Three hundred and fifteen years and counting.
On Jan. 26, 1700 one of the worldB次元官网网址檚 largest earthquakes occurred off the west coast of Vancouver Island. The undersea Cascadia thrust fault ruptured along a 1,000-kilometre length, from mid Vancouver Island to northern California, producing a megathrust earthquake with an estimated magnitude of 8.7.
B次元官网网址淲e know that these earthquakes do occur in a cyclic fashion so there is going to be another megaquake,B次元官网网址� said Stephen Johnston, director of the School of Earth and Ocean Sciences at the University of Victoria.
He said itB次元官网网址檚 impossible to accurately predict when the next megaquake will hit but they occur in cycles of between 300 and 700 years.
B次元官网网址淲hat a lot of research is going into of late is trying to pin down whether itB次元官网网址檚 closer to 300 years, in which case we should start being concerned, or if itB次元官网网址檚 closer to 700 years, in which case we can relax a bit,B次元官网网址� said Johnston.
The earthquake in 1700 collapsed houses of the Cowichan people on Vancouver Island and caused numerous landslides and a tsunami that swept across the Pacific, causing devastation in Japan.
According to Natural Resources Canada, the shaking from the earthquake was so violent on Vancouver Island that people could not stand and lasted so long it made them sick. The tsunami completely destroyed the winter village of the Pachena Bay people, leaving no survivors.
Geological evidence indicates that there have been 13 megaquakes over the past 6,000 years.
In contrast, the earthquake that struck northeast of Tofino on Jan. 8 was a magnitude 4.8, the largest onshore quake to have hit the region since 2000.
Johnston said thereB次元官网网址檚 a big difference between onshore quakes like the one that struck near Tofino and the megaquakes that occur offshore, typically at the subduction zone.
B次元官网网址淯sually the ones offshore are hugely more powerful. For example, the 1700 megaquake was probably on the order of 35,000 times more powerful than that Tofino earthquake, so itB次元官网网址檚 an enormous difference in power.B次元官网网址�
And itB次元官网网址檚 the offshore quakes that give rise to tsunamis, which are typically responsible for the greatest loss of life.
Johnston said the fault responsible for the 1700 quake lies about 100 kilometres west of Port Renfrew. B次元官网网址淎nd that is the same fault that will rupture again and cause the next megaquake.B次元官网网址�
The extent of the devastation caused by that megaquake remains unclear.
B次元官网网址淭here would be a huge tsunami and Port Alberni and all the other west coast towns would be at risk,B次元官网网址� said Johnston. B次元官网网址淚 think itB次元官网网址檚 obvious Tofino, much of it, would be inundated. Shaking is going to be a problem and old buildings that are not seismically upgraded B次元官网网址� you can think of brick buildings in downtown Victoria that were built a long time ago B次元官网网址� they would be liable to be heavily damaged.B次元官网网址�
He said those living closer to the west coast will likely see more violent shaking but a lot of local issues will also come into play.
B次元官网网址淚f youB次元官网网址檙e built, and much of Victoria is built, on bedrock, then there wonB次元官网网址檛 be such severe shaking. But if youB次元官网网址檙e built on recent sediments B次元官网网址� think or regions around here where itB次元官网网址檚 very flat lying and muddy B次元官网网址� those are lake beds that formed 10,000 years ago and they will shake like a bowl of jelly.B次元官网网址�
But Johnston said a smaller quake like the one that struck Tofino can prove to be just as deadly as a megaquake.
B次元官网网址淭hatB次元官网网址檚 the bigger concern, not the megaquake but something like the earthquake that took place near Tofino except maybe a little bit stronger than that.B次元官网网址�
He pointed to the 6.3 magnitude quake that rocked Christchurch, New Zealand in 2011, resulting in 185 deaths.
B次元官网网址淧eople die in magnitude 4 earthquakes and they donB次元官网网址檛 die because of the shaking, they die because a wall falls on them or something like that.B次元官网网址�
Very few earthquakes have been centred around Victoria. Johnston recalls a magnitude 2 quake hitting Langford a year or two ago, B次元官网网址渂ut it was so tiny that even people standing right on top of it couldnB次元官网网址檛 feel it.B次元官网网址�
He said the Nisqually earthquake that struck south of Seattle in 2001 caused significant tremors felt in Victoria. The aftermath of that quake has left Victoria reasonably well prepared.
B次元官网网址淲eB次元官网网址檝e had all of our public schools and a lot of the buildings at UVic and downtown seismically upgraded since then because we did have a good shake and it gave us a little taste of what might come.B次元官网网址�
Johnston said Island residents would be well-advised to have a plan in place in the event of a large earthquake but admits thatB次元官网网址檚 not something heB次元官网网址檚 put a lot of thought into.
B次元官网网址淚tB次元官网网址檚 something that you canB次元官网网址檛 get out of your mind but at the same time weB次元官网网址檙e all pretty happily living here and IB次元官网网址檓 not getting ready to move,B次元官网网址� he said.
editor@oakbaynews.com
