San Andrea and Cascadia faults may combine to produce continuous seismic hazards

They are two of the most destructive generators of large earthquakes on the West Coast: California’s San Andreas Fault and the Cascadia Subduction Zone off California’s north coast, Oregon, Washington and British Columbia.

The public often views these hazardous areas as separate entities.

But what if they had the ability to suffer successive disasters?

A groundbreaking new study recently published in the journal Geosphere describes this disturbing possibility.

The authors believe that over thousands of years, large earthquakes in the Cascadia subduction zone will soon be followed by large earthquakes north of the San Andreas Fault.

In 1700, an earthquake of approximately magnitude 9 on the Richter scale occurred in the Cascadia subduction zone. Based on archaeological evidence, the village sank and had to be abandoned, according to the U.S. Geological Survey. That earthquake was so strong that an entire section of the Pacific coastline dropped 5 feet. In the Pacific Northwest, Native American stories tell of “how the prairie became the sea” and how canoes were thrown into the trees.

Research shows that the Cascadia earthquake was followed by a Northern San Andreas Fault earthquake from Cape Mendocino toward San Francisco with a magnitude of about 7.9.

“This suggests that the San Andreas earthquake occurred very close to the Cascadia earthquake,” said Jason R. Patton, an engineering geologist at the California Geological Survey and co-author of the study.

There is evidence that the 1700 San Andreas Fault ruptured in earthquakes within hours to days of the Cascadia earthquake. “It could even be a few minutes, but we don’t know for sure,” said Chris Goldfinger, the study’s lead author, a paleoseismologist and professor emeritus of marine geology at Oregon State University.

A hypothetical earthquake or two could be “more than a one-in-a-million black swan probability,” Goldfinger said. “That’s true most of the time. The only exception in the past 2,500 years is 1906 — the only time” that there were no major earthquakes in the Cascadia subduction zone before a major earthquake hit the northern part of the San Andreas fault, according to an analysis of the available data.

The Great Cascadia Earthquake and the Northern San Andreas Fault Earthquake may also have occurred between 1425 and 1475; between 1175 and 1225; around 825 AD and 475 BC, according to Goldfinger.

The Cascadia subduction zone, located off the north coast of California, Oregon, Washington and British Columbia, is capable of producing magnitude 9 earthquakes.

(John Wesley Powell Center for Analysis and Synthesis/USGS)

The author’s conclusions are significant. Emergency managers have long feared a repeat of the 1906 San Francisco earthquake or the 1700 Cascadia earthquake and tsunami.

A repeat of the 1906 earthquake could kill thousands of people and cause hundreds of billions of dollars in property damage, according to one estimate.

A magnitude 9 earthquake in the Cascadia subduction zone could trigger tsunamis with wave heights that would wash away coastal towns, destroy US 101 and cause $70 billion in damage to a large swath of the Pacific Coast. More than 100 bridges will be destroyed, power lines downed and coastal towns isolated. According to a scenario published more than a decade ago, as many as 10,000 people would have died if residents had only had 15 minutes to escape to higher ground. The harbor could suffer significant damage.

The San Andreas Fault cuts through Juniper Mountain.

(Myung J. Chun/Los Angeles Times)

To be sure, the results of this study are only hypothetical. Patton said scientists can’t say for sure whether a major Cascadia quake would trigger an earthquake on the northern San Andreas Fault unless it happens in the future.

But the implications of the study, Goldfinger said, suggest that a large earthquake in the Cascadia subduction zone could cause the northern San Andreas fault to rupture in a major earthquake minutes to hours later, perhaps even months or years later.

“Any one of these major events would consume the resources of the entire country to deal with it,” Goldfinger said. “So if you have two of those, you double that.”

Another key implication is that most past earthquakes north of the San Andreas Fault probably started near Mendocino Point and then moved toward San Francisco. This scenario would have shaken San Francisco more severely than in 1906, when the epicenter was around the Golden Gate Strait but later moved away from the city.

Key to the study was the investigation of fragments of soil that scientists collected from ships collecting samples from deep below the ocean floor. Earthquakes can trigger undersea landslides and leave behind sediments called turbidites that become buried over time.

Typically, scientists expect earthquakes to produce a familiar pattern — coarse sand at the bottom, indicating massive landslides from a large earthquake, and finer silty sediment on top as lighter material settles, Goldfinger said.

But for decades, scientists have been puzzled by a mystery.

Only in one specific area off the California coast—around Noyo Canyon, very close to the San Andreas Fault but about 50 miles from the Cascadia Subduction Zone—the seafloor samples appeared to be inverted, with finer silt sediments at the bottom and coarser sand grains at the top. Goldfinger said scientists couldn’t explain it and the mystery was “very annoying.”

It took them more than 15 years to find a potential answer.

According to this interpretation, finer silty sediments were first deposited by earthquakes farther away in the Cascadia subduction zone. The Cascadia earthquake was about 50 miles away from Noyo Canyon, so the ground shook less at that distance, “and … the first thing it deposited was finer-grained sediment,” Goldfinger said.

Then, according to Goldfinger, an earthquake was triggered by the much closer San Andreas Fault, causing stronger shaking and causing the tumbling of coarser sand grains on top of the silty layer created by the earthquake.

When the researchers came up with this possible explanation, “suddenly it all made sense,” Goldfinger said.

In recent years, scientists have discovered more evidence of earthquakes on the northern San Andreas Fault in 1700, around the time of the Cascadia earthquake.

First, there is evidence of a major earthquake in 1700 along the northern San Andreas Fault from around Mendocino Point to San Francisco, including clues found in Lake Merced near the San Francisco Zoo and other locations north of the city.

Coincidentally, another group of scientists, using an entirely different approach—tree-ring patterns and other observations of ancient coast redwood trees—published a recent study showing that the last major earthquake in the northern San Andreas before 1906 occurred around 1700.

Goldfinger said this was “very good confirmation of our proposal”.

Earthquakes can trigger other earthquakes, which has been shown before. Scientists believe that the 6.1-magnitude Joshua Tree earthquake that occurred on April 22, 1992 caused aftershocks to continue to migrate north. They eventually triggered a 7.3-magnitude Landers earthquake in the Mojave Desert on June 28, strong enough to shake Denver, and a 6.3-magnitude quake in Big Bear a few hours later.

Whenever a large earthquake occurs, the crust around the ruptured fault is squeezed and stretched, Barton said. In some places, seismic strains have lessened; in others, conditions have worsened—bringing the region’s seismic faults closer to the possibility of collapse and causing another major earthquake.

Goldfinger and Patton were co-authors of a 2008 research article published in the Bulletin of the Seismological Society of America that found that large earthquakes in the Cascadia subduction zone slightly exacerbated seismic stresses on the northernmost segment of the San Andreas Fault.