Geologists have confirmed a previously unmapped active fault in Humboldt County, California, that could unleash a magnitude 7 or greater earthquake. The discovery raises concerns about seismic activity in a region already known for its geological instability.
The fault, named the Shively Fault, is located near the community of Shively within the Mendocino Triple Junction, an area where three tectonic plates converge, making it California's most seismically active zone. Researchers have mapped approximately 1.9 miles of the fault, which is currently sufficient to generate a magnitude 5.6 earthquake. However, scientists believe the fault may extend significantly farther underground, potentially increasing its capacity to produce a much larger seismic event.
Researchers are investigating whether the Shively Fault is connected to the San Andreas Fault system or the Cascadia Subduction Zone, both of which are major sources of seismic risk along the West Coast. The potential for a magnitude 7 earthquake could result in widespread and damaging shaking across parts of Northern California.
The discovery was made by Jason Patton, a geologist with the California Geological Survey, utilizing LiDAR technology for remote sensing. He was later joined by Mark Hemphill-Haley, a professor emeritus of geology at Cal Poly Humboldt, who assisted in excavating the site to confirm the fault's presence and characteristics. Their excavation revealed that the Shively Fault is a reverse fault, a type formed when tectonic forces push one block of Earth's crust over another.
By examining sediment layers exposed in a roughly 100-foot-long trench, geologists identified offsets and disturbances that indicate past earthquake activity. Dating these materials helps researchers reconstruct the timing and frequency of ancient earthquakes along the fault. The team is currently analyzing samples from the trench and expects to publish its full findings later this year.
If its seismic potential is fully confirmed, the Shively Fault could be added to California's official seismic hazard maps. This would enable engineers and emergency planners to better incorporate the newly identified risk into their assessments and preparedness strategies.
This discovery follows a recent significant earthquake in Northern California. On Wednesday, a magnitude 5.6 earthquake struck the region, with tremors felt across several hundred miles up and down the West Coast, reaching Oregon and parts of Nevada. The epicenter of that earthquake was near the Maacama Fault zone, a major strike-slip fault that is part of the larger San Andreas Fault system. The U.S. Geological Survey has warned of a high probability of aftershocks in the coming week, including a slim but non-zero chance of another earthquake exceeding magnitude 5.0.