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Vancouver Island could be ground zero for a 'Big One' earthquake.
It has long been known that the coastline of southern British Columbia, Washington, Oregon, and northern California is at risk of an eventual “major” earthquake, possibly accompanied by a large tsunami.
Such powerful earthquakes are expected to occur in the Cascadia subduction zone, where the Pacific plate is slowly sliding beneath the North American tectonic plate.
“Big” earthquakes in Cascadia occur about once every 500 years, the last one occurring in 1700.
The destructive power of strong earthquakes is usually in the 9.0 magnitude range, such as the 2011 earthquake in Japan, which also triggered a tsunami that led to the failure of a nuclear plant in Fukushima.
An exceptionally strong 9.0 earthquake. For every 1.0 magnitude increase, an earthquake carries a 10-fold magnitude of magnitude, representing an exponential growth in strength. For example, compared to a magnitude 6.0 earthquake, a magnitude 7.0 earthquake is 10 times stronger, a magnitude 8.0 earthquake is 100 times stronger, and a magnitude 9.0 earthquake is 1,000 times stronger.
The duration of earthquakes in general increases with their size; A magnitude 6.0 earthquake usually lasts for a few seconds, a magnitude 7.0 earthquake lasts for 15 seconds, a magnitude 8.0 earthquake can last for more than a minute, and a magnitude 9.0 earthquake can last for a few minutes.
The “huge” in Japan shook the Earth for about six minutes.
The strength of an earthquake and the damage it can cause is also determined by the location of the epicenter and its depth. Shallow earthquakes produce the most damaging waves, while deep earthquakes provide a buffer by forcing seismic waves to travel a longer distance.
According to a newly published study led by researchers from Columbia University, the northern part of the Cascadia subduction zone could be ground zero for the “big zone.”
New data and images collected by a tug ship over six weeks in 2021 from a detailed survey of the ocean floor now provide a much more accurate picture than the old, low-quality data dating back to the 1980s, which is what current modeling relies on.
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 that this section with the least friction between plates is most likely to rupture significantly — which could make it the most dangerous. Along the approximately 1000 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 may limit the extent to which the ocean plate slides under the continental plate, and limit the strength of the earthquake.
Moreover, this northern section has shallower subduction beneath the continental plate than the southern section.
“It takes a lot of 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 co-author of the study.
From the new data, the researchers also determined that the subduction zone is much more complex than originally thought.
The massive fault zone is not just one continuous structure between British Columbia and California, but is divided into at least four parts, with each part potentially insulated against the movements of the others to some extent. Based on previous results, it was believed that there was a risk of a strong earthquake along the entire coast.
“We can't say that this definitely means that only individual parts will rupture or that the whole thing will definitely end at once. But it does improve the evidence for fragmentary ruptures,” Tobin said.
Possible reasons why the rift zone is divided into four parts include different types of rocks that 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.
The researchers note that further study is needed to determine how their findings change the potential displacement of water when the ocean floor slides — creating a significant tsunami risk.
The team may publish practical assessments as early as next year that may impact building codes or other aspects of preparedness.
Currently, the structures are designed for optimal seismic performance based on three different earthquake frequencies, which are the potential to withstand the strength of an earthquake that occurs once every 475 years, once every 975 years, and once every 2,475 years. The latest building code standards require that all new buildings and structures be designed for an exceptionally strong earthquake event that occurs once every 2,475 years.
Between 1970 and 1985, building codes required seismic performance for once-in-100-year earthquakes. The previous once-in-475-year standard was in place from 1985 to 2005, when the once-in-2475-year standard was adopted.
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