Overview

• The Geosphere paper synthesizes deep-sea cores spanning roughly 3,100 years, identifying distinctive “doublet” turbidites that align across both fault systems.
• Evidence points to the January 1700 Cascadia magnitude-9 event being followed within minutes to days by a large northern San Andreas rupture potentially reaching around magnitude 7.9.
• Researchers report about 18 instances over roughly 3,000 years in which Cascadia earthquakes appear to precede San Andreas events, with 1906 standing out as an exception.
• Corroborating clues include lake and tree-ring records near San Francisco and a recurring rupture initiation near Cape Mendocino that could direct stronger shaking toward the Bay Area.
• Scientists stress the linkage remains a hypothesis pending future observation, while emergency planners note that near-synchronous quakes could overwhelm response capacity and elevate tsunami and shaking risks across multiple cities.