Aftershocks are continuing Morning morning after five earthquakes shook 200km off the west coast of Vancouver Island during the evening of Oct. 21.
“It’s not a surprise because this is one of the most seismically active parts of Canada,” John Cassidy, a seismologist and professor at the University of Victoria, said the next morning.
“What was different last night was to have three of this magnitude in 45 minutes.”
Tonight's M6.5-6.8 earthquakes ~200 km west of Vancouver Island were not felt by many people, but were easily recorded across #Canada from Victoria (VGZ) and Haida Gwaii (BNB) to Whitehorse, #Ottawa, and Halifax! Also recorded around the world. pic.twitter.com/gbEaUzzMOQ
— John Cassidy (@earthquakeguy) October 22, 2018
The shaking began at around 10:30 p.m., when three of the quakes were recorded within 45 minutes of each other at a magnitude of 6.6, 6.5, and 6.8. Two more quakes followed, measured at 4.9 and 4. While the shakes were felt in parts of the province, no injuries or damage has been reported.
“They were easily recorded all across Canada, and in fact around the world,” Cassidy said, adding that is typical of the magnitude. “If the earthquake is big enough the waves will circle the globe.”
This type of earthquake, caused by strikes movement, doesn’t generally cause a tsunami because the shift is horizontal and not vertical. Sarah Hunn of the City of Victoria’s emergency management team said they didn’t issue an alert on their subscription-based system because the Emergency Management BC and the national tsunami warning centre did not anticipate a tsunami. There were no alerts issued by the province’s automatic AlertReady system for these earthquakes. Usually, alerts are issued for events that require immediate action, such as evacuation.
At 470 km from Victoria, the earthquakes weren’t felt in the city only because of the distance. Cassidy pointed to a deadly earthquake in 2011 that killed 185 people in Christchurch, New Zealand, that was measured at a magnitude of 6.3 — less than Oct. 21’s earthquakes off Vancouver Island. People can feel earthquakes at magnitude 3. By magnitude 4, the shaking is 10 times stronger. At 5, it’s 100 times stronger.
“It’s that distance that is making all the difference here, that they’re several hundred kilometres away rather than 10 kilometres away like in Christchurch,” Cassidy said.
While seismologists can’t predict when earthquakes will occur, they are studying where they will strike, how often and how large they will be. This information is used by engineers building and designing infrastructure in high seismic zones like Victoria.
“If we have a better understanding of what to expect in the future and our infrastructure is designed to withstand that level of shaking, then it doesn’t matter when an earthquake happens. Buildings survive and people know what to do. If we’re prepared then you really reduce the impacts of those future earthquakes,” Cassidy said.
Cassidy said he’ll be studying these earthquakes in the coming days and weeks.