Seismologists have developed a new understanding of the tectonic complexity beneath Northern California, where the San Andreas fault meets the Cascadia subduction zone. The research, conducted by scientists from the U.S. Geological Survey (USGS), University of California, Davis, and University of Colorado Boulder, was published on January 15 in Science.
The study focuses on the Mendocino Triple Junction off the coast of Humboldt County, a region where three major tectonic plates converge. According to Amanda Thomas, professor of earth and planetary sciences at UC Davis and co-author of the study, “If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard.”
In this area, south of the junction, the Pacific plate moves northwest against the North American plate along the San Andreas fault. To the north, another plate known as Gorda (or Juan de Fuca) moves northeast beneath North America in a process called subduction.
Despite these general movements being mapped out for decades, researchers say that what happens at depth is far more complex than surface observations suggest. A notable example is a magnitude 7.2 earthquake in 1992 that occurred much shallower than expected.
David Shelly from USGS’s Geologic Hazards Center explained: “You can see a bit at the surface, but you have to figure out what is the configuration underneath.”
Shelly and colleagues used an array of seismometers throughout the Pacific Northwest to track swarms of very small “low-frequency” earthquakes occurring deep underground where tectonic plates interact. These micro-earthquakes are thousands of times weaker than anything people would feel on land.
To confirm their findings about how these plates move relative to one another below ground level, researchers analyzed how seismic activity responds to tidal forces caused by gravitational pulls from both the Sun and Moon. Thomas noted: “When tidal forces align with the direction in which a plate wants to move, you should see more small earthquakes.”
Their new model suggests that five distinct moving pieces are involved—two more than previously thought—and some are entirely hidden beneath Earth’s surface. One discovery was that part of North America has broken off at depth and is being pulled down alongside Gorda as it subducts under North America.
Additionally, south of this junction point lies another hidden feature: as it moves northward along its boundary with North America, part of Pacific plate drags a block known as Pioneer fragment underneath—an ancient remnant from what was once called Farallon plate.
Kathryn Materna from CU Boulder explained that this new arrangement clarifies why some earthquakes occur closer to Earth’s surface than earlier models predicted: “It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that,” she said. “The plate boundary seems not to be where we thought it was.”
The project received support through funding from National Science Foundation.
