Scientists map one of Earth’s top hazards in the Pacific Northwest

Scientists have mapped one of the most hazardous spots on the globe in unprecedented detail: a 600-mile geologic boundary just off the Pacific Northwest coast.

Along this fraught stretch, called the Cascadia subduction zone, two pieces of the Earth’s crust slide against each other, building up stresses capable of unleashing a catastrophic 9.0-magnitude earthquake and generating a tsunami, with waves as high as 40 feet.

Because scientists don’t know when that day will arrive, they prepare by trying to better understand the geological state of play.

To do that, a 235-foot ship cruised for 41 days along the coast of Oregon, Washington and British Columbia in summer of 2021, sending sound waves deep into the ocean and recording the echoes with a “streamer” — a 9-mile-long waterproof cable containing 1,200 specialized microphones. Similar to how doctors use ultrasound to see inside the body, they used the data to construct a comprehensive map of the underwater geology in a study published Friday in the journal Science Advances. The new resource will help scientists understand the range of earthquake and tsunami scenarios — and help policymakers devise building codes that protect people.

Scientists map one of Earth’s top hazards in the Pacific Northwest © Madelaine Lucas/University of Washington

A cross section of the Cascadia Subduction Zone shows the ocean floor plate (light gray) moving under the North American continental plate. © U.S. Geological Survey

The entire area, which stretches from Northern California to Vancouver Island, is at risk. But the scientists found that the geometry of the fault off the coast of Washington, where the fault is flat and smooth, closer to the surface and extends farther onshore, may be particularly at risk.

“I’m excited to use these results to make sure the shaking estimates I’m producing are as accurate as they can be,” said Erin Wirth, a seismologist with the United States Geological Survey who was not involved in the study. “I’ll be busy now.”

A quiet Cascadia subduction zone comes into focus

A map of the Cascadia Subduction Zone shows the depth of the fault. Colors indicate depth, with shallowest areas in yellow and orange, then progressively deeper with green, blue and purple. The wavy red line indicates the edge of continental rocks. © Science Advances, 2024/Science Advances 2024

For hundreds of years, the Cascadia subduction zone has been quiet. But on Jan. 26, 1700, the earth shook. Japanese history records show an “orphan” tsunami traveled across the Pacific, with no preceding quake. Native American oral histories describe earth shaking and marine flooding. Analysis of tree rings from “ghost forests” that died when the land abruptly subsided helped scientists pin down the date. Scientists estimate that on that day more than three centuries ago, a magnitude-9 quake struck.

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