Scientists have used swarms of minuscule earthquakes to map a hidden and intricate tectonic structure beneath northern California. This region, at the intersection of the San Andreas fault and the Cascadia subduction zone, involves five moving pieces rather than the expected three. The findings help explain past seismic events and improve hazard predictions.
In a study published on January 15 in the journal Science, researchers from the U.S. Geological Survey, University of California, Davis, and University of Colorado Boulder analyzed faint tremors too weak for humans to feel. These low-frequency earthquakes occur where tectonic plates slide against each other, providing clues about the subsurface at the Mendocino Triple Junction, offshore from Humboldt County.
The junction marks where three major plates meet: to the south, the Pacific plate slides northwest past the North American plate along the San Andreas fault; to the north, the Gorda plate—part of the Juan de Fuca plate—subducts eastward beneath the North American plate. However, the team found the setup is more complex, involving five components. A slab of the North American plate has detached and is sinking with the Gorda plate, while south of the junction, the Pacific plate drags the Pioneer fragment—a remnant of the ancient Farallon plate—under the North American plate along a nearly flat, invisible fault.
"If we don't understand the underlying tectonic processes, it's hard to predict the seismic hazard," said Amanda Thomas, a professor of earth and planetary sciences at UC Davis and coauthor of the study.
The researchers deployed a dense network of seismometers across the Pacific Northwest to capture these tiny events. They validated their model by observing how lunar and solar tidal forces modulate the earthquakes, increasing them when aligned with plate motion.
This revised view explains anomalies like the 1992 magnitude 7.2 earthquake, which struck at an unexpectedly shallow depth. "The plate boundary seems not to be where we thought it was," noted coauthor Kathryn Materna. The work, funded by the National Science Foundation, underscores the need for refined models in this high-risk area prone to major quakes.