The hidden threat of an earthquake beneath Seattle

Beneath Seattle, overlooked secondary faults may be more active than previously thought, rupturing on much shorter timescales than the main fault. These results suggest that seismic hazards in the region could be shaped by hidden, difficult-to-detect processes that are still not fully understood. Credit: SciTechDaily.com

A new study in the GSA Bulletin aims to better determine how often and where earthquakes occur beneath central Seattle.

In the Pacific Northwest, major fault systems such as the offshore Cascadian subduction zone tend to get most of the attention. However, smaller fault systems can also pose serious risks, and a new study in the GSA Bulletin examines the behavior of the complex network of faults directly beneath Seattle.

“My job as a paleoseismologist is to figure out when and how often these local faults rupture, which will help us roughly predict when we come into the next potential rupture window,” says Dr. Stephen Angster, a research geologist at the USGS Earthquake Science Center in Seattle and lead author of the new study.

Secondary errors reshape the understanding of risk

The research focuses on the Seattle Fault Zone (SFZ), an east-west fault system that extends beneath Bainbridge Island and the city of Seattle. While scientists have long recognized that the main fault ruptures on timescales exceeding 5,000 years, it is only recently that smaller secondary faults within the Salalah Free Zone have been detailedly mapped. These smaller structures are often excluded from standard risk models, and Angster aims to better understand their potential impact.

“When we created the National Seismic Hazard Model for the United States, we ignore these shorter faults, because they do not meet the minimum length requirements and are therefore considered to have low magnitude potential,” Angster says. “In the case of the Salalah Free Zone, we don’t fully understand the dynamics of rupture at depth, but they rupture frequently and close to home.”

A hidden strain accumulating beneath Seattle

The Seattle Fault Zone plays a key role in accommodating stress generated by crustal compression between Portland, Oregon, and Vancouver, British Columbia. This strain accumulates continuously over time and is released intermittently through earthquakes. Salalah Free Zone represents about 15% of the total breed in the region. Since these defects are not directly visible on the surface, understanding their behavior is a major challenge.

To investigate them, Angster and his colleagues rely on indirect methods that reveal subsurface features. These include magnetic surveys that reveal subtle differences in underlying rock, as well as high-resolution lidar imaging that can penetrate dense vegetation and reveal surface features such as slopes formed by past earthquakes. The team is also digging trenches through these features to determine the timing and magnitude of previous rupture events.

Repeated ruptures reshape the risk profile

By reconstructing the history of two recently identified secondary faults within the Salalah Free Zone, researchers found that these smaller faults erupt approximately every 350 years, much more frequently than the main fault.

“These secondary faulting events have dominated surface ruptures caused by earthquakes within the Salalah Free Zone for the past 2,500 years,” says Angster.

Evidence from radiocarbon dating and tree ring analysis suggests that the last rupture occurred in the 19th century. Going forward, Angster and his team aim to improve estimates of how often these events occur and assess the level of risk they pose to the nearly four million people living in the Seattle area.

“The thing about the Seattle Fault is that in a Cascadia event, we’ll be shaking hard and for a long time when that happens, but it probably won’t be as devastating to Seattle as a large event on the Seattle Fault,” Angster says. “I think we’re still trying to understand the size and potential of these smaller ruptures and the relationship between the rupture of the main fault and these smaller, more frequent ruptures.”

Reference: “Earthquakes from the Pleistocene to the 19th century on bend-moment reverse faults in the Seattle Fault Zone, Washington” by Stephen J. Angster, and Brian L. Sherrod, and Jesse K. Pearl, and Lydia M. Stich, Wes Jones, and Richard J. Blakely, 27 Jan 2026, GSA Bulletin.DOI: 10.1130/B38333.1

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