New Fault Near Redwoods Could Trigger Magnitude 7 Earthquake in Northern California

California's iconic redwood forests stand on a precarious foundation, concealed beneath a newly identified earthquake threat capable of triggering a devastating magnitude 7 tremor. Geologists have officially confirmed the existence of a previously unmapped active fault in Humboldt County, revealing evidence that it has generated at least four major earthquakes over the last 20,000 years.

Located near the small community of Shively within the Mendocino Triple Junction, this site marks where three tectonic plates collide, rendering it the most seismically active region in the state. Researchers warn that if mapping efforts confirm the fault extends beyond its currently identified section, it could unleash a magnitude 7 or greater event. Such an earthquake would likely produce widespread, damaging shaking across significant portions of Northern California. Currently, scientists have mapped approximately 1.9 miles of the fault, a length sufficient to generate a magnitude 5.6 quake, but they suspect the fracture runs much farther underground.

The discovery was spearheaded by Jason Patton of the California Geological Survey, who utilized LiDAR technology to map the terrain with exceptional accuracy. Patton was joined by Mark Hemphill-Haley, a professor emeritus at Cal Poly Humboldt, who helped excavate the site for closer inspection. "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."

By digging a trench roughly 100 feet long, 30 feet wide, and 15 feet deep, the team exposed the fault beneath the surface to examine sediment layers accumulated over millennia. Every rupture leaves subtle offsets in these layers, allowing researchers to date ancient earthquakes and estimate rupture frequency. The trench confirmed the fault is a reverse fracture, created when immense tectonic forces push one block of crust upward over another. The team continues to analyze collected samples before publishing complete findings later this year. If verified, the Shively Fault will be incorporated into California's official seismic hazard maps, enabling engineers and emergency planners to better account for the risk posed by this newly confirmed threat.

The urgency of this discovery is heightened by recent seismic activity. On Wednesday, California experienced its strongest earthquake in 40 years, with the US Geological Survey recording a magnitude 5.6 quake at 11:10 a.m. ET near Redwood Valley. While the strongest shaking was felt at the epicenter, tremors traveled more than 600 miles along the West Coast, reaching Coos Bay, Oregon in the north and Salinas, California in the south, as well as parts of western Nevada.

The USGS has issued stark warnings, stating there is a nearly 90 percent chance of another earthquake stronger than magnitude 3.0 striking the region within the next week. Experts further note a 40 percent risk of a quake exceeding magnitude 4.0 and a seven percent chance of an event over magnitude 5.0 hitting the West Coast this week. The epicenter of Wednesday's quake was located just seven miles from the Maacama Fault zone, a major active strike-slip fault capable of causing strong shaking in Mendocino and Sonoma counties. As this fault runs through rural communities and wine country, its connection to the larger, infamous 800-mile-long San Andreas Fault system only amplifies the concern for locals.