Cascadia megathrust earthquake could trigger San Andreas fault

When the tectonic subduction zone under the northwest of the Pacific moves, it does so dramatically. Not only is the soil shaking with an incredibly destructive magnitude earthquake 9+, but the event triggers tsunamis and landslides to worsen damage. Now a new study in the Geosphere suggests that the “really big” could also trigger a major earthquake in California.

“It is difficult to exaggerate what an M9 earthquake in the northwest of the Pacific would look like,” said Dr. Chris Goldfinger, paleoseologist at Oregon State University and the main author of the new study. “And therefore the possibility that a earthquake of San Andreas would follow is the territory of the film.”

Tectonic parameter

The United States Pacific Coast is defined by the limits of tectonic plates.

North of Cape Mendocino, California, Juan de Fuca’s plaque plunges under the continent in what geologists call a megathrust subduction area.

In the south, the Pacific and Northern plates are drunk against each other, which leads to periodic earthquakes like the famous destructive earthquake of San Francisco.

The prospect that the two could move at the same time redefines the concept of risk of earthquake in the western United States

An accidental discovery

The researchers’ objective came to such an astonishing conclusion when they left. In fact, the study was born due to a navigation error during a research cruise in 1999.

Originally, the plan was right to look at the offshore sedimentary file of cascadia earthquakes by piercing sediment nuclei and by conducting seismic surveys on the northwest margin of the Pacific. But one night, a graduate student entered the bad latitude for its night destination and the drilling ship found itself too far in the south. The researchers had left the margin of Cascadia and were now 90 kilometers south of Cape Mendocino – in the field of San Andreas.

“We found ourselves in northern California,” said Goldfinger. “When I woke up, I was quite hot. But, once we were there, I said to myself:” Well, take a nucleus here. “”

When they examined this nucleus, taken from the Noyo Canyon submarine off the Californian coast near Fort Bragg, they noticed something strange. Throughout the nucleus, some 3,000 years ago, a series of turbiditis or deposits from underwater landslides quickly called currents of turbidity. The turbidites have a characteristic overlapping, with grains of smaller sediment at the bottom and smaller grains at the top. But unusually, many turbidites of the nucleus of Noyo Canyon and the Cascadia nuclei were deposited by pairs.

“There were these major events of double and sandy sand double where he had a fine grain element, and in addition, there was a very coarse sandy unit. And we scraped our heads,” explains Goldfinger.

Once they had used radiocarbon to go out with the events of turbidite in the coast, they found another surprise. In the nuclei taken north and south of Cape Mendocino, more than half of the turbidites were deposited at the same time, in the radiocarbon dating error. The researchers estimated that it was too many to be a coincidence and had to be due to a common cause.

After having excluded other imaginable explanations, they ended up with the awareness that the first unit of each doublet in the Canyon of Noyo was a current of turbidity triggered by a large earthquake on the megathrust of Cascadia. The second doublet unit therefore had to be caused by a movement on the San Andreas nearby.

“A bulb continued and we realized that the Noyo channel probably recorded the earthquakes of Cascadia, and that a similar distance, the Cascadia sites probably recorded the earthquakes of San Andreas,” explains Goldfinger. “Well, what if Cascadia was triggered and triggered a low current of turbidity near San Andreas, then the San Andreas left a little later and triggered a very coarse sand deposit.

Cascade dangers

The moment between earthquakes is uncertain, because the upper turbidite could have eroded the sediments between the doublets. But, in certain deposits of turbidite, the researchers saw evidence that the second doublet unit was deposited in a few minutes or hours following the first, which raises the possibility that almost the entire coast of the American Pacific could experience a major earthquake almost simultaneously.

The potential impact of such a series of earthquakes raises questions about the preparation for this danger range for people’s life and infrastructure.

“I come at the origin of the Bay Area,” explains Goldfinger. “If I were in my hometown of Palo Alto, and Cascadia started, I think I would drive east.