Unraveling Seattle’s Geological Puzzle: The ‘Unzipping’ of Oceanic Crust 55 Million Years Ago

Seattle’s significant fault may have originated from the ‘unzipping’ of oceanic crust around 55 million years ago, according to recent findings. Magnetic data indicates that the fault line formed during a complex collision event, wherein the southern portion of a subducting volcanic island chain merged with the North American continent, separating from its northern counterpart.

Situated atop the Cascadia Subduction Zone, Seattle’s geological history reflects the convergence of tectonic plates, namely the Explorer, Juan de Fuca, and Gorda plates, beneath the North American Plate. This convergence led to the movement of volcanic islands towards the continent, culminating in a collision visible in Seattle’s bedrock.

Analysis of newly generated bedrock maps illustrates the chaotic nature of this collision, with the northern portion of the island chain subducting under the continent while the southern half accumulated on top. The transition zone where the islands shifted from subduction to obduction likely experienced significant stress, resulting in a gradual ‘unzipping’ akin to crustal tearing.

Lead author Megan Anderson, a geophysicist with the Washington Geological Survey, likened this process to a slow, ongoing tear in the Earth’s crust. Computer simulations conducted by Anderson and her team supported this hypothesis, revealing a correlation between the resulting crustal gash and Seattle’s fault line.

Further analysis combining magnetic, seismic, and geological data provided insights into the orientation of rocks surrounding the fault line. The distinct angular arrangements of rocks on either side of the fault suggest discontinuities in the geological structures, supporting the ‘unzipping’ theory.

These findings, published in the journal Tectonics, offer a fresh perspective on the origins of the Seattle fault line, potentially aiding in the refinement of earthquake hazard models. Despite the uncertainties surrounding the fault, historical seismic events like the Nisqually earthquake of 2001 underscore the importance of preparedness.

Anderson emphasized the need for continued research to enhance earthquake simulations and mitigate risks to communities, particularly considering the potential for a magnitude 7.2 earthquake along the Seattle fault line. This study underscores the ongoing efforts to understand and address seismic hazards in the region