Earthquakes can trigger a huge slide in Cascadia

On December 20, 2022, a 6.4 magnitude earthquake shook part of Northern California, causing damage in the town of Ferndale. This event, which collided with 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 to document post-seismic slip at the interface caused by ruptures in the subducting Gorda slab.”

Researchers have been seriously studying Cascadia because the fault is capable of producing devastating magnitude 9.0 earthquakes. How small shocks, such as the Ferndale earthquake, change the pressure at the subduction zone front, known as a major thrust, reveals the danger facing the Pacific Northwest.

“This is the first study to document post-seismic slip at the interface caused 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. “These fault interactions may be more common in subduction zones than previously thought.”

Three-way tectonic collision

Within the Cascadia Subduction Zone, the Juan de Fuca, Jorda, and Explorer plates are subducted beneath the larger North American plate along a 1,100 km (700 mi) stretch off the west coast of North America between British Columbia and northern California.

The subduction zone ends near Cape Mendocino, California. There, the Gorda, Pacific, and North American plates meet at the Mendocino triple junction—a triple tectonic collision. North of the triple junction, the Gorda plate is spreading away from the Pacific plate and subducting beneath the North American plate. To the south, the Pacific plate slides across the North American plate along the San Andreas Fault.

Lying between the Pacific and North American plates, the Gorda plate is subjected to enormous pressures and partially releases in frequent earthquakes, making the Cape Mendocino offshore one of the most seismically active areas in California.

In 1992, a 7.2 magnitude earthquake caused fires, landslides and a small tsunami. Most recently, a pair of earthquakes rocked the Petrolia community in 2021. An earthquake struck Ferndale the following year, causing two deaths, as well as damage to a bridge and 150 buildings.

And yet he moves

The authors of the new study explored how the Ferndale earthquake affected movement within the Mendocino trijunction. They used data from the Northern California Seismic Network and the Global Navigation Satellite System (GNSS) to determine 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. Although the quake generated numerous aftershocks along these faults and nearby faults, “aftershocks were noticeably absent from the subduction interface,” said David Shelley, a USGS geophysicist and first author of the study.

But after the shaking subsided, Cascadia's huge thrust was 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 quake, in contrast to the eastward displacement observed during the event.

The researchers simulated this post-seismic movement and found that the GNSS data agreed with about 4 centimeters (1.6 inches) of gradual slip along the Cascadia megathrust over a period of two months. This movement was likely caused by stress transmitted from the main shock.

“The observation of post-seismic slip on the megathrust is one of the most exciting discoveries in this paper,” Jung said. Interactions between sheet ruptures and slip at subduction interfaces have been reported from Japan but not previously described at the Mendocino Triple Junction.

Whose fault is it, anyway?

“We must take into account potential interactions between faults, including the possibility that large high-thrust earthquakes may nucleate when slip on the fault within the subducting plate.”

The study reaffirms how researchers view complex subduction zones. “Instead of thinking [subduction] “In fault systems in isolation, we must take into account potential interactions between faults, including the possibility that large earthquakes will nucleate when slip on the fault within the subducting slab,” Shelley said.

These fault interactions can affect seismic hazard estimates. The massive Cascadia force has generated magnitude 9.0 earthquakes in the past, most recently in 1700. The new study suggests that a fault in the Mendocino triple junction could trigger a powerful Cascadia earthquake, which could affect millions of people in cities like Portland. raw. Seattle, Washington; and Vancouver, British Columbia

The study also highlights how much is still unknown about Mendocino Triple Junction, Gong said. “Although significant progress has been made in densifying the seismic and geodetic monitoring network near the Triple Junction in recent years, a denser network is needed to better image faults and resolve important slip parameters.”

—Caroline Hassler (@carbonbasedcary), science writer

Citation: Hasler, C. (2024), Earthquakes can trigger strong slip in Cascadia, Eos, 105, https://doi.org/10.1029/2024EO240162. Published April 8, 2024. Text © 2024. The Authors. CC BY-NC-ND 3.0 Unless otherwise noted, images are subject to copyright. Any reuse without express permission of the copyright owner is prohibited.

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