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. "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." 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. 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.
Extract — continue reading at the source.
