Rio Dell Council Briefed on Hazards of ‘Tectonic Bumper Car Area’

Map showing earthquake intensities in the greater Rio Dell area. [Image from the presentation]

“I will try not to scare you,” Dr. Wendy Bohon of the California Geologic Survey’s Seismic Hazards and Earthquake Engineering division told the Rio Dell City Council at the start of her April 7 presentation on “seismic hazards.”

Rio Dell Mayor Debra Garnes had alluded to the “scary things” Bohon has said in the past to a council presiding over a city that bore the brunt of impacts from a Dec. 20, 2022 earthquake and another on Jan. 1, 2023.

Bohon’s presentation explained the causes and outcomes of earthquakes in what she described as the “tectonic* bumper car area” that is the North Coast.

Some faults are close to Rio Dell, such as the Salmon Fault north of the city and the Russ Fault to the south.

Map showing faults in the Rio Dell area

Bohon said the Salmon Fault has had three earthquakes in the last 1,700 years. Quakes from onshore faults have more serious impacts.

“All of these faults can potentially have earthquakes,” Bohon said, adding, “If you’re interested in earthquakes, this is a really good place to be.”

That’s because of the 11 magnitude 7-plus earthquakes occurring in California since 1900, five were in the greater Rio Dell area.

“So this is the place in California where you are most likely to feel earthquake shaking from a magnitude 7 earthquake,” Bohon said.

She summarized an earthquake history that includes magnitude 7 quakes in 1980 and 1992, and the 6.4 2022 quake, which was preceded by a 6.2 quake on the same date a year earlier.

Is there a significance to the precise one-year spacing? Bohon said there isn’t.

“It just is that you keep having bigger earthquakes,” she continued.

She explained that the intensity of an earthquake and its shaking can significantly vary depending on location. “The local rock and soil conditions make a huge difference,” she said.

A lower magnitude earthquake can cause a lot of damage in certain areas.

“What you feel depends on largely where the earthquake is and where you are,” Bohon said, adding, “When people say, it’s a rolling earthquake or a jolting one, what that’s telling us is how far away you were.”

Answering questions from councilmembers, Bohon emphasized the significance of impacts from earthquakes occurring with semi-regular frequency.

“I get a little frustrated with the media because they’re always like, ‘oh, the big one’s coming’ – and I would say the big one is the one that happens near you and causes damage in your community.”

Bohon said a magnitude 5 earthquake a mile below Rio Dell would cause “very strong shaking and that would be meaningful because it’s close to people.”

Councilmembers related their earthquake experiences.

Councilmember Frank Wilson talked about the late 2022 earthquake sequence, where the initial 6.4 quake was followed by a 5.4 magnitude quake on Jan. 1, 2023.

“The first one had a higher number and the second one had a lower number but it was very violent,” he said. “And what it did to our city, what it did to chimneys, including my own chimney and different things that took place – the magnitude thing doesn’t mean anything to me.”

Garnes said the northern part of Rio Dell was “hit super hard” by the Dec. 20, 2022 quake but the subsequent Jan. 1 quake “destroyed the southern end of town”

“My guess is that it was closer to you and that’s why you felt it more strongly even though it was a smaller magnitude earthquake,” said Bohon.

Her presentation included the “drop, cover and hold on” advice on what to do if an earthquake strikes and Garnes related that her former dining room table wasn’t big enough for everyone to get under.

“We bought a new dining room set based on being able to get under it in an earthquake,” she said.

City Manager Kyle Knopp said the city is encouraging people to sign up for the state’s Earthquake Brace and Bolt seismic retrofit program that was launched in 2023.

With regard to city infrastructure, Knopp said it’s a matter of “building to modern code” and the city “just had to adjust a water tank that’s currently under construction to meet the current highest standard.”

*Spelling corrected.