In the world’s tectonic nightmares, a giant earthquake on the San Andreas fault in California looms large. But farther north, another geologic fault — the Cascadia subduction zone — gives disaster planners even more to worry about. The possibility that Cascadia could shake the Northwest in the near future was highlighted in a 2015 New Yorker article that called the hypothetical event “The Really Big One.”
In a study published last month in the journal Geosphere, earthquake scientists suggest that the two faults could have an intertwined fate. Their paper suggests that Cascadia earthquakes of the recent geological past have triggered earthquakes along the San Andreas fault too.
“Cascadia is often talked about as, ‘When it goes, it’ll be the biggest disaster in North American history,’” said Chris Goldfinger, a professor emeritus at Oregon State University whose earlier warnings were highlighted in that New Yorker article. “But what if the San Andreas goes at the same time? It’s still going to be the biggest disaster in North American history, but it’s quite a bit bigger.”
The Cascadia subduction zone stretches from Vancouver Island in Canada to Northern California. In this region, an oceanic plate called Juan de Fuca dives beneath the continental plate of North America.
“It’s where there’s a lot of frictional contact that the trouble for people arises,” Dr. Goldfinger said.
With the new paper, he and colleagues have found evidence that the friction could cause problems for people farther south, near the San Andreas fault, which begins at Cape Mendocino south of Eureka, turns inland and terminates near the Salton Sea. Cascadia and San Andreas come together near Cape Mendocino.
Dr. Goldfinger first gathered data connecting the faults during a research cruise in 1999. His group had traveled to the southern end of Cascadia to collect sediment cores from the seabed, which can reveal a historical record of thousands of years of the fault’s earthquakes.
A graduate student on the night watch gave the ship a wayward waypoint.
When Dr. Goldfinger got up in the morning and looked at the navigation display, he was confused, then distraught.
They were halfway to San Francisco, just off the northern part of the San Andreas Fault, in a part of the ocean called the Noyo Channel.
The student, sleeping on a stool, awoke. “He looks up and he goes, ‘Oh, no,’” Dr. Goldfinger said.
They decided to gather samples anyway.
When they returned and analyzed the sample, they were looking for what paleoseismologists call “turbidites”: submarine landslide deposits left on the sea floor after an earthquake, with heavier material settled to the bottom and lighter material on top.
But the turbidites they saw were weird. “They were essentially upside down,” Dr. Goldfinger said. Going back thousands of years into the core’s sedimentary record, they found the same backwardness: The heavy sand was on top, and the lighter material was at the bottom.
In more data, gathered later and from more locations, they saw the same flipped fraternal twinning.
They pondered this anomaly, along with even more data, for years before coming up with an explanation. “The upside-down turbidites weren’t upside down,” Dr. Goldfinger said. They were from two separate events: The bottom layer in each sample was from a Cascadia earthquake — light material that had traveled far. The second was heavier material from a closer San Andreas quake.
They seemed to have been deposited one right after the other — indicating that soon after Cascadia’s friction had reached a breaking point, the San Andreas followed. “In the last, say, 2,500 years, all eight of the largest Cascadia earthquakes have a matching San Andreas earthquake, or appear to,” Dr. Goldfinger said.
“Appear to” would be key words to Lucy Jones, a research associate at the Seismological Laboratory at Caltech. It sure looks like a pattern, she said, and it very well may be — but she wants more proof it’s not random coincidence.
She would like to see more statistical analysis, and more quantification of uncertainty about these coincident events. “Statistical study is important because we do a really good job of fooling ourselves,” said Dr. Jones.
If the connection between the faults bears out, she added, it would make sense. “I’d be surprised if you didn’t see evidence of San Andreas earthquakes right after Cascadia ones,” she said (although having data to support even obvious, but uninvestigated, ideas is helpful).
After all, it makes intuitive sense that tectonic upheaval could affect nearby plates. Dr. Goldfinger admits the field of geology accepts ideas slowly, even when they make sense. “We understood general relativity before we knew why Africa and South America fit together,” he said.
But whether scientists are talking about San Andreas and Cascadia, or faults elsewhere in the world, one idea remains true: “The most dangerous day is the day after a big earthquake,” Dr. Jones said.