Overview
- In June, scientists dug a roughly 30-foot-wide, 15-foot-deep trench on Humboldt Redwood Company land in Shively and visually confirmed a reverse fault first flagged by lidar about four years ago.
- Layers exposed in the trench contain offsets the team interprets as four sizable earthquakes over roughly the last 20,000 years and preliminary measurements indicate the fault could be capable of magnitude‑7 or larger events.
- The investigation is a multi‑agency effort that includes the California Geological Survey, U.S. Geological Survey and Cal Poly Humboldt, and the work was funded by the USGS National Earthquake Hazards Reduction Program while researchers prepare to publish full results within the year.
- A magnitude 5.6 quake struck nearby Mendocino County while trenching was under way and scientists caution that single recent quakes do not by themselves define the new fault's length or connections to other regional structures.
- Major unknowns remain about how far the fault extends, whether it links to the San Andreas system or the Cascadia subduction zone, and how often it ruptures, and any update to official hazard maps will wait for dating, peer review and formal USGS evaluation.