Cascadia Fault Could Trigger Earthquakes on San Andreas with 'No Separation in Time,' Scientists Claim

The Cascadia and San Andreas Faults may be linked, with earthquakes on one triggering events in the other "with minimal or no separation in time," scientists have said.

Chris Goldfinger and Joel Gutierrez, from Oregon State University, say their evidence showing a relationship between the two goes back almost 3,000 years.

The controversial findings, which have not yet been published, will be presented at the American Geophysical Union fall meeting in San Francisco on Friday.

The San Andreas Fault forms part of the tectonic boundary between the Pacific and North American Plate. It stretches about 750 miles along the east coast and has the potential to produce major earthquakes.

It is one of the most dangerous faults in the U.S. as it is lined by heavily populated areas, including the cities of Los Angeles, San Francisco and San Diego.

Because there has not been a large earthquake at San Andreas for over 100 years, experts are concerned it could be due one relatively soon.

Meanwhile, the Cascadia Subduction Zone, which stretches from northern California to Canada, is known as a "megathrust" fault that can produce some of the biggest earthquakes in the world. In 1700 it produced an earthquake estimated to be between magnitude 8.7 and 9.2. According to the Canadian government, there have been 13 megathrust earthquakes at the subduction zone in the last 6,000 years. "Some have been as close together as 200 years and some have been as far apart as 800 years. The last one was 300 years ago," it said.

In their research, Goldfinger and Gutierrez looked at the two adjacent faults and their histories by using stratigraphic evidence—looking at layers of rocks. From this, they were able to look back at when earthquakes took place at each fault over 2,800 years. Their findings show that on multiple occasions, an earthquake at Cascadia would be followed by one at the northern part of the San Andreas Fault. "In one instance, a separation of [about] 100 years is observed, but in all other cases, time separation between the pairs is not observed," the research abstract said.

Goldfinger and Gutierrez say their research suggests earthquakes at Cascadia can trigger earthquakes at the northern end of San Andreas. According to Nature magazine, the researchers found that this happened at least eight times over 2,800 years. "This is mostly a circumstantial case," Goldfinger told the magazine. "I don't have a smoking gun."

The idea the two tectonic zones are linked was first proposed by Goldfinger over 10 years ago. However, at the time the team did not have enough geological evidence to back it up. By looking at the stratigraphic data, they were able to show concurrent events.

In an email to Newsweek, Goldfinger said: "This evidence has forced us to conclude, as we did earlier, that these two great faults are interacting, one triggering the other, space closely in time. The evidence clearly points to Cascadia first, then some short time later, the San Andreas goes. The evidence was not so strong when we had only radiocarbon, but now we are seeing the stacked beds together, and it's much stronger. Like most things in geology though, you never get to see the event, so there are always uncertainties.

"At this point, alternative explanations are few and require some tortured coincidences, a conclusion we've reached after nearly 20 years working on the problem."

Not everyone is convinced. Joan Gomberg, a seismologist at the U.S. Geological Survey, told Nature that the data could be interpreted differently, with a far less "sensational" conclusion. Gomberg says the layers being analyzed do not show exactly when or where earthquakes took place. "All this uncertainty leaves multiple, equally plausible interpretations on the table," the seismologist said.

John Vidale, a seismologist from the University of Southern California, told Northwest News Network that many of the details of the research are "still hazy." However, he added: "It's not so common to have linkage across two different kinds of faults, from a subduction zone onto a strike-slip fault. So this would be kind of new and interesting."

Goldfinger told Nature that he was putting the case out there for the two zones being connected. Speaking to Northwest News Network, he said: "When you have two big faults that connect directly, there's a pretty high probability they're going to interact in some way. So one fault triggering another, or even becoming synchronized with the other for a period of time, is not a fantastical scenario. It is actually a fairly likely scenario. It just isn't on the radar anywhere yet."

He told Newsweek that their findings could have implications for earthquake forecasting. If an earthquake were to hit Cascadia, it is possible San Andreas may experience one soon after—from just hours to several decades.

"It means that the almost incalculable damage that will take place from the next earthquake on either fault, could include both," he said. "Assessments that never include both faults at once, should begin to consider that as a possible scenario, and not a black swan scenario, but a probable one."

This article has been updated with additional comments from Chris Goldfinger.