A new seismic analysis of the Juan de Fuca tectonic plate beneath northern Oregon suggests the subducting slab lies closer to the surface than scientists previously believed. The finding could affect estimates of how strongly the region might shake during a future Cascadia megathrust earthquake.
The Juan de Fuca plate is being forced beneath the North American plate, creating the Cascadia subduction zone along the Pacific Northwest. This fault system has produced devastating magnitude 9 earthquakes in the past.
A Shallower Plate Beneath Oregon
“We estimate that the slab interface is about 20 kilometers [deep] near the coastline, which is about 5 kilometers shallower than previous estimates,” said Erin Wirth, a seismologist with the U.S. Geological Survey who presented the results at the 2026 SSA Annual Meeting.
“This could increase estimated peak ground acceleration — in other words, shaking intensity — from Cascadia megathrust earthquakes by approximately 9 to 17%,” along the coast of northern Oregon, she added.
The researchers also detected a deep sedimentary basin beneath Tillamook, Oregon. The new measurements provide the first direct seismological constraints on the basin’s shape and depth.
“Sedimentary basins can amplify ground shaking during an earthquake and have been well-studied in other parts of the Pacific Northwest such as the Seattle Basin,” Wirth explained. “Characterizing the presence of a sedimentary layer, as well as its likely thickness, helps scientists to more accurately estimate ground shaking from future earthquakes.”
Filling a Gap in Cascadia Research
The work helps address an important gap in scientists’ understanding of the Cascadia subduction zone. Northern Oregon experiences relatively little seismic activity compared with western Washington and northern California, leaving researchers with fewer earthquakes to use when studying the buried slab and the way seismic waves move through the region.
To get a clearer picture of the subduction zone beneath the northern Oregon coast, Wirth and her colleagues installed 192 temporary nodal seismometers during the summers of 2021 and 2022. The instruments were arranged in an array stretching from Tillamook to Portland.
The team combined those measurements with a complementary study based on seismic recordings collected offshore from Vancouver Island to northern California in 2021. That study also found evidence that the slab was shallower offshore than earlier estimates suggested.
Together, the datasets produced a more detailed map of the Cascadia subduction zone and could help scientists improve estimates of earthquake hazards across northern Oregon.
Why a Shallower Slab Matters
Earthquakes that rupture closer to the surface can produce stronger shaking than deeper events because the seismic energy has less distance to travel before reaching the ground above. In deeper earthquakes, that energy has more opportunity to spread out and weaken before it reaches the surface.
That helps explain why a shallower Juan de Fuca slab beneath northern Oregon could increase peak ground shaking in populated areas.
The sedimentary basin beneath Tillamook could add another source of amplification. In these basins, soft sediments can behave like a “bowl of jello.” Seismic waves can strongly shake the weaker material and become trapped along the basin’s edges, which may cause the shaking to continue for longer.
These amplified waves can pose a particular threat to tall buildings and other large structures.
Wirth said the team plans to use the nodal seismometer dataset to investigate the Tualatin Basin near Portland in greater detail.
