Megathrust Earthquakes
June 13, 2013

Using The Geological Record To Prepare For Future Megathrust Earthquakes

redOrbit Staff & Wire Reports - Your Universe Online

In an attempt to better prepare for future massive “megathrust” earthquakes along the west coast of Canada, researchers have prepared a record of earthquake history in southern British Columbia.

Their findings, which were published Wednesday in the Canadian Journal of Earth Sciences, will play an important role in helping scientists, government officials and the general public understand the size and frequency of tremblers along the Pacific Coast of North America. With that knowledge, they will be better able to predict how often and how intense the ground motion of future megathrust earthquakes might be.

Using a high-resolution age model, the study authors said that they have identified and dated disturbed sedimentary layers in a 40-meter marine sediment core raised from Effingham Inlet. They believe that those disturbances were caused by megathrust earthquakes that have occurred over the past 11,000 years.

“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,” Dr. Audrey Dallimore, Associate Professor at Royal Roads University and co-author of the study, said in a statement.

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

According to Dr. Dallimore and her colleagues, the last megathrust earthquake that originated from the Cascadia Subduction Zone occurred in 1700 AD, meaning that the region is at risk for another quake. Dr. Dallimore explained that they aren´t quite sure how quickly that would occur — it could be a matter of days or it could take centuries.

Regardless, she said, “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“¦ gives us our first glimpse back in geologic time, of the recurrence interval of large and megathrust earthquakes impacting the vulnerable BC outer coastline,” added Dr. Olav Lian, an associate editor of the Canadian Journal of Earth Sciences and a professor at the University of the Fraser Valley. “In addition to analyzing the Effingham Inlet record for earthquake events, this study site has also revealed much information about climate and ocean changes throughout the Holocene to the present.”

Those findings, Dr. Lian said, demonstrate the importance of analyzing the geological record when assisting earthquake planners and policy makers, as well as making sure that towns and cities throughout Canada are well prepared for future seismic events. He also noted that the research team´s findings support paleoseismic data discovered in offshore marine sediment cores in the US portion of the Cascadia Subduction Zone.