Seattle, Washington – A recent study published in Tectonics suggests that the origin of the Seattle fault zone, which poses a risk of destructive earthquakes to over four million inhabitants of the region, may have originated over 50 million years ago due to tectonic deformation. This new study sheds light on the fault system’s initial formation, aiming to enhance the prediction and understanding of potential hazards for this heavily populated area. The study was published in Tectonics, AGU’s journal for research exploring the evolution, structure, and change of Earth’s crust and upper mantle.
The dangerous Seattle fault zone may have originated from the continent’s edge splitting in half over 50 million years ago, offering a potential new explanation for the fault’s formation. Magnetic evidence indicates an ancient volcanic island interaction with the North American plate as a contributing factor to the fault’s formation.
The study’s lead author, Megan Anderson, a geophysicist with the Washington Geological Survey, expresses surprise at the findings, stating that the results predict a major ancient fault where the Seattle fault lies today. The research reveals that Washington in the Eocene looked significantly different from today, with a coastline well east of where Seattle currently sits and a chain of volcanic islands offshore.
The study suggests that around 55 million years ago, that island chain was pulled toward the continent. As it ran into the North American plate, part of it went up and over the crust while the rest was sucked under it. Between these two parts, the crust would have been under great strain and torn. That ancient tear zone set the geologic stage for the modern Seattle fault, the study authors posit.
The study’s findings aim to enhance hazard modeling for the densely populated Puget Sound region and offer a deeper understanding of the area’s potential risks and seismic activity. This could be critical in preparing and mitigating the impact of potential earthquakes in the region. The research also provides valuable insights into the region’s ancient faults and geologic structures, offering detailed information about the bedrock under and within the Seattle basin.
The discovery of this buried tectonic story may provide a great basis for further study of active fault geometry for the Seattle fault and other faults in western Washington. The findings hold potential for improving the accuracy of seismic ground shaking models in the area, ultimately contributing to better preparation and response strategies for seismic events.