On 20 December 2022, a magnitude 6.4 earthquake shook part of Northern California, causing damage in the town of Ferndale. The event, which struck on a fault within the subducting Gorda plate, likely triggered a months-long episode of slip along the nearby Cascadia Subduction Zone, according to a new paper published in Science Advances.
“This is the first study that documents postseismic slip on the interface triggered by ruptures in the subducting Gorda slab.”
Researchers have been studying Cascadia in earnest because the fault is capable of producing devastating magnitude 9.0 earthquakes. How smaller shocks such as the Ferndale quake change the stress on the subduction zone interface, known as the megathrust, reveals the hazard facing the Pacific Northwest.
“This is the first study that documents postseismic slip on the interface triggered by ruptures in the subducting Gorda slab,” said Jianhua Gong, an assistant professor of seismology at Indiana University Bloomington who was not involved in the study. “Such fault interactions might be more common in subduction zones than previously thought.”
A Three-Way Tectonic Collision
Within the Cascadia Subduction Zone, the Juan de Fuca, Gorda, and Explorer plates subduct under the larger North American plate along a 1,100-kilometer (700-mile) stretch off the west coast of North America between British Columbia and Northern California.
The subduction zone ends near Cape Mendocino, Calif. There, the Gorda, Pacific, and North American plates meet at the Mendocino Triple Junction—a three-way tectonic collision. To the north of the triple junction, the Gorda plate spreads away from the Pacific plate and subducts beneath the North American plate. To the south, the Pacific plate slides past the North American plate along the San Andreas Fault.
Caught between the Pacific and North American plates, the Gorda plate is under immense stress, partially released in frequent earthquakes, making offshore Cape Mendocino one of the most seismically active areas in California.
In 1992, a magnitude 7.2 earthquake caused fires, landslides, and a small tsunami. More recently, a pair of quakes rocked the community of Petrolia in 2021. The Ferndale earthquake struck the following year, causing two fatalities, as well as damage to a bridge and 150 buildings.
And Yet It Moves
The authors of the new study explored how the Ferndale earthquake affected movement within the Mendocino Triple Junction. They used data from the Northern California Seismic Network and the Global Navigation Satellite System (GNSS) to locate the sources of the main quake and its aftershocks.
The Ferndale earthquake struck along a roughly east–west oriented strike-slip fault in the subducting Gorda slab, according to the data. Though the earthquake generated many aftershocks along this and nearby faults, “aftershocks were notably absent from the subduction interface,” said David Shelly, a geophysicist at the U.S. Geological Survey and first author of the study.
But after the shaking subsided, the Cascadia megathrust slowly adjusted. GNSS data, which can be used to track ground motion, revealed that the crust south of the main shock fault moved westward in the months after the earthquake—opposite of the eastward displacement observed during the event.
The researchers simulated this postseismic motion and found that the GNSS data are consistent with around 4 centimeters (1.6 inches) of gradual slip along the Cascadia megathrust over 2 months. That motion was likely triggered by stress transferred from the main shock.
“The observation of postseismic slip on the megathrust is one of the most exciting discoveries of this paper,” Gong said. Interactions between slab ruptures and slip at subduction interfaces have been reported from Japan but have not been previously described at the Mendocino Triple Junction.
Whose Fault Is It, Anyway?
“We should consider potential interactions between faults, including the possibility that large megathrust earthquakes might nucleate as slip on a fault within the subducting slab.”
The study reaffirms how researchers view complex subduction zones. “Rather than considering [subduction] fault systems in isolation, we should consider potential interactions between faults, including the possibility that large megathrust earthquakes might nucleate as slip on a fault within the subducting slab,” Shelly said.
Such fault interactions could affect estimates of seismic hazard. The Cascadia megathrust has generated magnitude 9.0 earthquakes in the past, the most recent of which occurred in 1700. The new study suggests that faulting in the Mendocino Triple Junction could trigger a Cascadia megathrust earthquake, which could affect millions of people in cities such as Portland, Ore.; Seattle, Wash.; and Vancouver, B.C.
The study also highlights how much is still unknown about the Mendocino Triple Junction, Gong said. “Although great progress has been made to densify the seismic and geodetic observational network near the triple junction in recent years, an even denser network is needed to better depict faults and resolve important slip parameters.”
—Caroline Hasler (@carbonbasedcary), Science Writer