Megathrust Earthquake Strikes The North American West Coast — When Will The Inevitable Happen?

When will the next megathrust earthquake strike the Pacific coast of North America? It’s an important question for those living in the region, and also more generally for the governments and economies of the United States and Canada, but it is a difficult one to answer. But now, new research may finally be helping to bring some clarity to this subject — the first truly comprehensive and well-dated record of earthquake history along the southern coast of British Colombia.

Image Credit: Dan Anthon, Royal Roads University
Image Credit: Dan Anthon, Royal Roads University

Such a record gives us a much more accurate understanding of the size and frequency of large earthquakes — especially megathrust earthquakes — along the Pacific coast of North America. The new work was done by utilizing a new high-resolution age model to identify and date the “disturbed sedimentary layers in a 40-meter marine sediment core raised from Effingham Inlet. The disturbances appear to have been caused by large and megathrust earthquakes that have occurred over the past 11,000 years.”

Dr Audrey Dallimore, Associate Professor at Royal Roads University, and a study co-author, explains: “Some BC coastal fjords preserve annually layered organic sediments going back all the way to deglacial times. In Effingham Inlet, on the west coast of Vancouver Island, these sediments reveal disturbances we interpret were caused by earthquakes. With our very detailed age model that includes 68 radiocarbon dates and the Mazama Ash deposit (a volcanic eruption that took place 6800 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. However, it appears that the time between major shaking events can stretch up to about a 1,000 years.”

“The last megathrust earthquake originating from the Cascadia subduction zone occurred in 1700 AD. Therefore, we are now in the risk zone of another earthquake. 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.”

“This exceptionally well-dated paleoseismic study by Enkin et al., involved a multi-disciplinary team of Canadian university and federal government scientists, and a core from the 2002 international drill program Marges Ouest Nord Américaines (MONA) campaign,” says Dr. Olav Lian, an associate editor of the Canadian Journal of Earth Sciences, professor at the University of the Fraser Valley and Director of the university’s Luminescence Dating Laboratory. “It gives us our first glimpse back in geologic time, of the recurrence interval of large and megathrust earthquakes impacting the vulnerable BC outer coastline. It also supports paleoseismic data found in offshore marine sediment cores along the US portion of the Cascadia Subduction Zone, recently released in an important United States Geological Survey (USGS) paleoseismic study by a team of researchers led by Dr. Chris Goldfinger of Oregon State University.”

Additionally, the Effingham Inlet site has provided further information on the climate of the time period when these earthquakes occurred — providing data that is of great use in other fields.

As a side note — the Cascadia Earthquake of 1700 is well accounted for by oral traditions passed down by the American Indian tribes which lived in the area.

“They had practically no way or time to try to save themselves. I think it was at nighttime that the land shook. … I think a big wave smashed into the beach. The Pachena Bay people were lost. … But they who lived at ‘House-Up-Against-Hill’ the wave did not reach because they were on high ground. … Because of that they came out alive. They did not drift out to sea with the others.”

The new research was published in the Canadian Journal of Earth Sciences.

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