Cascadia Megaquake Could Trigger the San Andreas — And Serve As a Warning for the Bay Area

Researchers warn that a massive rupture on the Cascadia subduction zone in the Pacific Northwest could increase stress on the northern San Andreas fault, potentially triggering near-consecutive major earthquakes that would send destructive seismic waves up and down the West Coast, including through the Bay Area.

What the Study Found

Lead author Chris Goldfinger, an Oregon State University marine geologist and Bay Area native, and his colleagues analyzed deep-sea sediment cores from the seafloor in both regions. They compared turbidites — distinct sediment layers deposited by underwater landslides often triggered by strong earthquakes — and found that nine very large Cascadia events over the past 3,000 years were followed by earthquakes on the northern San Andreas, including the 1700 rupture.

The Cascadia subduction zone runs from Vancouver Island, Canada, to Cape Mendocino in Northern California. The team notes Cascadia is capable of producing magnitude 9+ events (the last such event occurred around 1700). They estimate the northern San Andreas has roughly a 37% chance of a magnitude ~7+ earthquake in the next 50 years.

How The Link Works — And What It Means

When one fault ruptures, it can transfer stress to nearby faults, potentially advancing the timing of earthquakes on those faults. The study’s turbidite correlations suggest a historical pattern of Cascadia ruptures preceding northern San Andreas events. However, the time gap between such paired events is highly uncertain — it can range from minutes to decades, possibly even up to 50 years in some cases.

"If Cascadia went off, it would serve as a warning system to the Bay Area," Goldfinger said, emphasizing that a Cascadia rupture could increase the probability the San Andreas might follow.

Risk, Response, and Preparedness

• Emergency strain: A simultaneous or closely spaced pair of large quakes would heavily tax national and regional emergency response systems, reducing available resources for rescue, medical care, aviation and maritime logistics.
• Warning time: Local early-warning systems could provide seconds to minutes of automated alerts depending on location; Seattle might get up to a couple of minutes for a southern Cascadia rupture, while many Bay Area locations could get only seconds for a nearby San Andreas event.
• Tsunami threat: A Cascadia-generated tsunami would be catastrophic for the Pacific Northwest and could affect Japan, but it would have very limited impact on the Bay Area shoreline.

Q&A Highlights With Chris Goldfinger

Q: What is your worst fear about synchronized Cascadia and San Andreas ruptures?

A: The biggest concern is that two large events would draw down emergency resources nationwide. Airports and ports could be damaged, hindering both air and sea response.

Q: How much warning could Bay Area residents get after a Cascadia quake?

A: The time between a Cascadia event and a possible San Andreas rupture is unpredictable — it might be minutes, years, or decades. In the paired events studied, only the 1700 sequence has relatively tight timing (estimated between 20 minutes and one year).

Q: Should people evacuate the Bay Area after a Cascadia quake?

A: Goldfinger suggests considering temporary relocation if feasible, but he does not expect mass highway evacuations since the warning window cannot be precisely defined. Planning and preparedness are more practical than panic-driven flight.

Q: Could a Cascadia quake trigger formal early warnings or tsunami advisories?

A: That would be a societal and policy decision, not purely a scientific one. Systems like MyShake or official tsunami advisories could be used, but coordinated decisions would be required.

Takeaway

The research identifies a historical connection between major Cascadia ruptures and subsequent northern San Andreas activity. While that link could provide an advance signal to Bay Area officials and residents, it does not constitute a precise prediction: timing remains highly uncertain. The finding underscores the importance of robust early-warning systems, resilient infrastructure, and community preparedness along the entire West Coast.