New fault capable of massive earthquakes discovered in California

Amid the towering redwoods of rural Humboldt County, geologists this month cracked open an 100-foot-long trench to search for the signs of ancient earthquakes. 

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Jason Patton, a geologist for California Geological Survey, first found evidence of a fault in this area about four years ago using lidar, a remote sensing method. He teamed up with Mark Hemphill-Haley, a professor emeritus of geology at Cal Poly Humboldt, to dig up the ground and take a thorough look.

Now, after scrutinizing the trench throughout June, the scientists have not only confirmed the existence of a fault but also believe it is still active after four sizable earthquakes over the past 20,000 years. 

“We felt really confident but we needed to go explore by excavating it,” Hemphill-Haley told SFGATE. “We have since discovered that it’s a very well-presented reverse fault.”

The team, which includes geologists from the U.S. Geological Survey, local geotechnical consultants and undergraduates from Cal Poly Humboldt, is still gathering data from the trench — which spans 30 feet wide and 15 feet deep on a Humboldt Redwood Company property — and intends to publish results within the year. Trenching is a method used since the 1970s in paleoseismology, the study of old earthquakes, but it works best in remote places without paved surfaces. They worked as a magnitude 5.6 temblor hit the neighboring county on Wednesday. 

USGS funded the project under the National Earthquake Hazards Reduction Program. Once the findings are complete, USGS can add this fault to the seismic hazard model for California, Hemphill-Haley said.

The trench is in the small unincorporated community of Shively, but the scientists do not know how far the new fault might extend. Shively is in a particularly seismically active area where three tectonic plates converge in what is known as the Mendocino Triple Junction. As the meeting place between the San Andreas Fault system and Cascadia Subduction Zone, which are capable of massive, potentially interconnected temblors, the region is key to understanding where significant earthquakes can strike in California. 

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“We’re trying to see if there is a linkage between the San Andreas and Cascadia, which has been speculated about recently, and how this fault might be linked to that if there is a connection,” Hemphill-Haley explained. “And then the question is whether this fault can produce large earthquakes.”

Based on observations from the trench so far, the team thinks the fault could produce a very large earthquake of magnitude 7 or greater. For reference, one of the biggest recent earthquakes in California was the 7.1 Ridgecrest quake in 2019.

Melanie Michalak, a tectonic geologist who teaches at Cal Poly Humboldt, lives in the area and visited the Shively trench. She’s not involved directly but believes that this kind of research is invaluable for California as a whole.

“Sometimes damaging earthquakes happen on faults that we didn’t know were there, or we suspected but didn’t know much about,” she told SFGATE. “So for scientists to map out and find every potential active fault in the state is such powerful information.”