Cascadia and San Andreas Faults May Rupture Together, Thousands of Years of Sediment Evidence Suggests
Background on the Two Fault Systems
The Cascadia subduction zone lies offshore from northern California through Oregon, Washington, and into British Columbia, where the Juan de Fuca plate slides beneath the North American plate. The northern San Andreas fault runs through California. Both are capable of producing magnitude-8+ earthquakes, but until recently, scientists had little reason to suspect they might trigger each other.
Sediment Evidence
Researchers examined ocean sediment layers that preserve a historical record of past ground shaking. When a large earthquake occurs, it disturbs seafloor sediments in characteristic ways, leaving distinct signatures that can be dated. The team identified several intervals where sediment layers suggested seismic events on both fault systems occurred within minutes to hours of each other. One notable cluster aligns with the well-documented Cascadia earthquake of 1700, which had an estimated magnitude near 9.
Implications for Earthquake Hazard Assessment
If the two fault systems can rupture in rapid succession, existing emergency response plans—typically designed around a single major quake—may need revision. Coastal communities in Washington, Oregon, and northern California could face overlapping crises, straining resources like search-and-rescue teams, hospital capacity, and supply chains simultaneously. Scientists emphasize that the probability of such a synchronized event in any given year remains low, but the potential consequences warrant closer study.
Ongoing Research
Geologists plan to refine the timing estimates using additional sediment cores and dating techniques. Better understanding of how stress changes propagate between the two fault systems could improve long-term hazard models for the Pacific Northwest and northern California.