The “big one” might not come alone: Two major West Coast faults may strike back to back

A recent study suggests that the Cascadia subduction zone and the San Andreas fault can become synchronized, with a major earthquake on one fault potentially followed by seismic activity on the other. In some cases preserved in the geologic record, the two systems may have ruptured only minutes or hours apart.

"We're used to hearing the 'Big One' - Cascadia - being this catastrophic huge thing," said Chris Goldfinger, a marine geologist at Oregon State University and lead author of the study. "It turns out it's not the worst-case scenario."

3,100 Years of Earthquake History

To investigate the relationship between the faults, Goldfinger and his colleagues studied deep sea sediment cores containing about 3,100 years of geologic history.

These cylindrical samples preserve layers of material that accumulated on the ocean floor over centuries. Of particular interest were deposits called turbidites, which can form when underwater landslides send large amounts of sediment rushing downhill. Powerful earthquakes are one of the events capable of triggering these submarine landslides.

By comparing turbidite layers associated with the Cascadia and San Andreas fault systems, the researchers found striking similarities in their timing and structure. Those patterns suggest that major earthquakes on the two faults have sometimes occurred unusually close together.

Pinning down the exact amount of time between ancient earthquakes is difficult. However, Goldfinger said the team identified three events during the past 1,500 years when Cascadia and the northern San Andreas appear to have ruptured within minutes to hours of each other.

The most recent of those episodes occurred around 1700.

A Potential West Coast One-Two Punch

The possibility of closely spaced earthquakes could have major consequences for disaster preparation.

Even one major rupture along Cascadia or the San Andreas could create an enormous emergency response challenge. If both systems ruptured within a short period, multiple major metropolitan areas could be dealing with severe damage at nearly the same time.

"We could expect that an earthquake on one of the faults alone would draw down the resources of the whole country to respond to it," Goldfinger said. "And if they both went off together, then you've got potentially San Francisco. Portland, Seattle and Vancouver all in an emergency situation in a compressed timeframe."

Scientists have considered for decades whether separate fault systems can synchronize their behavior. Direct observations, however, are extremely rare.

Only one example has previously been observed, involving earthquakes in Sumatra that occurred three months apart in 2004 and 2005.

An Accidental Discovery at Sea

Goldfinger has been investigating the possibility of linked earthquakes for decades, and the roots of the new research stretch back to an ocean expedition in 1999.

At the time, Goldfinger and his colleagues were collecting sediment cores from the Cascadia subduction zone off Oregon and northern California. A navigational mistake carried their vessel off its intended route, placing the researchers about 55 miles south of Cape Mendocino, California, inside the San Andreas fault region.

Rather than abandon the detour, the team decided to drill a sediment core there.

That unexpected sample eventually revealed something unusual.

Turbidites normally show a predictable arrangement. Coarser material settles first and forms the lower portion of a layer, while finer sediment gradually settles above it.

In this core, however, the researchers found the reverse. Fine, silty material appeared at the bottom, while coarse, sandy sediment sat on top.

The Clue Hidden in Upside Down Sediment

The unusual layering suggested that two separate events had occurred in rapid succession.

The researchers concluded that the fine sediment at the bottom had been deposited following a major Cascadia subduction zone earthquake. The coarse sediment above it appeared to have resulted from subsequent movement on the nearby San Andreas fault.

To investigate further, the team used radiocarbon dating to estimate the ages of turbidite deposits from that core and from others collected both north and south of Cape Mendocino.

Cape Mendocino is particularly important because it lies near the region where the northern San Andreas system and the Cascadia subduction zone converge.

The expanded analysis strengthened the case that the unusual sediment structures were produced by earthquakes on both fault systems occurring close together in time.

The researchers call these distinctive paired layers "doublets."

According to the team, earthquakes on the two separate fault systems provide the best explanation for how the doublets formed, rather than aftershocks or other processes.

A New Look at West Coast Earthquake Risk

If Cascadia and the northern San Andreas can influence one another, earthquake planning may need to account for a scenario in which major disasters unfold across a much larger portion of the West Coast within a compressed period of time.

The findings do not mean that every large earthquake on one fault will trigger the other. Instead, the sediment record suggests that closely spaced ruptures have occurred in the past and that interactions between the two systems deserve greater attention.

Other authors of the paper are: Ann Morey, Christopher Romsos and Bran Black of Oregon State's College of Earth, Ocean, and Atmospheric Sciences; Jeff Beeson of the National Oceanic and Atmospheric Administration Oregon State; Maureen Walzcak, University of Washington; Alexis Vizcaino, Springer Nature Group in Germany; Jason Patton, California Department of Conservation; and C. Hans Nelson and Julia Gutiérrez-Pastor, Instituto Andaluz de Ciencias de la Tierra in Spain.