February 4, 2023

When the earthquake began at 2:34 a.m. on December 20, the epicenter was just off the Ferndale coast. But a combination of factors, including the way the fracture was moving inland on an east-north-east track, placed the quake’s bullseye on Rio Dell.

The magnitude 6.4 quake, which reached as far away as Redding and the Bay Area, plunged the entire region into darkness and caused an estimated roughly $10 million in damage across the country, most of which was attributable to the small Town of Eel River Valley where the quake took place reached an intensity rarely seen.

More than 70 buildings across the country were marked red, meaning they were unsafe for occupation, while another 114 received yellow markings, indicating significant damage — the vast majority of those in Rio Dell — killing about 170 residents as of December 1, 2020 Jan. 2 were displaced, or about 5 percent of the city’s population.

“The key point is that earthquakes are not explosions that send energy out evenly in all directions,” said Cal Poly Humboldt geology professor emeritus Lori Dengler, a world-renowned earthquake expert, who outlined three main reasons why an area further from the Earth is away The epicenter was hit harder than others that were closer.

Among these, she said, was the nature and orientation of the fault, in this case a so-called strike slip — in which the two sides of the fault move horizontally along each other in an east-west direction — resulting in “S wave energy (the outward and reciprocation, which is the strongest part of an earthquake’s shaking) at Rio Dell and Fortuna.”

Another reason was the direction of the fracture, which began offshore and moved inland towards Hydesville, directing energy into Rio Dell, and the Doppler effect – the way seismic waves change frequency, when moving toward or away from a location, Dengler said.

“The seismic waves coming at you are compressed and therefore sharper and stronger,” she said, adding that in places where the fracture was moving away, like Petrolia, the motion was more of a rolling effect.

These factors, along with the region’s geology, likely created a type of perfect storm, causing Rio Dell to feel stronger tremors than nearby Fortuna and locations even closer to the quake’s origin.

“Rio Dell lies on the boundary between the sediments and relatively softer rocks in the Eel River drainage basin and the hard Franciscan bedrock,” Dengler said. “Such interfaces can cause additional reverberations and amplification.”

The tremor shook the city with the third-highest intensity ever recorded in California, with a maximum ground acceleration – measured in G-force – of 1.48 g, as documented by a seismic station located at the US Highway 101 overpass Located on Painter Street.

In comparison, a reading of 0.919 g was recorded at Smith Lane and Fortuna Boulevard in Fortuna, 0.450 g was recorded at the College of the Redwoods, and a seismic station at Harris and Dolbeer streets in Eureka recorded a peak acceleration of 0.309 g center for Engineering Strong Motion Data, a partnership of the US Geological Survey, the California Geological Survey and the Advanced National Seismic System.

The highest intensity ever recorded in the state — 2 g — also occurred in Humboldt County, during the 7.2-magnitude Cape Mendocino earthquake in 1992, although Dengler said “that was a weird, single sharp peak.” The No. 2 position currently goes to the 1994 Northridge Tremor with a magnitude of 6.7 and an acceleration level of 1.82g as measured in Tarzana.

“This was also an unusual situation, with speculation that a large landslide slid during the quake and amplified the quake,” Dengler said. “Accelerations above 1 g are extremely rare in earthquakes – only about 15 worldwide.”

The world’s strongest was 3.2g during the 7.8 magnitude Kaikoura quake in New Zealand, but Dengler said it’s important to understand that “those high readings are always outliers,” noting that most accelerations at were recorded below 1 g at the groundbreaking event of 2016.

“We start to see damage to structures at around 0.3g – especially if the shaking continues,” she said. “One of the reasons we didn’t see any further damage on December 20 was that the strong shaking only lasted about eight seconds.”

For comparison, the devastating 1964 Alaskan earthquake with a magnitude of 9.2 shook for four to five minutes, according to the USGS.

While Dec. 20 wasn’t the largest earthquake to strike Humboldt County in recent memory, Dengler said that magnitude is just one piece of the puzzle when it comes to intensity, and “just a function of fault size, the amount of misalignment and the amount of warping is how tightly the two sides are held together.”

“We have a series of magnitude 8 and magnitude 9 earthquakes that erupted very slowly and people hardly felt them,” she said, noting that “how quickly the rupture occurs is important to the strength of the shaking.”

“This earthquake ruptured a small portion of the fault, but that appears to have happened very quickly — about 10 seconds,” Dengler said.

Hundreds of aftershocks have rattled the Humboldt County area in recent weeks, most notably the 5.4 on New Year’s Day that struck 9 miles southeast of Rio Dell on another fault that was moving in a different direction, causing more damage and further traumatizing townspeople trying to pick up the pieces after the December 20 quake.

“The 5.4 was very short and the direction of fracture propagation probably doesn’t matter,” Dengler said. “But orientation is important, and this earthquake was almost perpendicular to the 6.4 and focused its S-wave energy in a different direction. Some people at Rio Dell felt the first was stronger and others in a slightly different place said the 5.4 was stronger.”

As of Jan. 24, the USGS reported that there were 27 aftershocks of magnitude 3 or greater “strong enough to be felt nearby,” and one of magnitude 5 or greater — Jan. 1 — “which is large are enough to do harm.”

The agency’s aftershock forecast says the region has a 1 percent chance of experiencing one or more magnitude 5 — or greater — aftershocks through February 23.

“There will likely be minor aftershocks over the next month, with up to four magnitude 3 or greater aftershocks. … The number of aftershocks will decrease over time, but a large aftershock may temporarily increase the number of aftershocks,” the forecast said.

Is Humboldt, one of California’s most seismically active areas with a long history of large earthquakes — 40 or more greater than magnitude 6.0 in the last 100 years due to offshore tectonic plate collisions — seeing above-average earthquake activity? Dengler said the short answer is no.

Looking at annual earthquake activity since 1980, the only standout year is 1992, she said, with 2021 and 2022 being similar, noting that both events were contained in the magnitude 6.0 range.

As for the current aftershock sequence from the Dec. 20 quake, it appears to be unwinding, Dengler said. About 10 have been recorded in the last week or so, including a 3.3 off the coast of Ferndale on January 22nd.

However, she noted that there have been other earthquakes in the region that could be residual effects of the 6.4, including a 3.7 on January 19 and a 4.2 the next day near Weitchpec and a 3, 6 near Fieldbrook on January 10th.

“These earthquakes are not aftershocks — they’re about 40 miles north of the aftershock zone,” Dengler said. “It is possible that they fall into the category of events triggered by stress changes caused by the December 20th event. We’ve seen a few earthquakes now…in areas where we don’t see frequent tremors. They’re not unprecedented – we’ve had magnitude 5 tremors in this area before – so I’m not ready to say for sure there’s a connection. It will be interesting to see if we have more of that.”

Kimberly Wear (she/she) is the digital editor at the Journal. You can reach her at (707) 442-1400, extension 323 or [email protected]

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