Scientists have discovered that pieces of a long-lost tectonic plate are sliding beneath the North American continent in the southern part of the Cascadia subduction zone. This leftover plate fragment could pose new earthquake hazards to the region.
A new study published in the journal Science on Thursday (January 15) reveals that the Pioneer Fragment, the remnant of an oceanic plate that disappeared beneath the North American Plate about 30 million years ago, is now stuck to the floor of the Pacific Ocean and moving northwest with the plate.
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Some evidence suggests that earthquakes in the Cascadia subduction zone could trigger earthquakes along the San Andreas, potentially magnifying the hazard posed by the Cascadia Fault.
Study lead author David Shelley, a geophysicist at the U.S. Geological Survey in Golden, Colorado, said the new findings do not reveal any risks, but are a step toward understanding the relationship.
The Pioneer fragment “effectively increases the contact area between the Pacific plate and the subduction zone,” Sherry told Live Science.
Sherry and his colleagues investigated the Mendocino triple junction using small low-frequency earthquakes and tremors. Tremors are a type of earthquake tremor that occur deep within the Earth’s crust and cannot be felt without sensitive seismometers. “Those are very small events, but they occur frequently on the largest faults,” Sherry said.
By analyzing these phenomena, the researchers determined the direction of the plate’s subtle movements. At Mendocino, the Pacific Plate slides northwest against the North American Plate and collides with the Golda Plate as it is pushed under North America. This is a complex situation, and there are competing explanations for exactly where all the debris is and where the fault line runs.
Sherry and his colleagues discovered that the situation is further complicated by the fact that surprising fragments of the long-gone Farallon Plate are still impacting the triple junction. This ancient tectonic plate began sinking beneath North America 200 million years ago during the breakup of the supercontinent Pangea. Juan de Fuca is one of the remnants of Farallon. But now researchers have discovered another piece of debris stuck to the Pacific plate. This remnant, the Pioneer Fragment, is not sinking, but moving sideways to the continent.
Meanwhile, fragments of the Golda plate that were scraped off the North American plate when the two collided may now be returning to the Golda plate like a “tectonic hot potato” and sinking beneath North America, Sherry said.
This bit of geological confusion may explain why the 1992 Cape Mendocino quake, one of the largest triple-junction earthquakes, had a shallower origin than scientists expected. Because of the extra fragments, “the fault may not be along the oceanic crust itself; it may be shallower than that,” Sherry said.
In addition to increasing the surface area of the Pacific Plate that interacts with Cascadia, the Pioneer fragment itself could cause earthquakes. Between this fragment and the North American plate is a nearly horizontal fault, like a layer of ice on a cake.
“We don’t know if that fault will cause a major earthquake, but it’s a fault that is not currently included in hazard models,” Sherry said. “So that’s something we need to consider in the future.”
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