Scientists excavate hidden Humboldt fault after Mendocino quake rattles county

Scientists are excavating a previously unknown fault deep in a Humboldt County forest after the June 24 magnitude 5.6 earthquake in neighboring Mendocino County rattled homes, injured people and damaged businesses across the North Coast. The trenching is meant to show whether the buried fault belongs in the region’s shaking risk picture.

Humboldt sits near the Mendocino triple junction, where the Pacific, North America and Juan de Fuca/Gorda plates meet. The nearby Mendocino fracture zone has produced damaging earthquakes before, including a magnitude 7.1 in 1923 that caused some damage and a small tsunami, and another magnitude 7.1 in 1994 that did little damage onshore.

On December 5, 2024, an offshore Cape Mendocino magnitude 7.0 earthquake damaged several homes in the Ferndale-Fortuna area, damaged one road and caused a gas leak at a school in Rio Dell. Widespread power outages followed across Humboldt County, and hundreds of aftershocks were recorded offshore in a rapid-response deployment, which USGS called the fastest in U.S. history.

USGS says some surface-faulting events return only every thousands to tens of thousands of years, which means active faults can stay hidden until scientists map them directly. Those maps feed the National Seismic Hazard Maps, the federal standard used to help gauge earthquake risk.

USGS also uses Did You Feel It, a public reporting tool that gathers what people experienced and how much damage they saw after a quake.