When most people think of earthquake risks in the U.S., California immediately comes to mind.
But new research suggests that the state’s seismic dangers might be far more complicated—and potentially more severe—than previously imagined.
Scientists studying Northern California have uncovered hidden fault lines that could make the region far riskier than current models indicate.
The Mendocino Triple Junction’s Secrets
For years, experts believed the Mendocino Triple Junction was a meeting point of three major tectonic plates: the San Andreas Fault to the north, the Cascadia Subduction Zone to the south, and the Mendocino Fault to the east.
This junction has long been recognized as one of the most earthquake-prone areas in the U.S., capable of producing a magnitude 8.0 quake.
Now, researchers reveal that it’s not just three plates converging—it’s at least five distinct plates or fragments hidden deep underground.
This discovery significantly complicates our understanding of the region’s seismic activity and suggests that the earthquake hazard may be higher than previously estimated.
Why the Hidden Faults Matter
Because the junction sits off the coast and affects both the San Andreas and Cascadia systems, understanding its true structure is critical.
If current models fail to account for these hidden faults, millions of people along the West Coast could be at greater risk than anticipated.
Scientists liken this hidden underground system to an iceberg: most of it remains invisible beneath the surface.
Amanda Thomas, a geophysicist at the University of California, Davis, explained, “If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard.”
Simply put, missing these hidden faults in earthquake models could lead to underestimating how much stress is building underground—and that stress could be released suddenly, triggering larger-than-expected quakes.
Clues from Past Earthquakes
The team’s suspicion that the junction was more complex than previously thought dates back to a magnitude 7.2 earthquake in 1992.
That quake occurred at a much shallower depth than models predicted, hinting at hidden structures below the surface.
David Shelly from the US Geological Survey’s Geologic Hazards Center said, “You can see a bit at the surface, but you have to figure out what the configuration is underneath.”
Tracking the Invisible Movements
To map these hidden faults, researchers used a network of seismometers across the Pacific Northwest.
They monitored tiny “low-frequency” quakes deep underground—so small they are thousands of times weaker than what humans can feel.
By comparing these events with tidal forces from the sun and moon, the team could confirm their model.
When gravitational forces aligned with the motion of tectonic plates, the frequency of these minor quakes increased, providing key insights into underground plate movement.
A Far More Complex Plate System
The new findings reveal a tectonic puzzle far beyond a simple three-plate junction.
At the southern end of the Cascadia Subduction Zone, part of the North American plate has broken off and is sinking along with the Gorda plate.
Meanwhile, further south, the Pacific plate is pulling the Pioneer fragment—a remnant of the ancient Farallon plate—northward beneath the continent.
This horizontal fault between the Pioneer fragment and North America is completely invisible from above, making it a hidden but potentially dangerous feature.
The updated model also explains the shallow depth of the 1992 quake, as the subducting surface is higher than previously assumed.
As researcher Materna noted, “It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that. The plate boundary seems not to be where we thought it was.”
The Road Ahead
These discoveries highlight the need for a major revision in earthquake risk assessments along the West Coast.
By understanding these hidden faults, scientists hope to improve predictive models and prepare communities for potential future quakes.
What lies beneath Northern California is far more intricate than anyone realized—and the stakes for millions of residents could not be higher.
Share on Facebook «||» Share on Twitter «||» Share on Reddit «||» Share on LinkedIn