A new study suggests that two major fault systems on the West Coast—the Cascadia subduction zone and the northern San Andreas fault—may sometimes rupture in quick succession. This conclusion is based on evidence found in ocean sediments that span nearly 3,100 years of geological history. Researchers identified unusual layers in these sediments, indicating that major earthquakes on the two faults have occurred minutes to hours apart. One such event may have occurred around the time of the massive Cascadia earthquake in 1700. If this pattern repeated today, it could lead to simultaneous emergencies across several major West Coast cities. The study, led by Chris Goldfinger, a marine geologist at Oregon State University, analyzed deep-sea sediment cores that contain layers of material accumulated over centuries. These cores preserve records of past geological events. Of particular interest were deposits called turbidites, which form when underwater landslides send large amounts of sediment cascading down the seafloor. These landslides can be triggered by powerful earthquakes, making turbidites valuable clues for understanding past seismic activity. By comparing the timing and structure of turbidite layers from the Cascadia and San Andreas fault systems, researchers found striking similarities. The patterns suggest that major earthquakes on the two faults have sometimes occurred very close together in time. The team identified three such events in the past 1,500 years, with the most recent occurring around 1700. These findings could have significant implications for disaster preparedness, as simultaneous major earthquakes could overwhelm emergency response efforts in multiple cities at once. The idea that separate fault systems could synchronize their behavior has been around for decades, but direct evidence has been rare. Only one previously known instance involved earthquakes in Sumatra that occurred three months apart in 2004 and 2005. Goldfinger has studied the possibility of linked earthquakes for many years, and the new research traces back to an ocean expedition in 1999. During that trip, a navigational error led the team to collect a sediment core from the San Andreas fault region, which ultimately revealed unusual sediment layers suggesting two closely spaced earthquakes. The team used radiocarbon dating to determine the ages of the turbidite layers, both from the core collected near Cape Mendocino and others from surrounding areas. Cape Mendocino is a key location because it lies at the convergence of the northern San Andreas fault and the Cascadia subduction zone. The analysis confirmed that the unusual sediment structures were likely caused by earthquakes on both fault systems occurring in close succession. The researchers refer to these paired layers as "doublets," and they believe the best explanation for their formation is simultaneous earthquakes on the two separate fault systems rather than aftershocks or other processes. The study does not suggest that every large earthquake on one fault will trigger the other. However, the sediment record indicates that closely spaced ruptures have occurred in the past and that interactions between these two fault systems warrant further attention. Understanding such interactions could improve preparedness for potential future seismic events on the West Coast.