Twitter @RS_JSkropanic and on Facebook. Join Jessica on Get Out! Nor Cal Recreation Facebook Group. To support and sustain this work, subscribe today. Thank you very much.

This article originally appeared on Redding Record Searchlight: Why powerful earthquakes are shaking Northern California’s coast