Though a swarm of about a dozen earthquakes jiggled an area south of Humboldt Bay this weekend, experts say nothing about it indicates a larger temblor is on the horizon.
Still, former Humboldt State University geology professor Lori Dengler, director of the Humboldt Earthquake Education Center, says the swarm is a good reminder for North Coast residents that a strong quake could occur at any time.
“Earthquakes on the North Coast may be inevitable but they can be prepared for,” Dengler wrote in a report for the education center and the Redwood Coast Tsunami Work Group based on data from the U.S. Geological Survey and the seismology lab at the University of California, Berkeley. “Stockpiling food and water, developing a family or business emergency plan and reducing the hazard from items that may topple or slide will help insure (sic) that you and your family will survive and thrive.”
The swarm began with a magnitude 2.9 earthquake at 4:30 p.m. Friday in the Mendocino triple junction region about 11 miles west of Petrolia, according to Dengler’s report. As of Monday, it included three temblors in the magnitude 4 range and five in the magnitude 3s, according to Dengler.
The largest was a magnitude 4.5 earthquake that occurred at 2:18 p.m. Sunday eight miles west of Petrolia, according to Dengler’s report.
Residents in the Cape Mendocino area felt the stronger temblors, according to Dengler. Some as far away as the Humboldt Bay area and Shelter Cove also felt them, she wrote.
The earthquake swarm occurred in one of the most seismically active areas in the Lower 48. The Gorda, Pacific and North American plates, along with the San Andreas falt, the Mendocino fault and the Cascadia subduction zone fault systems meet in the Mendocino triple junction region, according to Dengler.
Centered on Cape Mendocino and the town of Petrolia, the triple junction is about 25 miles in diameter, Dengler wrote.
“The triple junction is a smushy area of numerous folds and faults — so many that we don’t even try to name them,” Dengler told the Triplicate via email Monday. “It’s the most seismically active part of the state — at least two or three earthquakes (occur) every week.”
According to Dengler, though the North Coast isn’t at any more risk of a damaging earthquake as it was before last weekend’s swarm struck, most of the area’s damaging earthquakes in historic times have been centered on faults within the Gorda plate offshore of Humboldt and Del Norte counties. Onshore faults are also capable of producing damaging temblors, she said.
“Humboldt and Del Norte counties also sit atop the southern portion of the Cascadia subduction zone, a fault system capable of producing an earthquake of magnitude 9,” she said, noting earthquake information in her report may change as more information is available. “A strong earthquake could occur at any time and will likely damage road, lifeline and communication infrastructures. Plan to be self-sufficient for at least a week or longer.”
A strong earthquake could also be the first warning that a tsunami is on its way, Dengler said. For more information about the tsunami risk to Del Norte County and Crescent City, visit www.preparedelnorte.com.