Geologists have confirmed the existence of a previously unmapped active fault in California's Humboldt County that could produce a magnitude 7 or greater earthquake. The discovery, made by researchers from the California Geological Survey and Cal Poly Humboldt, centers on the Shively Fault.
This newly identified fault lies within the Mendocino Triple Junction, a region known for its high seismic activity due to the collision of three tectonic plates. Evidence gathered indicates the Shively Fault has caused at least four major earthquakes over the past 20,000 years. Researchers currently estimate that the mapped 1.9 miles of the fault could generate a magnitude 5.6 earthquake. However, they believe the fault may extend further underground, potentially increasing its capacity for a much larger event.
Jason Patton, a geologist with the California Geological Survey, initially identified the fault using LiDAR technology, a remote sensing method that employs laser pulses to map terrain. To confirm its nature and extent, he collaborated with Mark Hemphill-Haley, a professor emeritus of geology at Cal Poly Humboldt. The team excavated a trench approximately 100 feet long, 30 feet wide, and 15 feet deep to examine the subsurface.
Their excavation revealed that the Shively Fault is a reverse fault, a type formed when tectonic forces push one block of Earth's crust upward over another. By studying sediment layers within the trench, geologists can identify offsets and disturbances caused by past ruptures, allowing them to estimate when ancient earthquakes occurred and their frequency. The team is currently analyzing samples from the trench, with plans to publish their complete findings later this year.
If the Shively Fault is officially recognized and added to California's seismic hazard maps, it could provide crucial information for engineers and emergency planners. The potential for a magnitude 7 or larger earthquake raises concerns for Northern California, where such an event could cause widespread and damaging shaking.
This discovery comes shortly after Northern California experienced its strongest earthquake in 40 years on Wednesday. The US Geological Survey reported a magnitude 5.6 quake near Redwood Valley, with tremors felt as far north as Oregon and south into California, as well as parts of western Nevada. Experts have warned of a high probability of aftershocks in the coming week, including a slim but possible chance of another quake exceeding magnitude 5.0. The epicenter of Wednesday's earthquake was near the Maacama Fault zone, which is part of the larger San Andreas Fault system.