We are all warned of the “Big One,” a magnitude 9 mega quake forecast to hit Metro Vancouver at some point in the future. The history of these massive earthquakes off the southern BC coast has prompted researchers to investigate just how strong the tremors really were.
Vancouver sits above the Cascadia subduction zone where the continental North America plate overrides the oceanic Juan de Fuca plate. The boundary between the plates, from northern California to Vancouver Island, is currently locked and under strain. If the entire locked zone releases, it will cause a “giant” — magnitude 9 or higher — earthquake and create catastrophic damage.
The difference is critical.
“The earthquake magnitude scale is not a linear scale. It’s more of a logarithmic scale. So the difference in the energy released between a magnitude 8 and a magnitude 9 earthquake is about 30 times,” explained Clague.
Currently, the notional idea of geologists is that the entire length of the Cascadia subduction zone, some 1,100 km, ruptures during each earthquake, meaning it releases quakes that are magnitude 9 or higher.
The magnitude of an earthquake carries implications for determining the risk. For example, earthquakes that rupture portions of a subduction zone are typically in the magnitude 8–8.8 range. This contrasts with an earthquake that ruptures all or nearly all the fault zone, which can exceed magnitude 9.
“Three of the last eight earthquakes at the northern end of the subduction zone appear to have been giants [and] two seem to have been restricted to this northern segment, and were probably magnitude 8–9 events,” Hutchinson said, summarizing his findings. “. . . [T]he status of the other three is unclear; they may have been giants, or they may not.”
The problem the researchers faced is that they “date these earthquakes using radiocarbon dating. It’s a technique that has uncertainty built in,” according to Hutchinson. As a result, the researchers couldn’t conclusively say that the ancient quakes that have occurred off BC’s southern coasts were all giant.
“If we had a dating tool that could tell us to the nearest year when an earthquake occurred . . . we would know for sure whether that quake involved a segment of, or the entire subduction zone,” Clague said.
“In reality, however, the only dating tool we have is radiocarbon or carbon dating . . . and unfortunately that tool yields dates that are only approximate. There is analytical uncertainty . . . that results in the age being imprecise.”
It remains uncertain as to whether all the earthquakes that have occurred in BC were all giants. However, the federal government says that of all the regions in Canada, the coast of BC is the most at risk for earthquakes.