Off the coasts of southern British Columbia, Washington, Oregon, and northern California lies the Cascadia Subduction Zone, where the Pacific Ocean floor is slowly diving under North America. This area hosts a megathrust fault, where tectonic plates move against each other in a dangerous manner. It is believed that big earthquakes occur roughly every 500 years in this region, with the last one happening in 1700. Scientists have been studying the subterranean structures and mechanics of the zone to understand where quakes are most likely to occur, how big they might be, and what warning signs they might produce.
A newly published study has greatly advanced this effort by providing a comprehensive survey of the complex structures beneath the seafloor along the entire Cascadia Subduction Zone. The study, published in Science Advances, was led by marine geophysicist Suzanne Carbotte from Columbia University’s Lamont-Doherty Earth Observatory. The researchers utilized the latest geophysical instruments to gather data during a 41-day cruise in 2021, providing a new framework for earthquake and tsunami hazard assessment in the region.
The study revealed that the megathrust fault zone is not one continuous structure but is divided into at least four segments, each potentially isolated from the movements of the others. The buried features, including large faults, divide these segments and might prevent movement from one segment to the next. The segmentation appears to be caused by the variety of rocks in the overriding North American continental plate, which causes the incoming oceanic plate to bend and twist in different ways. This segmentation could impact the size and distribution of future earthquakes.
Researchers focused on a specific segment that runs from southern Vancouver Island alongside Washington state to the Oregon border, which appears to be smoother and more likely to rupture along its entire length at once, potentially making it the most dangerous section. The shallow subduction angle in this segment also means that the earthquake-prone interface between the plates could extend directly under Washington’s Olympic Peninsula, which could increase shaking on land in places like Tacoma and Seattle.
While the implications of the study are still being analyzed, the data is already being used to assess tsunami hazards. Research scientist Kelin Wang from the Geological Survey of Canada is using the data to model seafloor features off Vancouver Island that could generate tsunamis during a quake. The results of this modeling will be used to analyze tsunami hazards on land. Practical assessments that could impact building codes and preparedness measures may be published as early as next year, given the complexity of the data and its implications.
Overall, the study provides a wealth of new information about the Cascadia Subduction Zone’s structure and behavior, offering insights into the potential risks posed by future earthquakes and tsunamis in the region. By improving our understanding of the zone and its segmentation, scientists and authorities can better prepare for and mitigate the impacts of these natural disasters when they occur. Ongoing research and modeling efforts will continue to refine our knowledge of the area, ultimately enhancing our ability to protect vulnerable communities along the Pacific Northwest coast.