Twin faults in the Pacific Northwest may be connected to produce a 'mega-earthquake'

For decades, residents in the Pacific Northwest and California have trained for the arrival of the impending “Big One,” a catastrophic +8.0 magnitude mega-earthquake that could level the entire Pacific coastline.

New research hints that this deadly and massive earthquake might be triggered by two volatile faults striking in unison, with one quake nudging the other into devastating motion.

Twin faults may share stress fissures

If you live anywhere from Northern California to British Columbia, you have probably heard of the Cascadia subduction zone that lies beneath the Pacific Ocean, as well as the San Andreas Fault.

The massive 700-mile offshore Cascadia Fault, which starts near Cape Mendocino and ends at Vancouver Island in Canada, is made up of one tectonic plate that is forced beneath another.

The San Andreas Fault is a major 800-mile strike-slip fault. It extends from the northern end of the Gulf of California through much of Western California and passes into Tomales Bay within the Pacific Ocean, near San Francisco. 

Because of these two crustal blocks sliding past each other horizontally, Cascadia’s volatile makeup can produce magnitude 9.0 earthquakes and higher.

On January 26, 1700, one such catastrophic event was recorded. The 1700 Cascadia Earthquake had an estimated magnitude of 8.2 – 9.2, triggering a powerful tsunami that traveled as far as Japan.

A long, careful look at the seafloor’s layered memory shows that the two famous faults may share stress fissures across time and distance.

New evidence of layered events

The research was led by Chris Goldfinger at Oregon State University. The team recovered deep-sea cores of sediment drilled from the ocean floor, which capture about 3,100 years of earthquake history, and measured how those layers lined up across both fault systems.

In the cores, the researchers found repeated paired deposits called doublets. These are two stacked layers formed by closely timed events that place a fine unit beneath a coarser unit in an unusual upside-down order.

The pattern appears most clearly near the Cape Mendocino triple junction, the point where three tectonic plates meet, and fades away in opposite directions along each fault. This is a clue that two sources are involved.

The record indicates 18 earthquake deposits in southern Cascadia and 19 in the Noyo Channel along the northern San Andreas during the past three millennia.

The data also recorded 10 pairs lining up closely in time, with median age differences averaging 63 years. 

Some deposits suggest the second layer followed within minutes to hours, a tight linkage that matters for emergency planning in several West Coast cities. The 1700 Cascadia event anchors the older end of this pattern, according to the USGS.

Two faults rupturing close together

Earthquakes can trigger submarine landslides that rush downslope and settle as turbidites, sediment layers formed underwater when shaken loose material cascades down continental slopes.

The deposits form distinct beds that let scientists read sequences of events when faults themselves lie offshore and are difficult to study directly.

Using these beds as an earthquake proxy, or substitute indicator, is a pillar of paleoseismology, the study of ancient earthquakes using geological records.

The approach has been tested extensively on Cascadia, where event beds correlate between canyons and along hundreds of miles. 

Methods include correlation of layer structure, regional mapping, and calibrated radiocarbon to separate local slope failures from widespread shaking events.

By comparing the timing and internal structure of the beds on either side of the Mendocino area, the authors argue that two large faults sometimes ruptured close together.

The distinctive upside-down doublets in the Noyo Channel are interpreted as a fine unit from a more distant Cascadia quake, followed by a coarser unit from a nearer San Andreas rupture.

Syncing faults and a mega-earthquake

Scientists have long suspected that one large earthquake can alter stress fissures and advance failure on a neighboring fault.

A well-documented modern example sits in the Indian Ocean, where a magnitude 9.0 class event in December 2004 was followed approximately three months later by a magnitude 8.6 to 8.7 rupture farther south along the same subduction margin.

The USGS explains the differences between these events and notes their close timing, offering a useful analog for how sequences can cascade.

Fault systems may influence each other

The Cascadia/San Andreas case is different because it involves two distinct fault systems that meet at the coast – not one long subduction interface.

Still, the underlying physics of stress fissure changes acting on nearby faults provides a plausible path for short lags between big events.

The new paper infers partial synchronization, meaning the faults may sometimes influence each other but not always rupture together, and emphasizes that recurrence near the junction is not the sum of both faults.

That observation lines up with the idea that paired layers represent two beds stacked together, not an inflated event count.

Mega-earthquake could be devastating

History shows how severe single events can be. The Great 1906 San Francisco Earthquake, which registered at a magnitude of 7.9, began near San Francisco at 5:12 a.m. on April 18, 1906, and ruptured roughly 296 miles of the San Andreas, producing violent shocks across the region.

Now, consider a scenario in which Cascadia ruptures first and the northern San Andreas follows soon after. It could trigger a mega-earthquake of catastrophic proportions.

Even if the second rupture occurs days or months later, response capacity, supply chains, medical systems, and transportation networks could be stretched across multiple states at once.

The researchers emphasize that their evidence speaks to partial synchronization and stress fissure transfer, not inevitability.

Still, the pattern they document pushes hazard planners to consider multi-fault timelines when they exercise response plans for Portland, Seattle, Vancouver, San Francisco, and smaller coastal communities.

Not a matter of if, but when

The looming threat of the “Big One” has kept volcanologists, seismologists, scientists, and residents along the Pacific coastline on edge for decades.

Scientists say it’s likely not a matter of if a catastrophic mega-earthquake may strike the west coast of the U.S., but when.

“It’s kind of hard to exaggerate what an M9.0 earthquake would be like in the Pacific Northwest,” said Goldfinger.

The team’s findings describe in depth how the signal is strongest near the Mendocino area and fades along each system, which is what one would expect if two sources are stacked in time. 

“We infer that the stratigraphy is best explained by earthquakes on both systems spaced closely in time, beginning with the Cascadia subduction zone, as opposed to aftershock sequences, hydrodynamic generation, or other causes,” wrote the study authors.

Hazard models and communication may need to reflect a non-additive recurrence near the junction and allow for paired events in emergency drills.

That framing is consistent with bed counts and layer structure and avoids overstating the total event rate.

Until a mega-earthquake does strike, diligent observations and monitoring will continue, and the public will stay informed and continue preparedness training.

The study is published in the journal Geosphere.

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