The Oregon Fault may be closer to the surface than scientists think

Scientists have discovered new details beneath northern Oregon that may change earthquake risk estimates and reveal previously unseen structures deep underground. Credit: SciTechDaily.com

A new study suggests that northern Oregon may experience stronger shaking during the Great Cascadia Earthquake than previous models predicted.

A new analysis of the Juan de Fuca tectonic plate beneath northern Oregon suggests that the plate lies closer to the surface than scientists previously thought. This finding could affect estimates of how strongly the region might shake during a future massive Cascadia earthquake.

The Juan de Fuca plate slides beneath the North American plate, forming the Cascadia subduction zone along the northwestern Pacific Ocean. This fault system has produced massive magnitude 9 earthquakes in the past.

“We estimate that the slab frontage is about 20 kilometres [deep] “The earthquake is located near the coast, and is about 5 kilometers deeper than previous estimates,” said Erin Wirth, a seismologist with the US Geological Survey who presented the results at the 2026 Sub-Saharan Annual Meeting.

“This could increase the estimated peak ground acceleration — in other words, the intensity of shaking — from large Cascadia earthquakes by about 9 to 17%,” she added, along the northern Oregon coast.

USGS scientist Erin Wirth deploys decadal seismometers in northern Oregon. The goal of this study is to better understand the risks associated with the Cascadia Subduction Zone. Credit: Erin Werth

The researchers also identified a deep sedimentary basin beneath Tillamook, Oregon, providing the first direct seismic measurements of its shape and depth.

“Sedimentary basins can increase ground shaking during an earthquake, and it has been well studied in other parts of the Pacific Northwest such as the Seattle Basin,” Wirth explained. “Determining the presence of a sedimentary layer, as well as its likely thickness, helps scientists more accurately estimate ground shaking from future earthquakes.”

Filling a major gap in Cascadia research

The study helps address a long-standing gap in scientists’ understanding of the Cascadia subduction zone beneath northern Oregon. Compared to western Washington and northern California, the region experiences relatively little seismic activity, leaving researchers with fewer earthquakes to study for clues about plate structure and the movement of seismic waves underground.

To explore the region in more detail, Wirth and her colleagues deployed 192 temporal measurements of decadal earthquakes between Tillamook and Portland during the summers of 2021 and 2022. The team also combined data from a separate marine seismic study conducted in 2021 from Vancouver Island to northern California, which similarly indicated a lower depth of offshore plates.

USGS scientist Erin Wirth deploys decadal seismometers in northern Oregon. The goal of this study is to better understand the risks associated with the Cascadia Subduction Zone. Credit: Erin Wirth Hidden underground structures beneath Oregon

Combined, the datasets produced a more detailed picture of the subduction zone beneath northern Oregon, which could improve estimates of future earthquake hazards.

Shallow earthquakes generally produce stronger vibrations at the surface because seismic energy has less distance to attenuate before reaching populated areas. This means that the shallow Juan de Fuca slab could have increased shaking intensity during the Great Cascadia Earthquake.

A newly identified sedimentary basin beneath Tillamook may also amplify seismic activity. Wirth compared the effect to a “jelly pot,” where the soft sediment vibrates violently and traps seismic waves along the edges of the basin. This process can prolong the duration of vibration and may pose greater risks to high-rise buildings and other large structures.

Wirth said the team plans to continue analyzing data from the decadal seismometer deployment to study the Tualatin Basin near Portland.

Meeting: 2026 Annual Meeting of the Seismological Society of America

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