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The West Coast of North America is a geologically tumultuous zone where tectonic plates collide, subducting under and scraping past one another. Over the eons, this activity has regularly caused major earthquakes. New research reveals that some of these seismic events may have happened in sync along the coast’s two major faults: the San Andreas Fault and the Cascadia Subduction Zone.

A team of researchers analyzed a trove of seafloor sediment from the region where the faults meet off the coast of northern California. The researchers’ findings, published recently in Geosphere, reveal that the fault systems have produced several synchronized earthquakes over the past 3,000 years.

Chris Goldfinger, an Oregon State University marine geologist and lead author of the new paper, compares the process to tuning an analog radio, in which the device’s oscillators are synced up to convert incoming signals. “When you tune an old radio, you’re essentially causing one oscillator to vibrate at the same frequency as the other one,” he says. “When these faults synchronize, one fault could tune up the other and cause earthquakes in pairs.”

The Cascadia Subduction Zone, where the Juan de Fuca and Gorda plates slide underneath the North American Plate, stretches all the way from Vancouver Island to northern California to meet the San Andreas Fault. That fault extends south for 750 miles along a boundary where the North American and Pacific plates slide past each other.

Since 1999, Goldfinger and his team have been drilling into the seafloor at this tectonic crossroads, known as the Mendocino Triple Junction, to pull up cores that show a cross-section of the sediments that have built up there. For the new study, the researchers examined more than 130 sediment cores that record roughly 3,000 years of geological history. Many of the cores contained layered sediments known as turbidites, which are created by marine landslides that move large amounts of material around the ocean floor. Many of these landslides are caused by earthquakes, making turbidite layers a useful proxy for pinpointing past seismic events.

Most turbidites have coarser sediment layers at the bottom and finer silt-like sediment at the top, similar to what you get when you swirl a bucket of sand at the beach. But the turbidites in samples from the Mendocino Triple Junction “seem to be upside down with all the sand at the top,” Goldfinger says. “And as far as we know, gravity hasn’t changed.”

As they investigated the puzzling features, Goldfinger realized the cores contained two turbidites stacked on top of each other. This provides evidence of two separate earthquake events happening in quick succession—as the first earthquake was settling a layer of silt over the ocean floor, a second shock sent another avalanche of sand over top.

Some of the layered turbidites are so closely spaced that these events could have happened anywhere from within minutes to decades of each other. Analysis of the ages of shells in the sediments suggest there were at least eight large earthquakes along the San Andreas Fault over the past 3,000 years that occurred within decades of significant quakes along the Cascadia Subduction Zone.

Meng Wei, a marine geologist and geophysicist at the University of Rhode Island, says the idea that fault systems near each other could synchronize has been floating around for years and has been seen at smaller fault boundaries over short periods. But he says the new paper is impressive for illustrating that the phenomenon is possible with larger fault systems over thousands of years.

Though the Cascadia and San Andreas systems have apparently been linked for millennia, there seems to be some variability when it comes to the timing between successive quakes. Wei, who was not involved in the new study, says it is possible that the two faults could produce shaking within a few years of each other at some point in the future, but more research is needed to gauge how one quake triggers another. “Even if these two faults are synchro, the time interval between earthquakes can still be decades,” he adds.

The two systems are also not in perfect sync. The team discovered that some temblors, including the 1906 earthquake that devastated San Francisco, were one-off events that were caused exclusively by movements along the northern San Andreas Fault.

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