Double Seismic Threat on the West Coast of the United States

SadaNews - Two of the most dangerous fault systems on the West Coast of the United States may be more interconnected than scientists previously thought, with a new study suggesting that the Cascadia Subduction Zone and the San Andreas Fault could potentially synchronize, leading to earthquakes occurring within minutes or hours of each other.

This rare synchronization could significantly increase the magnitude of a major disaster on the West Coast, as instead of one massive earthquake, multiple regions could be struck nearly simultaneously, according to the report published by Science Daily.

Chris Goldfinger, a marine geologist at Oregon State University and the lead researcher in the study, states: "We have become accustomed to hearing the term 'devastating earthquake' - Cascadia - as being a huge disaster, but it turns out that it is not the worst-case scenario possible."

Evidence from the Deep Seas

To investigate this possibility, Goldfinger and his colleagues examined samples of sediments taken from the ocean floor, which preserve about 3,100 years of geological history. The research team focused on turbidite sediments, which are layers of sediment left behind by underwater landslides often triggered by earthquakes.

By comparing the layers of turbidite sediments from regions affected by the two fault systems, the researchers identified similarities in their structure and timing, indicating a possible synchronization between the Cascadia and northern San Andreas faults.

Determining the precise timing between earthquakes on the two faults is challenging. However, Goldfinger noted three instances over the past 1,500 years, including the most recent event in 1700, where data indicates that the earthquakes occurred within minutes to hours of each other.

A Broader Disaster Scenario

This potential linkage poses significant implications for earthquake preparedness, as Goldfinger explained: "One earthquake on one fault alone can be expected to drain the entire state's resources to respond to it. If both earthquakes occur together, [it is likely that] San Francisco, Portland, Seattle, and Vancouver would all be in emergency status within a very short period."

Scientists have long contemplated the idea of fault interaction in this manner, but real-world evidence has been scarce. The only documented example occurred in Sumatra, where two major earthquakes struck three months apart in 2004 and 2005.

A Serendipitous Discovery Leading to a Scientific Breakthrough

Goldfinger's interest in this question dates back decades, including a pivotal moment during a research cruise in 1999. While collecting sediment samples from the Cascadia Subduction Zone off the coasts of Oregon and Northern California, the research team accidentally deviated from their course. They ended up about 55 miles south of Cape Mendocino in California, inside the San Andreas Fault zone.

Rather than abandon the site, the researchers decided to collect a core sample from there as well. The findings they uncovered were completely unexpected.

"Double Layers"

Under normal circumstances, turbidite sediments exhibit a consistent pattern, with coarse materials settling at the bottom and finer sediment layers accumulating on top. But in this unexpected core sample, the pattern was reversed. Coarse sandy material settled above fine clay sediments.

This unusual composition suggests a two-phase process. It is likely that the lower fine layer formed first during a major Cascadia earthquake, while the coarse material above appears to be the result of a subsequent event along the nearby San Andreas Fault.

To confirm this idea, the researchers used radiocarbon dating on this core sample and other samples collected near Cape Mendocino, where the two fault systems meet. The results supported the notion that these reversed layers, which the researchers termed "double layers," formed as a result of earthquakes occurring close in time, rather than seismic aftershocks or unrelated events.