Researchers with the U.S. Geological Survey have completed a seismic analysis of the Juan de Fuca plate in northern Oregon that suggests the subducting slab sits approximately 20 kilometers below the surface near the coastline. This represents a reduction of roughly 5 kilometers from earlier scientific estimates. The findings were presented at the 2026 SSA Annual Meeting and indicate that ground shaking intensity from potential Cascadia megathrust earthquakes could be higher than previously calculated, with peak ground acceleration potentially increasing by 9 to 17 percent along the northern Oregon coast.
The research team also identified a previously uncharacterized deep sedimentary basin beneath Tillamook, Oregon. Understanding the presence and dimensions of such basins is important for earthquake hazard assessment, as sedimentary layers can amplify ground motion during seismic events. The material in these basins can behave in ways that trap and prolong seismic waves, creating extended shaking that poses particular risks to large structures and tall buildings.
To gather the new data, scientists deployed 192 temporary nodal seismometers across northern Oregon during the summers of 2021 and 2022, arranging them in an array extending from Tillamook to Portland. This deployment addressed a significant research gap, as northern Oregon experiences relatively limited seismic activity compared to western Washington and northern California, making direct observations of the subduction zone challenging. The team combined their findings with complementary offshore seismic recordings collected in 2021 from Vancouver Island to northern California, which corroborated evidence that the slab was shallower than earlier estimates.
The proximity of the subducting plate to the surface has direct implications for earthquake intensity because seismic energy from shallower ruptures travels a shorter distance to reach the ground, resulting in stronger shaking. In contrast, energy from deeper earthquakes dissipates over longer distances before reaching populated areas. The researchers indicated that future investigations will focus on applying their dataset to examine the Tualatin Basin near Portland in greater detail.
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