When the earth began to shake at 2:34 a.m. on Dec. 20, the epicenter was just off the coast of Ferndale. But a combination of factors, including how the rupture traveled inland on an east-north-east trajectory, placed the quake's bullseye on Rio Dell.

Felt as far away as Redding and the Bay Area, the magnitude 6.4 quake plunged the entire region into darkness and is estimated to have caused some $10 million in damage countywide, most of which was centered in the small Eel River Valley city, where the shaking reached a level of intensity rarely seen.

More than 70 structures countywide have been red tagged, meaning they were unsafe for occupation, while an additional 114 received yellow tags, indicating significant damage — the vast majority of those in Rio Dell — leaving approximately 170 residents displaced as of Jan. 2, or about 5 percent of the city's population.

"The main point is that earthquakes are not explosions that send out energy equally in all directions," said Cal Poly Humboldt geology professor emeritus Lori Dengler, a world-renowned expert on earthquakes who laid out three main reasons why an area farther away from the epicenter was hit harder than others closer by.

Among those, she said was the type and orientation of the faulting, in this case what's known as a strike-slip — with the two-sides of the rupture moving horizontally along each other in an east-west direction — which "focused S-wave energy (the back-and-forth motion that's the strongest part of an earthquake's shaking) at Rio Dell and Fortuna."

Another was the direction of the fault rupture, which started offshore and moved inland toward Hydesville, channeling the energy right into Rio Dell, and the Doppler effect — the way that seismic waves change frequency when moving toward a location or away from one, Dengler said.

"The seismic waves coming toward you are compressed and hence sharper and stronger," she said, adding that in places where the rupture was traveling away, like Petrolia, the motion was more of a rolling effect.

Those factors, along with the region's geology, likely combined to create a sort of perfect storm, resulting in stronger shaking being felt in Rio Dell than in nearby Fortuna and places even closer to the quake's origin.

"Rio Dell is at the boundary between the sediments and relatively softer rocks in the Eel River basin and the hard Franciscan bedrock," Dengler said. "Such interfaces can cause additional reverberations and amplifications."

The quake shook the city with the third-highest intensity ever recorded in California, with a peak ground acceleration — measured in g-force — of 1.48g, as documented by a seismic station located in the U.S. Highway 101 overpass at Painter Street.

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

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

"That was also an unusual situation where it is speculated that a large landslide slipped during the quake, amplifying the shaking," Dengler said. "Accelerations over 1g are extremely rare in earthquakes — only 15 or so worldwide."

The strongest in the world was 3.2g during the magnitude 7.8 Kaikoura quake in New Zealand, but Dengler said it's important to understand that "these high values are always outliers," noting most of the accelerations in the recorded-breaking 2016 event registered below 1g.

"We start seeing damage to structures at around 0.3g — especially if the shaking is sustained," she said. "One of the reasons we didn't see more damage on Dec. 20 was because the strong shaking only lasted about eight seconds."

For perspective, the devastating 1964 Alaska earthquake, a magnitude 9.2, shook for four to five minutes, according to USGS.

While Dec. 20 was not the largest earthquake to hit Humboldt County in recent memory, Dengler said magnitude is just one piece of the puzzle when it comes to intensity and is "only a function of the size of the fault, the amount of offset and how tightly the two sides are held together."

"We have a number of examples of magnitude 8 and 9 earthquakes that ruptured very slowly and people hardly felt them," she said, noting that "how fast the rupture occurs is important to the strength of shaking."

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

Hundreds of aftershocks have rattled the Humboldt County region in the ensuring weeks, most notably the 5.4 on New Year's Day that hit 9 miles southeast of Rio Dell on a different fault that moved in a different direction, causing more damage and further traumatizing city residents still trying to pick up the pieces after the Dec. 20 quake.

"The 5.4 was very short and rupture propagation direction probably doesn't play a role," Dengler said. "But orientation is important and this earthquake was nearly perpendicular to the 6.4 and focused its S-wave energy in a different direction. Some people in Rio Dell felt the first was stronger and others in a slightly different location said the 5.4 was stronger."

As of Jan. 24, the USGS reported there have been 27 magnitude 3 or higher aftershocks, "which are strong enough to be felt nearby," and one of magnitude 5 or higher — Jan. 1 — "which are large enough to do damage."

The agency's aftershock forecast indicates there is a 1 percent chance the region will see one or more aftershocks in a magnitude 5 range — or higher — between now and Feb. 23.

"There will likely be smaller aftershocks within the next month, with up to four magnitude 3 or higher aftershocks. ... The number of aftershocks will decrease over time, but a large aftershock can temporarily increase the number of aftershocks," the forecast states.

So is Humboldt, one of the most seismically active areas of California with a long history of large earthquakes — 40 or more above magnitude 6.0 in the last 100 years due to the clashing of tectonic plates off the coast — 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 both included events in the magnitude 6.0 range.

As to the current aftershock sequence from the Dec. 20 quake, that seems to be winding down, Dengler said. Around 10 have been recorded in the last week or so, including a 3.3 off the coast of Ferndale on Jan. 22.

But, she noted, there have been other earthquakes in the region that may be residual impacts from the 6.4, including a 3.7 on Jan. 19 and a 4.2 the next day near Weitchpec and a 3.6 near Fieldbrook on Jan. 10.

"These earthquakes are not aftershocks — they are about 40 miles north of the aftershock zone," Dengler said. "It is possible they fall into the category of events triggered by stress changes produced by the Dec. 20 event. We've now seen a couple of earthquakes ... in areas where we don't see frequent quakes. They aren't unprecedented — we've had magnitude 5 quakes in this area before — so I'm not ready to say for sure that there is a relationship. It will be interesting to see if we have more of them."

Kimberly Wear (she/her) is the digital editor at the Journal. Reach her at (707) 442-1400, extension 323, or kim@northcoastjournal.com.