October 12, 2025
SAN FRANCISCO – The "Big One," the long-feared catastrophic earthquake destined to strike California, may not be a single event but a devastating one-two punch, according to a sobering new study. Researchers have uncovered the strongest evidence yet that a massive rupture along the Cascadia Subduction Zone could trigger a second major quake on the northern San Andreas Fault just minutes later.
The study, published in the journal Geosphere, warns that this tandem earthquake scenario would create a disaster of unprecedented scale, stretching from Northern California to the San Francisco Bay Area. Such an event would cripple hundreds of miles of the West Coast simultaneously, overwhelming emergency response services and endangering millions of people.
"We've always thought of these as separate hazards," said a researcher familiar with the study's implications. "This new evidence forces us to consider a linked, cascading event that is far more destructive than our previous models."
The Cascadia Subduction Zone is a colossal 600-mile fault running from Northern California to British Columbia. It is capable of producing magnitude 9.0 or higher "megathrust" earthquakes, among the most powerful on the planet. The infamous San Andreas Fault, a different type of fault, carves a path more than 800 miles through California, from the Gulf of California to north of San Francisco.
By reviewing geological evidence from past seismic events, scientists found a startling correlation between tremors at the two sites. Their hypothesis centers on a massive Cascadia earthquake that occurred in the year 1700. That event was so powerful it caused a well-documented "orphan tsunami" in Japan. The Geosphere study now suggests this megathrust quake also set off a chain reaction, triggering a massive earthquake on the northern portion of the San Andreas Fault at nearly the same time.
The findings are particularly alarming given the seismic tension building along both faults. Major quakes on the Cascadia zone happen approximately every 220 years, making the fault more than a century overdue for a major rupture. The last great quake on the northern San Andreas Fault was the devastating 1906 San Francisco earthquake.
While seismologists cannot predict the exact timing of earthquakes, the historical patterns suggest both systems are primed for significant activity.
A dual-quake scenario presents a nightmare for emergency planners. A Cascadia quake would likely cause catastrophic damage in the Pacific Northwest, while a nearly simultaneous San Andreas event would devastate population centers in Northern California. The combined impact would sever transportation corridors, communication lines, and supply chains across a vast region, making rescue and relief efforts exceedingly difficult.
"This is a fundamental shift in how we need to view seismic risk in the West," the study implies. "The 'Big One' might not be one event, but two, and preparing for that reality is a challenge of a whole new magnitude."