Survey of Cascadia Subduction Zone reveals subterranean structure

New data covering the Cascadia subduction zone – a megathrust fault, which are capable of generating the world’s largest earthquakes – has got scientists rethinking the seismic potential of the region.

The Cascadia subduction zone (CSZ) stretches more than 900km from coast of Vancouver Island in Canada to Northern California in the US. In this region, called the Cascadia subduction zone (CSZ), the Pacific Ocean floor is slowly being forced eastward underneath North America.

That is, until the plates catch and lock against each other, building up stress until its finally release in a powerful earthquake and, potentially, an ensuing tsunami.

A new study, updating data for the first time in nearly 50 years,  has produced the first comprehensive survey of the Cascadia Subduction Zone’s subterranean structures. By better understanding the CSZs mechanics, scientists will be better able to identify places most susceptible to quakes, how big they might be and what warning signs they might produce. 

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A schematic cross section of the Cascadia Subduction Zone shows the ocean floor plate (light grey) moving under the North American continental plate, along with other features. Credit: U.S. Geological Survey

“The models currently in use by public agencies were based on a limited set of old, low-quality 1980s-era data,” says Suzanne Carbotte, a marine geophysicist at Columbia University’s Lamont-Doherty Earth Observatory in the US, who led the research.

“The megathrust has a much more complex geometry than previously assumed. The study provides a new framework for earthquake and tsunami hazard assessment.”

Researchers conducted a seismic imaging survey during a 41-day cruise aboard a research vessel in 2021.

One of the key findings, and potentially good news for ‘quake observers, is that the megathrust fault zone is not just 1 continuous structure but is instead divided into at least 4 segments.

These segments are divided by buried features including big faults where 2 segments slide against each other perpendicularly to the shore. This might help buffer against movement on one segment translating to the next, which is important as longer ruptures translate to bigger earthquakes.

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Sub-seafloor map of the Cascadia Subduction Zone, showing depth of the fault between the eastward-moving Juan de Fuca place and the North American plate. Yellow/orange indicates shallow depths; green, deeper; blues/purples deepest. Diagonal black lines approximate divisions between different segments of the zone. Wavy red line to right indicates the seaward edge of rigid continental rocks that apparently cause the zone to break into these segments. Credit: Modified from Carbotte et al., Science Advances, 2024

“We can’t say that this definitely means only single segments will rupture, or that definitely the whole thing will go at once,” says Harold Tobin, a geophysicist at the University of Washington and coauthor of the study.

“But this does upgrade evidence that there are segmented ruptures.”

A consortium of state and federal agencies and academic institutions has already began poring over the data to sort through the implications.

The researchers say that practical assessments that could affect building codes or other aspects of preparedness may be published as early as next year.

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