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Scientists have presented strong evidence that major earthquakes along the Cascadia Subduction Zone can rapidly trigger large quakes on California’s San Andreas Fault—raising the risk of near-simultaneous disasters along the U.S. West Coast, according to sediment core analysis and radiocarbon dating from Oregon State University researchers.
October 22, 2025
Source:
Live Science
Research Reveals Tectonic Earthquake Link
Scientists from Oregon State University have provided compelling new data showing that major earthquakes along the Cascadia Subduction Zone offshore Oregon and Washington can trigger powerful quakes along California’s San Andreas Fault. This discovery, outlined in reports from Live Science and ScienceDaily, has urgent implications for seismic risk along the entire U.S. West Coast.
How the Evidence Was Found
Sediment core analysis: Researchers extracted sediment cores both from Cascadia and the northern San Andreas, uncovering nearly identical earthquake-triggered sediment layers.
Serendipitous data: A research vessel’s navigational error allowed unique core samples directly linking earthquake timing across both regions.
Radiocarbon results: Analysis showed event layers formed within hours or days of each other multiple times over 3,000 years—far more often than chance allows (ScienceAlert).
Geological convergence: Both fault systems meet at the highly active Mendocino Triple Junction in Northern California.
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Source:
NBC News
Impacts of Linked Megaquakes
Synchrony and Triggering
At least three major events on the San Andreas Fault appear to have quickly followed massive Cascadia quakes, with the triggering direction flowing from north to south. The most recent such event occurred in 1700, when a powerful Cascadia quake caused tsunamis as far as Japan.
Short intervals: In some cases, the lag between Cascadia and San Andreas quakes could be just hours.
Disaster response risk: Experts warn that emergency systems could be quickly overwhelmed by two nearly simultaneous megaquakes. Models predict thousands of collapsed buildings and thousands of fatalities in Northern California alone.
The research findings have been cited by Oregon Public Broadcasting (OPB) and discussed in federal disaster preparedness plans such as the California Earthquake and Tsunami Response Plan.
Read More
Source:
Live Science
Preparedness and Scientific Consensus
Expert Statements and Caveats
Chris Goldfinger, lead researcher, noted, "If Cascadia went off, I would take that pretty seriously as a very clear advance warning for the northern San Andreas."
Reliability: While absolute dating of ancient earthquakes contains some uncertainty, matching sediment layers across faults point to a real link; coincidence is extremely unlikely.
Preparedness: Agencies now recommend planning for rapid, successive megaquakes—a shift from prior assumptions that saw such sequences as rare.
Public Guidance
Experts urge residents of California and the Pacific Northwest to review emergency plans in light of these findings.
For more background, see analyses from Live Science, ScienceDaily, and OPB.
How often do scientists monitor the Cascadia and San Andreas Faults for potential triggers?
Monitoring is continuous with networks of seismometers, GPS, and ocean-bottom sensors maintained by U.S. Geological Survey and regional agencies.
What are the potential consequences if both the Cascadia and San Andreas Faults were to trigger simultaneously?
How reliable are the sediment cores in predicting future earthquakes?
What measures are being taken to prepare for a potential double earthquake event?
How do scientists determine the timing of past earthquakes from sediment cores?
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