A seismologist with Natural Resources Canada says it may not take a massive earthquake to threaten the Colliery dams, but said the risk depends on the amount of ground shaking the structures are subjected to.
Dr. Garry Rogers is based out of the western office of Earthquakes Canada in Sidney. He said seismic standards for objects like dams are a complex subject to tackle, but said there are many conditions that determine how an earthquake would affect the dams, located in Nanaimo's historic Harewood neighbourhood.
The B.C. Dam Safety Section has determined that the dams do not meet current safety standards, and has previously stated the dams must meet a 1:10,000 earthquake event design standard.
Rogers said that standard is twice as strong as buildings in Nanaimo built to the current building code, which is a 1:2,500 standard. But how strong an earthquake that would mean is another question.
"The difficulty is, the structure really doesn't care whether it's an earthquake right under its feet, or a slightly bigger earthquake 10 kilometres away, or a slightly bigger, bigger earthquake 50 kilometres away," he said. It depends how far away and how deep and shallow the earthquake is, he added.
For example, a 1:10,000 event could mean an earthquake up to between six and 6.5 on the Richter scale if the earthquake took place directly at the dams, or a larger earthquake - for example, a magnitude seven event, 25 kilometres away, Rogers estimated.
"It's the shaking level that you need, not the magnitude," he said. "The likelihood is that it's going to be something some distance away, in which case the magnitude's going to be bigger."
Members of the Colliery Dams Preservation Society scored a major victory earlier this month after council voted to cancel issuing a tender to remove the lower and middle dams this summer.
The city has commissioned several engineering reports on the dams, including a 2010 seismic assessment and a 2012 inundation study that looked at the consequences of dam failure in the area.
Residents striving to protect the structures have alleged the scope of the reports did not include key information about the structural integrity of the dams, and they also say that the city did not properly assess other alternatives to address public safety concerns other than removing the structures.
Sean Williams, a retired clearance diver for the Royal Canadian Navy who specialized in demolition and a former Department of Fisheries and Oceans biologist, did a dive at the middle dam. He presented council with photos from the site and said the structures appear to be based on bedrock and contain reinforcing metal structures.
He also said the concrete appeared to be in good condition and added the site had significant ecological value. More than 30 other people also made presentations to council this month.
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