VANCOUVER — The last massive earthquake that shook the south coast of British Columbia took place on the cold winter night of Jan. 26, 1700, say researchers who have been able to use sediment samples taken from the sea floor off the coast of Vancouver Island to reveal the Pacific coast’s seismic history.
In a study published Wednesday, the team said the region that stretches from the northern tip of Vancouver Island down the coast to northern California has experienced 22 major earthquakes over the last 11,000 years, and is due for another.
Audrey Dallimore, of the School of Environment and Sustainability at Royal Roads University and the author of the study, said the research showed earthquakes occur every 500 to 1,000 years.
The last one took place 313 years ago.
“What that means is we’re due for another subduction zone earthquake either tomorrow — or 700 years from now,” she said.
“(It) may happen within our lifetimes and will certainly happen at some time over the life of our communities and our infrastructures.”
Researchers extracted a sediment core from the sea floor of Effingham Inlet, in Barclay Sound on the west coast of Vancouver Island, and used radiocarbon dating to determine when large or so-called megathrust earthquakes occurred on what is known as the Cascadia subduction zone.
“The sediments preserved on the bottom of Effingham Inlet resemble the rings of a tree,” Dallimore said, explaining that each year is represented by a thin layer of sediment.
“These layers have given us a story of what happened in Effingham Inlet year by year back all the way to the end of the last glaciations about 11,000 years ago.”
By radiocarbon dating interruptions in the sediment, researchers determined large earthquakes also took place about 1,200 and 4,000 years ago.
The first seismograph was installed in Victoria in 1898, so written records of B.C. seismic activity goes back only a little more than a century but Japanese written records confirm that a tsunami occurred from a magnitude-8 or 9 quake along the North American coast about 9 p.m. on Jan 26, 1700.
A megathrust earthquake occurs when a piece of the earth’s crust is forced underneath another plate.
The quakes are of magnitude 9 or greater, and both the 2004 Indian Ocean earthquake that caused the tsunami in
Indonesia and the 2011 earthquake and tsunami in Japan were megathrust events.
There has never been a megathrust earthquake along the west coast in the written history of Canada, but the study confirmed First Nations oral histories and found that megathrust earthquakes occur about every 500 years in the region, although they can stretch out for up to 1,000 years.
A magnitude 7.7 earthquake that occurred off the west coast of Haida Gwaii last October was the second-largest ever recorded in Canada but it was not a megathrust quake.
B.C. forms part of the North American portion of what is called the Pacific “Ring of Fire,” a 40,000 km horseshoe of ocean trenches and volcanoes where 90 per cent of the world’s earthquakes take place.
According to Natural Resources Canada, the Geological Survey of Canada records more than 1,000 earthquakes in western Canada each year. More than 100 magnitude 5 or greater earthquakes have been recorded in the ocean west of Vancouver Island in the past 70 years.
Scientists cannot predict when earthquakes will happen, said John Clague, a professor of earth sciences at Simon Fraser University.
But this and other studies show they are inevitable in this region, he said.
And while a megathrust quake is rare and may occur up to one every millennium, other, smaller quakes occur more frequently.
Previous research suggests massive quakes occur in clusters, and it’s unclear where B.C. is in the cluster interval, Clague said.
“They are inevitable and although it may not occur in my lifetime it certainly will occur when Seattle, Vancouver, Victoria are thriving cities,” he said.
“So we do have to prepare for these things. Societally, we owe it to our children and our grandchildren and so on to be as ready as we can for what is inevitable.”
The research by experts at Royal Roads, the Geological Survey of Canada, Natural Resources Canada, Fisheries and Oceans, UBC and the University of California is published in the current issue of the Canadian Journal of Earth Sciences.
Read more: http://www.ctvnews.ca/canada/b-c-in-risk-zone-for-coastal-mega-earthquake-study-1.1322063#ixzz2m6sWicqR