Overview

• Peer-reviewed research in Geosphere examined more than 130 seafloor sediment cores from the Mendocino Triple Junction to build a 3,000-year earthquake record.
• Stacked turbidite deposits provide evidence that large events on the Cascadia Subduction Zone and the northern San Andreas Fault often occurred minutes to decades apart.
• Age analyses indicate at least eight northern San Andreas earthquakes happened within decades of major Cascadia ruptures over the study period.
• Geological signals suggest a San Andreas quake closely followed the well-documented 1700 Cascadia megathrust event, though the precise interval remains unresolved.
• Scientists stress the systems are not perfectly synchronized—major quakes like 1906 were San Andreas-only—and the study does not estimate current probabilities, prompting calls to update emergency planning.