New studies have revealed strong new evidence. In fact, a significant earthquake on the Cascadia megathrust would immediately increase the chances of a major seismic event on the San Andreas fault. This discovery has profound implications for how earthquake hazards are understood throughout the entire western United States. The Cascadia megathrust is located under the Pacific Northwest. Here, the Juan de Fuca plate plunges under the North American continent, priming the region for massive, catastrophic earthquakes.
Research team leader and geologist Chris Goldfinger. They studied sediment cores taken from the undersea Noyo Canyon close to Fort Bragg, California. Their analysis illustrated just how destructive a magnitude 9 or greater quake on the Cascadia megathrust would be. It can initiate tsunamis and landslides, leading to even more devastation to coastal communities.
Understanding the Cascadia Megathrust
The Cascadia megathrust subduction zone extends along the coast of Washington, Oregon, and Northern California. In this area, the Juan de Fuca plate is gradually being subducted underneath the North American plate. This interaction generates huge amounts of seismic strain that can be released by large earthquakes. Based on paleoseismic records, we know that great earthquakes have been produced along this boundary about every 300 to 600 years.
In this new investigation, scientists looked in detail at sediment cores collected from the Noyo Canyon. These cores were made up of turbidites, deposits laid down by high-energy, sub-marine mass-wasting events commonly referred to as underwater landslides or turbidity currents. The deposition of these deposits denotes a history of strong seismic activity and provides great understanding to the geologic past of the region.
During development of the Noyo Canyon core, the researchers noticed that many of its turbidites occurred in pairs. They found the same patterns in other cores drilled from the Cascadia region. This combination revealed that all five seismic events happened at the same time. One even comes from the Cascadia megathrust, but the other is due to the San Andreas fault. These findings go against established assumptions of when seismic activity occurs. They fundamentally change our understanding of their effects at these two very different tectonic plate boundaries.
The Connection Between Cascadia and San Andreas Faults
The study’s results point toward a concerning possibility: a large earthquake on the Cascadia megathrust may not only occur independently but could instigate an earthquake on the San Andreas fault almost simultaneously. Completing this link dramatically reshapes the way scientists understand earthquake hazards up and down the entire Pacific coast of the United States.
More than half of the turbidites analyzed were deposited at similar times, within a margin of error for radiocarbon dating methods used. It has long been assumed that a big earthquake fired the first unit in each pair of turbidites on the Cascadia megathrust. The second unit perhaps due to subsequent shifts along the San Andreas fault line. This unusual double event indicates that the seismic effects of one fault can have a major effect on activity on the other.
Chris Goldfinger’s message was clear—when the next magnitude 9 cook off occurs on the Cascadia megathrust, it will have catastrophic consequences. He characterized it as being in “movie territory,” meaning apocalyptic results for surrounding areas. He warned that such an event might trigger a sequence of earthquakes along the San Andreas fault, further amplifying risks to California and neighboring states.
Implications for Earthquake Preparedness
From these findings, the implications of this research are deep, calling us to reconsider how we approach preparedness efforts. Either fault is capable of producing a large, damaging earthquake almost simultaneously. This opportunity demands that we take a more holistic view to disaster preparation across the Pacific shoreline.
Historically scientists have treated these two fault systems as completely different entities. This study shows that these two aspects of investigative journalism are closely intertwined in ways we didn’t quite appreciate. These findings urge policymakers and emergency response teams to develop a comprehensive coordinated response plan. Given the risks, this plan should address the cascading impacts that an earthquake on either fault would set off.
An accidental navigational blunder on a 1999 scientific research cruise led to the original research. This – somewhat cringe-inducing – mistake led to sediment core collections from the wrong Noyo Canyon onto the Noyo Canyon! This fortuitous error turned out to have lasting positive effects, allowing scientists to make connections between the history of past earthquakes.