A new study indicates that the Cascadia subduction zone and the northern San Andreas fault may trigger earthquakes in close succession, potentially amplifying disaster risks along the U.S. West Coast. Researchers identified evidence of such synchronization in ocean floor sediment cores spanning 3,100 years. This connection could affect cities from San Francisco to Vancouver simultaneously.
Two major fault systems on North America's West Coast—the Cascadia subduction zone and the San Andreas fault—may be more interconnected than previously thought, according to research led by Chris Goldfinger, a marine geologist at Oregon State University. The study, published in Geosphere, analyzed turbidites in sediment cores from the ocean floor, revealing patterns that suggest earthquakes on one fault can trigger events on the other within minutes or hours. Goldfinger noted three instances in the past 1,500 years, including 1700, where this occurred, based on similarities in sediment layers near Cape Mendocino, California, where the faults meet. These unusual 'doublets'—reversed layers of coarse and fine sediment—point to back-to-back quakes rather than aftershocks, confirmed by radiocarbon dating. Materials provided by Oregon State University. Journal reference: C. Goldfinger et al., Geosphere, 2025; 21 (6): 1132, DOI: 10.1130/GES02857.1. Goldfinger warned that a synchronized event could overwhelm response resources. 'We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it,' he said. 'And if they both went off together, then you've got potentially San Francisco, Portland, Seattle, and Vancouver all in an emergency situation in a compressed timeframe.' The finding builds on decades of research, including a 1999 cruise where an off-course core sample provided key evidence. The study involved collaborators from Oregon State, NOAA, University of Washington, and institutions in Germany and Spain. While scientists have theorized fault interactions, real-world evidence has been limited, with Sumatra's 2004-2005 quakes as a rare example three months apart. This research underscores the need for enhanced West Coast earthquake preparedness.
