Swarms of tiny earthquakes reveal hidden structure of California faults

A study published in Science used swarms of very small (about magnitude 0) earthquakes to map the Mendocino Triple Junction and found a complex, five-piece geometry that includes a buried Pioneer fragment and may represent an unaccounted seismic hazard.

Scientists analyzed swarms of very small earthquakes to image structures at the Mendocino Triple Junction off the Humboldt County coast. The work, published in Science, focused on tiny events that are far too small to feel. The junction is where the Pacific, North American and Gorda plates meet and has produced several large earthquakes in recent decades. Researchers say studying these microquakes offers a different window into plate motion and fault geometry. Key findings: - The study describes the triple junction as composed of five pieces rather than three simple lines. - Two of those pieces appear to be moving downward and out of view from the surface. - The North American plate has lost a chunk that the Gorda plate is pulling down as it sinks. - South of the junction, the Pacific plate is dragging a buried rock body called the Pioneer fragment northward beneath North America; the boundary with that fragment is horizontal and not visible at the surface. - The authors note this geometry could create a potential unaccounted earthquake hazard in the region, and they used swarms of roughly magnitude‑0 events to reach these conclusions. - The area has a history of large quakes, including a magnitude 7.0 offshore event in December 2024. Summary: The study changes the three-dimensional view of the Mendocino Triple Junction and highlights previously unrecognized structures beneath the seafloor. Experts quoted in the report say the findings could affect how interactions among the Cascadia subduction zone, the San Andreas system and nearby faults are understood. Undetermined at this time.