A piece of an ancient tectonic plate has been found sliding beneath the North American continent in the Cascadia subduction zone. This discovery adds a potential new earthquake risk for the area.
Recent research, published in the journal Science, identified the “Pioneer Fragment.” This oceanic plate fragment disappeared under North America about 30 million years ago. Now, it’s stuck to the Pacific Ocean floor and moving northwest along with that plate.
This discovery is significant because it occurs at the Mendocino triple junction, a key area where the famous San Andreas Fault meets the Cascadia subduction zone. Here, the North American and Pacific plates slide past each other, while oceanic plates dive below North America. Such tectonic shifts can trigger earthquakes of magnitude 9 or higher, according to the Pacific Northwest Seismic Network.
Interestingly, some studies suggest that quakes in the Cascadia zone could activate the San Andreas Fault, increasing overall risks. “While our findings don’t clearly define this risk, they help clarify the complex relationships involved,” said David Shelly, a geophysicist at the U.S. Geological Survey. He explained that the Pioneer Fragment increases the contact area between the Pacific Plate and the subduction zone.
Shelly and his team studied tiny, low-frequency earthquakes at Mendocino. These subtle tremors, hard to detect without specialized equipment, can indicate significant tectonic activity. The Pacific Plate is sliding northwest against North America, pressing against the Gorda Plate as it dips below. But this area is intricate, with many possible explanations for how these plates interact.
The research team also found that an ancient remnant of the Farallon Plate still impacts tectonic movements in this region. This plate began subducting under North America around 200 million years ago. The Juan de Fuca Plate is one remnant of this Farallon Plate, but now researchers see the Pioneer Fragment as another piece that’s moving sideways, not subducting.
Additionally, scraps of the Gorda Plate have seemingly returned to their original location, like a “tectonic hot potato,” possibly subducting again under North America. This geological complexity may help explain why a major earthquake in 1992 had a shallower origin than expected. The presence of these extra tectonic pieces means the fault lines might not be following the oceanic crust as previously thought.
Beyond just increasing the active area between the Pacific Plate and Cascadia, the Pioneer Fragment could potentially cause earthquakes. Between this fragment and the North American Plate lies a nearly horizontal fault. “We don’t know if this fault can produce large earthquakes, but it’s not included in current risk models,” Shelly noted. “This aspect warrants further investigation.”
In light of these discoveries, it’s crucial to keep an eye on seismic activities in the region. With the potential for new earthquake risks on the horizon, ongoing research will play a vital role in understanding and addressing these geological challenges.
