Researchers at the U.S. Geological Survey have completed a detailed seismic analysis of the Juan de Fuca tectonic plate in the region beneath northern Oregon. The study found that the subducting slab, which forms the Cascadia subduction zone along the Pacific Northwest coast, lies approximately 20 kilometers deep near the shoreline. This represents a significant revision from earlier scientific estimates, which placed the slab at roughly 25 kilometers depth.
The findings carry important implications for earthquake hazard assessments in the region. According to seismic modeling conducted by the research team, the shallower plate position could elevate estimates of peak ground acceleration during a future Cascadia megathrust earthquake by somewhere between 9 and 17 percent along the northern Oregon coast. The researchers also identified a previously unmapped deep sedimentary basin beneath Tillamook, Oregon, providing the first direct seismological measurements of its dimensions.
The study addresses a notable knowledge gap in Cascadia subduction zone research. Northern Oregon generates relatively limited seismic activity compared to western Washington and northern California, which had constrained scientists’ ability to study the buried slab and understand how seismic waves propagate through the area. To gather more comprehensive data, the research team deployed 192 temporary nodal seismometers across summers in 2021 and 2022, arranging them in a network extending from Tillamook to Portland. This effort was complemented by offshore seismic recordings collected from Vancouver Island to northern California in 2021, which similarly indicated a shallower slab than previously estimated.
The physical mechanisms underlying these findings relate to basic earthquake physics. When ruptures occur closer to Earth’s surface, the released seismic energy travels a shorter distance before reaching populated areas, resulting in stronger shaking. The identified sedimentary basin presents an additional complication, as soft sedimentary material can amplify seismic waves and trap them within basin boundaries, potentially extending the duration of ground motion. Such amplification effects pose particular risks to tall buildings and large infrastructure. The research team intends to apply their dataset to investigate the Tualatin Basin near Portland in subsequent analyses.
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