Cascadia's Buried Fault Is Shallower Than Thought, Raising Oregon Quake-Shaking Estimates

A new seismic survey of the fault beneath northern Oregon suggests the region's earthquake hazard may be higher than previously estimated, because the subducting tectonic plate lies closer to the surface than scientists thought.

The Juan de Fuca plate is slowly sliding beneath the North American plate off the Pacific Northwest coast, forming the Cascadia subduction zone — a fault system that has produced 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 (USGS), who presented the findings at the 2026 Seismological Society of America (SSA) Annual Meeting. She said the shallower depth could increase estimated peak ground acceleration — shaking intensity — from a Cascadia megathrust earthquake by roughly 9 to 17% along the northern Oregon coast.

The researchers also found a deep sedimentary basin beneath Tillamook, Oregon, and produced the first direct seismological measurements of its shape and depth. Wirth said such basins can amplify ground shaking during an earthquake, much as a similar basin does beneath Seattle.

To map the area, Wirth's team installed 192 temporary seismometers between Tillamook and Portland during the summers of 2021 and 2022, and combined the results with a separate offshore study stretching from Vancouver Island to northern California, which also found the slab to be shallower than expected.

Earthquakes that rupture closer to the surface tend to produce stronger shaking, because the seismic energy has less distance to spread out and weaken before reaching the surface. Soft sediment in a basin can further trap and prolong the shaking, posing a particular risk to tall buildings. The team next plans to study the Tualatin Basin near Portland using the same seismometer data.