Two of the most closely watched and potentially catastrophic fault systems on North America’s West Coast may be far more interconnected than geologists previously realized, fundamentally challenging how scientists view regional earthquake hazards and disaster response preparedness.
A recent comprehensive study suggests that the Cascadia subduction zone and the San Andreas fault are capable of becoming synchronized. According to the research, a major earthquake on one of these colossal fault systems could potentially be followed almost immediately by seismic activity on the other. In several distinct instances preserved within the deep geological record, the two massive fault systems appear to have ruptured mere minutes or hours apart from one another.
"We’re used to hearing the ‘Big One’ – Cascadia – being this catastrophic huge thing," said Chris Goldfinger, a marine geologist at Oregon State University and lead author of the study. "It turns out it’s not the worst-case scenario."
The implications of this newfound connectivity extend far beyond academic geology, carrying profound consequences for emergency management agencies, civil engineers, and millions of residents inhabiting the Pacific Rim and California coastlines.
3,100 Years of Earthquake History
To investigate the complex relationship between the two fault systems, Goldfinger and his multidisciplinary team of researchers embarked on a deep-dive analysis of ocean sediment cores. These cylindrical geological samples preserve a continuous, layered archive of material that has accumulated on the ocean floor over thousands of years, offering a window into prehistoric natural disasters.
Of primary interest to the researchers were specific geological deposits known as turbidites. These unique layers can form when powerful underwater landslides send massive amounts of sediment rushing rapidly down submarine slopes and canyons. While various phenomena can initiate these undersea flows, high-magnitude earthquakes are among the primary triggers capable of setting off widespread submarine landslides across continental margins.
By meticulously comparing turbidite layers associated with the Cascadia subduction zone and the northern segments of the San Andreas fault system, the research team discovered striking similarities in both their timing and their internal structure. These repeating patterns strongly suggest that major, ground-shaking earthquakes on the two distinct fault systems have occasionally occurred in unusually rapid succession.
Pinning down the exact amount of time that elapsed between ancient prehistoric earthquakes is inherently difficult given the limits of geological resolution. However, Goldfinger noted that the research team successfully identified three distinct geological events during the past 1,500 years where Cascadia and the northern San Andreas appear to have ruptured within minutes to hours of each other.
The most recent of those severe paired episodes is estimated to have occurred around the year 1700, a timeframe that aligns with historical and geological records of the massive Cascadia earthquake that generated a tsunami felt across the Pacific Ocean.
A Potential West Coast One-Two Punch
The realistic possibility of closely spaced, multi-fault earthquakes could have severe, far-reaching consequences for disaster preparation and emergency response infrastructure along the entire western seaboard of North America.
Even a single major rupture along either the Cascadia subduction zone or the San Andreas fault would create an astronomical emergency response challenge, threatening modern infrastructure, disrupting critical supply chains, and risking thousands of lives. If both massive fault systems were to rupture within a short, compressed period of time, multiple major metropolitan areas could find themselves dealing with catastrophic, widespread damage simultaneously.
"We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it," Goldfinger said. "And if they both went off together, then you’ve got potentially San Francisco, Portland, Seattle, and Vancouver all in an emergency situation in a compressed timeframe."
For decades, seismologists and geologists have debated whether separate, neighboring fault systems are capable of synchronizing their mechanical behavior. Despite theoretical models suggesting it might be possible, direct observations of fault synchronization in the modern era have remained extremely rare.
Prior to this study, only a single modern example had been reliably documented, involving a devastating sequence of major earthquakes in Sumatra that occurred three months apart in 2004 and 2005. The new research suggests that Cascadia and the San Andreas may share an even tighter, more immediate coupling.
An Accidental Discovery at Sea
Dr. Goldfinger has been investigating the intricate possibility of linked earthquakes for decades, and the unexpected origins of this recent breakthrough trace back to a routine oceanographic expedition conducted in 1999.
At the time, Goldfinger and his colleagues were aboard a research vessel collecting sediment cores from the Cascadia subduction zone off the coasts of Oregon and northern California. During the voyage, a navigational mistake inadvertently carried their research vessel off its intended course, placing the ship approximately 55 miles south of Cape Mendocino, California, well inside the territory of the San Andreas fault region.
Rather than abandoning the navigational detour and returning immediately to their original grid, the scientific team made the pivotal decision to drop anchor and drill an exploratory sediment core right there.
That unexpected sample eventually revealed something extraordinary that would alter the trajectory of Goldfinger’s research for years to come.
Under normal geological conditions, marine turbidites display a very predictable structural arrangement. Coarser, heavier material settles out of the water column first, forming the lower, dense portion of a sedimentary layer, while finer silt and clay gradually settle out above it over a longer period.
In this anomalous core, however, the researchers discovered the exact reverse arrangement. Fine, silty material appeared at the bottom of the layer, while coarse, sandy sediment sat directly on top of it.
The Clue Hidden in Upside-Down Sediment
This unusual and counterintuitive layering suggested to the geologists that two separate, high-energy events had occurred in rapid, succession, disrupting the normal sequence of sedimentation.
The research team ultimately concluded that the fine sediment found at the bottom of the core had been deposited following a major earthquake originating on the Cascadia subduction zone. Meanwhile, the coarse sediment resting directly above it appeared to have resulted from subsequent movement and shaking on the nearby San Andreas fault system.
To investigate this hypothesis further and test the broader regional extent of the phenomenon, the team utilized advanced radiocarbon dating techniques to estimate the ages of turbidite deposits retrieved from that specific core as well as from numerous other cores collected both north and south of Cape Mendocino.
Cape Mendocino serves as a critical geological junction because it lies in close proximity to the region where the northern San Andreas fault system and the Cascadia subduction zone converge.
This expanded analytical effort significantly strengthened the case that the unusual, inverted sediment structures were produced by earthquakes on both independent fault systems occurring exceptionally close together in time. The researchers have designated these distinctive paired sedimentary layers as "doublets."
According to the study’s authors, major earthquakes occurring on the two separate fault systems provide the most scientifically sound explanation for how these distinct doublets formed, ruling out common alternatives such as standard aftershocks or localized submarine slope failures.
A New Look at West Coast Earthquake Risk
If the Cascadia subduction zone and the northern San Andreas fault are indeed capable of influencing one another and triggering near-simultaneous ruptures, long-term earthquake planning and hazard mitigation strategies may need to be fundamentally re-evaluated.
Emergency planners may soon need to account for complex multi-hazard scenarios in which catastrophic disasters unfold across a much larger geographic expanse of the West Coast within a severely compressed timeframe, straining national and international disaster relief capabilities to their absolute limits.
The new findings do not imply that every large earthquake on one fault will automatically trigger a corresponding rupture on the other. Instead, the preserved sediment record clearly indicates that closely spaced, synchronized ruptures have occurred multiple times in the prehistoric past and that complex mechanical interactions between the two major systems deserve significantly greater scientific attention.
Other co-authors contributing to the published research paper include Ann Morey, Christopher Romsos, and Bran Black of Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences; Jeff Beeson of the National Oceanic and Atmospheric Administration and Oregon State University; Maureen Walzcak of the University of Washington; Alexis Vizcaino of the Springer Nature Group in Germany; Jason Patton of the California Department of Conservation; and C. Hans Nelson and Julia Gutiérrez-Pastor of the Instituto Andaluz de Ciencias de la Tierra in Spain.
