Seattle is best known for the seismic hazard posed by the massive Cascadia subduction zone just offshore. But researchers say another threat lies much closer to the city itself. New research published in GSA breaking news Researchers investigated the complex fault system that runs directly beneath Seattle and nearby areas, revealing that small faults in the region may be much more active than previously known.
“My job as a paleoseismologist is to figure out when and how often these local faults rupture, which helps us roughly predict when the next potential rupture will occur,” says geologist Dr. Stephen Unster of the U.S. Geological Survey’s Earthquake Science Center in Seattle.
Fault hidden beneath Seattle
This study focuses on the Seattle Fault Zone (SFZ), an east-west fault system that cuts through Bainbridge Island and Seattle. Scientists have long known that Seattle’s main fault causes large earthquakes only at intervals of very long periods, typically 5,000 years or more. But more recently, geologists have identified numerous small secondary faults associated with this system.
These shorter faults are generally not included in national seismic hazard calculations because they are considered too small to cause very large earthquakes. Unster says that can lead to important hazards being overlooked.
“When we create the U.S. national seismic hazard model, we exclude these short faults because they don’t meet the minimum length requirements and are considered less likely to have a magnitude,” Angster says. “In the case of SFZs, the fracture mechanics at depth are not fully understood, but fractures are occurring more frequently and in much closer proximity.”
Why the Seattle Fault Zone is important
The Seattle Fault Zone helps absorb strain caused by compression of the Earth’s crust between Portland, Oregon, and Vancouver, British Columbia. Stress is constantly building up in this region, and earthquakes periodically release stress. Researchers estimate that the SFZ accounts for about 15% of the total strain across the Pacific Northwest.
Studying fault systems is difficult because most faults are hidden underground and cannot be observed directly at the surface. To investigate them, scientists rely on indirect techniques that reveal clues beneath the landscape.
Researchers use magnetic surveys to detect subtle changes in the underlying rock. They will also analyze detailed lidar images that can penetrate dense forests and expose cliffs left by ancient earthquakes. These cliffs form when the ground moves during the rupture of a fault.
To find out when these earthquakes occur, scientists dig trenches across cliffs and study the layers of soil and sediment that have shifted.
evidence of frequent earthquakes
Using these methods, Angster and his colleagues reconstructed the history of two newly identified secondary faults within the Seattle Fault Zone. Their findings suggest that these small faults rupture approximately every 350 years, much more frequently than the major faults themselves.
“Seismic surface rupture within the SFZ has been dominated by these secondary fault events for the past 2,500 years,” Angster said.
The most recent destruction probably occurred in the 19th century. Researchers used radiocarbon dating and tree ring evidence from trees that died after the earthquake to pinpoint the timing.
Growing concerns about Seattle
The researchers hope future studies will provide a clearer picture of how dangerous these secondary faults are to the Seattle metropolitan area, which is home to about 4 million people.
“The important thing about the Seattle Fault is that when the Cascadia earthquake occurs, it shakes quite violently and for a long time,” Unster says. “But it’s unlikely to be as destructive to Seattle as a large earthquake on the Seattle Fault. I think we’re still trying to wrap our heads around the size and potential of these smaller faults, and the relationship between ruptures on the main fault and these more frequent smaller ruptures.”
