The link between the Cascadia and San Andreas earthquakes was discovered 30 years after the missing ship stumbled across key data

A “major earthquake” in the Cascadia subduction zone in the Pacific Northwest could trigger a similarly dangerous quake on the San Andreas Fault in California, a new study suggests.

The findings are based on sediments taken from the seafloor off the coast of Cape Mendocino, California, and offshore Oregon. At Cape Mendocino, California's famous San Andreas Fault ends and the Cascadia Subduction Zone begins.

These are two very different systems, but the sediment record indicates that in the past, at least three San Andreas earthquakes have occurred within hours to two days after the large Cascadia earthquakes. Perhaps another seven or so incidents occurred within decades to years or less. However, there are doubts in dating and records.

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But if the two fault systems are in fact coinciding, it could be a real problem for disaster relief, said the study's first author, Chris Goldfinger, a paleontologist and professor emeritus at Oregon State University.

“Having these things side by side is going to be really difficult,” Goldfinger told Live Science. “There will not be enough resources to respond to even one of them very well, and two of them will be very difficult.”

Possibility of an earthquake

Cascadia can create very powerful earthquakes. It is known that in 1700, the region experienced an earthquake believed to be between 8.7 and 9.2 magnitude, which sent devastating tsunami waves to Japan. These earthquakes are caused by the movement of three oceanic plates (Explorer, Juan de Fuca, and Gorda) sliding under the North American continent.

On the other hand, the San Andreas Fault is a strike-slip fault where the rock masses on either side of the fault move past each other horizontally. The largest known earthquake north of the San Andreas was the 1906 San Francisco earthquake with a magnitude of approximately 7.9. Because the fault passes through densely populated areas, it can cause a significant amount of damage, as happened in the 1989 Loma Prieta earthquake that killed 63 people.

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The two fault systems meet off the coast of Mendocino in an area known as the “triple junction.” Goldfinger and his colleagues were on a research expedition in 1999 to drill core samples from the ocean floor in Cascadia, looking for signs of ancient earthquakes. When large earthquakes occur on Earth, they can trigger underwater flows of sand and sediments known as turbidites. Turbidity follows a pattern where coarse sediment settles out of the water first, creating a layer. It is followed by fine sand and silt to form another layer.

However, on that cruise, confusion resulted in the ship traveling 60 miles from where it was supposed to be. The scientists, who were trying to take a nap between work, did not realize the error until the ship arrived.

“I was like, 'Oh no, we're halfway to San Francisco,'” Goldfinger recalls.

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He decided to take a core sample from that place anyway. When the team later analyzed the sample, they realized it contained a mystery. The turbidites in the sample did not have a coarse layer at the bottom and a fine layer at the top, as was usual.

“This original core of San Andreas had sediment that looked like it was upside down,” Goldfinger said. “The sand was up there.”

The evidence is upside down

The researchers had no explanation for this volatile pattern. They also, at first, had no explanation for another strange mystery of these marine samples: cores taken south of the triple junction, on a world north of San Andreas, seemed to show earthquakes that eerily matched the timing of earthquakes taken north of the triple junction in Cascadia. They found that during the past 1,300 years, there were 18 turbidites potentially caused by earthquakes in Cascadia and 19 offshore in the northern San Andreas. Ten of them appear to have been deposited within 50 to 100 years of each other.

Core sampling indicates that at least three earthquakes in the Cascadia subduction zone were followed by events in the San Andreas fault within just hours or days. (Image credit: Chris Goldfinger)

Even stranger, in three cases, the coarse sand of the upper layer of these upwelling turbidites was mixed with the fine sand of the lower layer, indicating that the upper layer had settled while the lower layer was still in motion. This means that the two layers were deposited within hours to days of each other. These three events included the Cascadia earthquake of 1700, as well as earthquakes that occurred 1,200 years ago and 1,500 years ago.

It took many years to conduct additional radiocarbon dating, collect corresponding earthquake records from other sources such as lake beds in California and ponder the meaning of these strange “double” turbidites. But eventually, Goldfener said, he realized that these San Andreas turbidities might represent two different earthquakes: one, from the distant Cascadia region, which shed only light silt and sand, and the second, from a San Andreas quake that occurred soon after that was locally stronger and could move coarser material.

“That would explain it,” Goldfinger said. “That would explain the matching ages…that's what broke the dam.”

The researchers argue in their new paper, published Sept. 29 in the journal Geosphere, that large earthquakes in Cascadia could transfer stress to the neighboring San Andreas, which would then trigger the San Andreas earthquake not long after.

Harold Tobin, a seismologist at the University of Washington who was not involved in the research, said earthquakes triggering each other were not previously unheard of. But most of these examples occur in the same fault zone.

“There are really no examples that I can think of where two different types of plate boundary faults are as closely coupled as suggested in this paper,” Tobin told Live Science. He said the work was done carefully, but “for me, the jury is still out on whether or not there could be other explanations for the sediment deposits.”

Cascadia and the northern San Andreas region are very seismically active, and many other faults can trigger earthquakes, Tobin said. Sedimentary deposits are difficult to interpret, and there are inherent uncertainties in radiocarbon dating.

“There are a lot of reasons why this system is complicated,” Tobin said. “This is an interesting set of observations, but it will take more detailed work to confirm them.”

Goldfinger said he hopes this work will inspire Cascadia geologists and San Andreas geologists to work more closely together to do this detailed work.

“We all have a lot to learn from each other,” he said. “I hope this will raise the quality of science on both sides.”

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