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Study identifies recurrences of megathrust quakes off B.C. coast


June 12, 2013   by Canadian Underwriter


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British Columbia is currently in the risk zone for a large or megathrust earthquake, based on a recurrence interval of such events identified in a new study.

B.C.

The study, published in the Canadian Journal of Earth Sciences, dated the disturbances in sedimentary layers off the B.C. southern coast over the last 11,000 years, identifying 22 earthquake shaking events.

The analysis of the geologic record allows researchers to predict the frequency and intensity of ground motion from major earthquakes along the coast, according to the study.

“Some B.C. coastal fjords preserve annually layered organic sediments going back all the way to deglacial times,” according to one of the study’s authors, Audrey Dallimore, an associate professor at Royal Roads University.

“In Effingham Inlet, on the west coast of Vancouver Island, these sediments reveal disturbances we interpret were caused by earthquakes,” she said.  

“With our very detailed age model that includes 68 radiocarbon dates and the Mazama Ash deposit (a volcanic eruption that took place 6,800 yrs ago), we have identified 22 earthquake shaking events over the last 11,000 years, giving an estimate of a recurrence interval for large and megathrust earthquakes of about 500 years,” she explained.

“However, it appears that the time between major shaking events can stretch up to about a 1,000 years,” she noted.

The last megathrust quake from the Cascadia subduction zone (which includes the B.C. coast) was in 1700 AD, putting the province in the risk zone of another earthquake, according to Dallimore.

“Even though it could be tomorrow or perhaps even centuries before it occurs, paleoseismic studies such as this one can help us understand the nature and frequency of rupture along the Cascadia subduction zone, and help Canadian coastal communities to improve their hazard assessments and emergency preparedness plans.”


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