December 5, 2024 Earthquake: Offshore Cape Mendocino, California, United States

Several homes damaged in the Ferndale-Fortuna area; one road damaged and a gas leak occurred at a school at Rio Dell; widespread power outages occurred in Humboldt County. A small tsunami with a wave height of 9 cm was recorded at Arena Cove.

The December 5, 2024, M 7.0 Offshore Cape Mendocino earthquake occurred approximately 70 km southwest of Ferndale, northern California. This earthquake occurred west of the Mendocino triple junction – the region where the Pacific, North America, and Juan de Fuca/Gorda plates meet – along the Mendocino Fault demarking the transform boundary between the Pacific and Juan de Fuca/Gorda plates. Focal mechanism solutions and aftershock distributions indicate that the earthquake occurred on an east-west striking right-lateral strike-slip fault, accommodating the eastward motion of the Juan de Fuca/Gorda plate relative to the Pacific plate.

In the Mendocino triple junction region, the northern terminus of San Andreas plate boundary (the transform boundary between the Pacific and North America plates) meets the southern terminus of the Cascadia subduction zone (the convergent boundary between the Juan de Fuca/Gorda plates and the North America plate) and the eastern terminus of the Mendocino fracture zone (the transform boundary between the Pacific and Juan de Fuca/Gorda plates). The relative motions of the Pacific and Juan de Fuca/Gorda plates with respect to the North America plate result in the Mendocino triple junction migrating northward, leading to the San Andreas plate boundary lengthening in the wake of the retreating southern end of the Cascadia subduction zone. Seismicity in this region reflects the relative motions across the three plate boundaries, as well as the effects of Mendocino triple junction migration and the corresponding plate boundary reorganization.

This area is thus one of the more seismically active regions of California and its surroundings. Of the 11 earthquakes magnitude 7 and larger in California since 1900, five (including the 2024 event) occurred in the vicinity of the Mendocino triple junction. Along the Mendocino fracture zone, the December 5, 2024, earthquake occurred in a similar location to a magnitude 7.1 earthquake in 1923 that caused some damage and triggered a small tsunami. Further west along the Mendocino fracture zone, a magnitude 7.1 earthquake occurred in 1994 that did little damage to onshore regions. The most damaging earthquakes in this region of northern California have occurred onshore, within the North America plate east of the three major plate boundary faults associated with the Mendocino triple junction. These include a magnitude 7.2 earthquake in April 1992 near Petrolia, California (a reverse faulting earthquake), and a magnitude 7.2 earthquake in November 1980 near Eureka, California (a strike-slip earthquake). Both events caused extensive regional damage, and the 1992 earthquake triggered a small tsunami recorded in California and Hawaii. More recently, the Petrolia/Ferndale region experienced a magnitude 6.2 earthquake on December 20, 2021, followed by a magnitude 6.4 earthquake one year later, on December 20, 2022.

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Several homes damaged in the Ferndale-Fortuna area; one road damaged and a gas leak occurred at a school at Rio Dell; widespread power outages occurred in Humboldt County. A small tsunami with a wave height of 9 cm was recorded at Arena Cove.

Tectonic Summary

The December 5, 2024, M 7.0 Offshore Cape Mendocino earthquake occurred approximately 100 km southwest of Ferndale, California, off the coast of northern California in the vicinity of the Mendocino Fracture Zone. This earthquake occurred in the vicinity of the Mendocino triple junction – the region where the Pacific, North America, and Juan de Fuca/Gorda plates meet. Focal mechanism solutions indicate that rupture occurred as a result of strike-slip faulting on a steeply dipping fault striking either east-southeast or north-northeast. The location, depth and faulting mechanism indicate that this event likely occurred on or near the Mendocino Fracture Zone, a fault zone that strike east-southeast and forms the boundary between the Pacific Plate to the south and the subducting Gorda Plate to the north.

Earthquakes are common in the region around the Mendocino triple junction. Oblique motion between the southern Juan de Fuca/Gorda plate and Pacific plate causes north-south compression within the Gorda plate and right-lateral translation along the boundary between the plates. The M 6.4 Ferndale earthquake occurred approximately 75 km to the east-northeast on December 20, 2022. A M 6.2 event occurred approximately 90 km east of the 2024 event, on December 20, 2021. In the past century, there have been at least 40 other earthquakes of M6 or larger, including five earthquakes M7 or larger, within 250 km of the December 5, 2024, earthquake. These prior earthquakes primarily occurred along the Mendocino transform fault, in the Cascadia subduction zone, or within the Juan de Fuca/Gorda plate.