It has long been known that the coastline of southern British Columbia, Washington, Oregon, and northern California is at risk for an eventual “Big One” earthquake, potentially accompanied by a major tsunami.
Such mega-thrust earthquakes are predicted for the Cascadia Subduction Zone, where the Pacific Ocean’s plate slowly slides beneath North America’s tectonic plate.
Cascadia’s “Big One” earthquakes occur roughly once every 500 years, and the last one occurred in 1700.
The devastating strength of mega-thrust earthquakes are generally in the range of magnitude 9.0, such as the 2011 earthquake in Japan, which also caused the tsunami that triggered the failure of a nuclear plant in Fukushima.
An earthquake with a magnitude of 9.0 is exceptionally powerful. For each 1.0 magnitude increment, an earthquake carries 10 times the strength, representing exponential growth in its potency. For instance, compared to a magnitude 6.0 earthquake, a magnitude 7.0 earthquake is 10 times more powerful, a magnitude 8.0 is 100 times more powerful, and a magnitude 9.0 earthquake is 1,000 times more powerful.
Also, the duration of earthquakes generally climbs with magnitude; a magnitude 6.0 earthquake typically lasts for a few seconds, magnitude 7.0 lasts for possibly 15 seconds, magnitude 8.0 can last for more than a minute, and magnitude 9.0 can last for a handful of minutes.
The “Big One” in Japan shook the ground for about six minutes.
The potency of an earthquake and the damage it can cause are also determined by its epicentre location and depth. Shallow earthquakes deliver the greatest damaging waves, while deeper earthquakes provide a buffer by forcing seismic waves to travel over a longer distance.
According to a newly published study led by Columbia University researchers, the northern section of the Cascadia Subduction Zone could be ground zero of the “Big One.”
New data and imagery gathered by a towing vessel over six weeks in 2021 from a detailed scan of the ocean floor now provides a far more accurate picture than the old, low-quality 1980s-era data, which is what current modelling depends on.
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. (Modified from Carbotte et al., Science Advances, 2024)
The new findings show that the ocean floor along the subduction zone between southern Vancouver Island and the Washington-Oregon border is very smooth, which could mean this section with less friction between the plates is more likely to significantly rupture — potentially making it the most dangerous section along the almost 1,000-km-long strip of the entire subduction zone. In contrast, the southern section off the coast of Oregon and northern California is “relatively rough,” which could limit how far the ocean plate will slip under the continental plate, and limit the strength of the earthquake.
Furthermore, this northern section has a shallower subduction under the continental plate than the southern section.
“It requires a lot more study, but for places like Tacoma and Seattle, it could mean the difference between alarming and catastrophic,” said Harold Tobin, a geophysicist at the University of Washington and study co-author.
From the new data, researchers also determined that the subduction zone is much more complex than originally thought.
The mega-thrust fault zone is not just one continuous structure between BC and California, but it is divided into at least four segments, with each segment possibly insulated against the movements of the others to an extent. Based on previous findings, it was believed that there was a risk that a mega-thrust earthquake could occur along the entire coast.
“We can’t say that this definitely means only single segments will rupture or that definitely the whole thing will go at once. But this does upgrade evidence that there are segmented ruptures,” said Tobin.
Possible reasons for the division of the fault zone into four segments include varying kinds of rocks formed at different times, with some being denser than others, causing some areas of the ocean plate to interact with the continental plate in different ways.
Researchers state further study is needed to determine how their latest findings change the potential displacement of water when the ocean floor slips — the resulting major tsunami risk.
The team may publish practical assessments as early as next year that could affect building codes or other aspects of preparedness.
Currently, structures are designed for optimal seismic performance based on three different earthquake frequencies—the probability of withstanding the magnitude of an earthquake happening once every 475 years, once every 975 years, and once every 2,475 years. The latest building code standards require all new buildings and structures to be designed for the exceptionally powerful once-in-2,475-year earthquake event.
Between 1970 and 1985, the building codes required seismic performance for once-in-100-year earthquakes. The previous once-in-475-year standard was in practice from 1985 to 2005, when the once-in-2,475-year standard was adopted.