Study suggests Cascadia megathrust quake could trigger a San Andreas rupture

A new study in Geosphere reports that sediment cores taken from the seafloor off northern California contain unusual double layers that likely represent two large earthquakes occurring in close succession: a magnitude-9 Cascadia megathrust event and a subsequent rupture of the San Andreas Fault. The research team, led by a paleoseismologist at Oregon State University, used radiocarbon dating to show the paired deposits formed almost simultaneously, possibly within minutes or hours.

This pattern mirrors similar deposits found in Cascadia, suggesting a historic chain reaction between the two fault systems. The authors warn that a repeat scenario could leave the Pacific Northwest devastated before California has time to recover, posing a severe challenge for disaster response. The scientist involved says the discovery has changed his personal emergency strategy, opting to head east from the Bay Area if the Cascadia quake occurs. The findings underscore a heightened risk of compounded seismic hazards along the West Coast.