Double seismic threat on the west coast of the United States

 

the west coast

Double seismic threat on the west coast of the United States

Two of the most dangerous fault systems on the U.S. West Coast may be more closely linked than scientists previously thought, with a new study suggesting that the Cascadia Subduction Zone and the San Andreas Fault may synchronize, causing earthquakes within minutes or hours of each other.

The rare synchronization could greatly increase the scale of a major disaster on the West Coast. Instead of one massive earthquake, multiple areas could be hit at almost the same time, according to Science Daily.

"We've become accustomed to hearing the term 'Cascadia' as a catastrophic event, but it turns out it's not the worst possible scenario," says Chris Goldfinger, a marine geologist at Oregon State University and lead researcher on the study.

Evidence from the deep sea

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

By comparing the turbidity sediment layers from the areas affected by the two fault systems, the researchers identified similarities in their structure and timing, which suggest a possible synchronization between the Cascadia Fault and the North San Andreas Fault.

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

A wider disaster scenario

This potential correlation threatens major implications for earthquake preparedness, as Goldfinger explained: "One earthquake on a fault alone can be expected to drain the entire nation's resources to respond to it. If the two earthquakes occur together, [it is likely] that San Francisco, Portland, Seattle, and Vancouver will all be in a state of emergency within a very short period of time."

Scientists have long considered the idea of ​​faults interacting in this way, but empirical evidence has been scarce. The only documented example occurred in Sumatra, where two major earthquakes struck three months apart in 2004 and 2005.

A chance discovery leads to a scientific breakthrough

Goldfinger's interest in this question dates back decades, including a pivotal moment during a research expedition in 1999. While collecting sediment samples from the Cascadia subduction zone off the coasts of Oregon and Northern California, the research team accidentally veered off course. They ended up about 55 miles south of Cape Mendocino, California, within the San Andreas Fault.
Instead of abandoning the site, the researchers decided to collect a core sample from it as well. Their findings were completely unprecedented.

"Double layers"

Under normal conditions, turbid sediments exhibit a consistent pattern, with coarse material settling at the bottom and finer sediment layers accumulating on top. However, in this unexpected core sample, the pattern was reversed. Coarse sandy material settled on top of fine clay sediments.

This unusual structure suggests a two-stage process. The fine-grained underlying layer likely formed first during the Great Cascadia Earthquake. The coarser material above appears to have originated from a later 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 inverted layers, which the researchers termed "double layers," were formed by closely spaced earthquakes, rather than by aftershocks or unrelated events.