Seismologists have used swarms of very small earthquakes to gain new insight into the Mendocino Triple Junction, a complex region off Northern California where three tectonic plates meet. The research, conducted by scientists from the U.S. Geological Survey, University of California, Davis, and University of Colorado Boulder, was published on January 15 in Science.
“If we don’t understand the underlying tectonic processes, it’s hard to predict the seismic hazard,” said Amanda Thomas, professor of earth and planetary sciences at UC Davis.
The Mendocino Triple Junction is where the Pacific plate moves northwest against the North American plate along the San Andreas fault to the south. To the north, the Gorda (or Juan de Fuca) plate subducts beneath North America. This intersection has been linked to significant seismic activity; for example, a magnitude 7.2 earthquake occurred in 1992 at a shallower depth than previously expected.
David Shelly of USGS Geologic Hazards Center likened studying this area to observing an iceberg: “You can see a bit at the surface, but you have to figure out what is the configuration underneath,” Shelly said.
Shelly and colleagues used seismometers across the Pacific Northwest to detect thousands of low-frequency earthquakes that occur as these plates interact deep underground—events far too weak for humans to feel. They tested their model by examining how tidal forces from the Sun and Moon influence earthquake frequency when aligned with plate movement direction.
The study proposes that five moving pieces—not just three plates—are involved in this region’s tectonics. Two components are hidden below ground level. At Cascadia’s southern end, researchers found part of North America has broken off and is descending with the Gorda plate beneath North America. South of the junction, they discovered that a mass called the Pioneer fragment is being dragged under North America by movement of the Pacific plate; this boundary lies horizontally and cannot be seen from above.
The Pioneer fragment was once part of an ancient Farallon plate mostly lost over time along California’s coast.
Kathryn Materna explained that their model accounts for why some earthquakes are shallower than previously believed: “It had been assumed that faults follow the leading edge of the subducting slab, but this example deviates from that,” Materna said. “The plate boundary seems not to be where we thought it was.”
This research received funding support from a National Science Foundation grant.
