USGS analysis finds Juan de Fuca slab 5 km shallower under Oregon coast

August 30, 2026 at 05:00 AM2 min

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A new seismic analysis by the U.S. Geological Survey finds the Juan de Fuca plate lies about 5 kilometers shallower beneath northern Oregon than previously estimated. The finding could raise peak ground acceleration from a future Cascadia megathrust earthquake by 9 to 17 percent along the coast. The study also identifies a deep sedimentary basin beneath Tillamook that may amplify shaking.

Key Facts

The slab interface is about 20 kilometers deep near the Oregon coastline, roughly 5 kilometers shallower than previous estimates.
Peak ground acceleration from Cascadia megathrust earthquakes could increase by 9 to 17 percent along the northern Oregon coast.
Researchers installed 192 temporary nodal seismometers between Tillamook and Portland during the summers of 2021 and 2022.
The study provides the first direct seismological constraints on the shape and depth of a sedimentary basin beneath Tillamook, Oregon.
The findings were presented at the 2026 SSA Annual Meeting by USGS seismologist Erin Wirth.

Shallower Slab Beneath Oregon

Erin Wirth, a seismologist with the U.S. Geological Survey, presented the results at the 2026 SSA Annual Meeting. She estimated the slab interface is about 20 kilometers deep near the coastline, about 5 kilometers shallower than previous estimates. The shallower depth could increase estimated peak ground acceleration from Cascadia megathrust earthquakes by approximately 9 to 17 percent along the coast of northern Oregon. The researchers also detected a deep sedimentary basin beneath Tillamook, Oregon, providing the first direct seismological constraints on its shape and depth. Sedimentary basins can amplify ground shaking during an earthquake, and similar basins such as the Seattle Basin have been well studied in the Pacific Northwest.

Filling a Research Gap

Northern Oregon experiences relatively little seismic activity compared with western Washington and northern California, leaving fewer earthquakes to study the buried slab. Wirth and her colleagues installed 192 temporary nodal seismometers during the summers of 2021 and 2022, 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 offshore study also found evidence that the slab was shallower than earlier estimates suggested. Together, the datasets produced a more detailed map of the Cascadia subduction zone and could help improve earthquake hazard estimates across northern Oregon.

Why Depth Matters

Earthquakes that rupture closer to the surface can produce stronger shaking than deeper events because seismic energy has less distance to travel before reaching the ground. In deeper earthquakes, that energy has more opportunity to spread out and weaken before it reaches the surface. The Cascadia subduction zone has produced magnitude 9 earthquakes in the past, and the new findings refine hazard models for the region.