Scientists find new fault line running under the Greater Victoria area in British Columbia

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Scientists have discovered a 72-kilometre fault line on Vancouver Island, in a region known as the Saanich Peninsula, just north of the provincial capital, Victoria. The finding, which was reported in the journal Tectonics, includes evidence of an earthquake, somewhere between magnitude 6.1 and 7.6, that struck the region thousands of years ago.

The newly identified fault has been named the XEOLXELEK-Elk Lake Fault, or XELF. It crosses Saanich Peninsula within Greater Victoria and as such poses a hazard to the region’s roughly 400,000 inhabitants, the scientists say.

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“Therefore, determining whether it produced recent large earthquakes is important for updating regional earthquake hazard models and increasing earthquake preparedness,” the team wrote in their paper.

Using geophysics, fault-fold modelling and good old fashioned digging, the team, with members from Canada, the United States and France, determined that a single large earthquake occurred on the XELF between 2,300 and 4,700 years ago.

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“A similar future earthquake … could cause major damage to the Greater Victoria area,” they write, adding that a tsunami is also possible, since part of the fault lies underwater. “Thus, our results can improve future earthquake hazard assessments.”

The paper doesn’t give an estimate for when the next earthquake might occur on the fault or how strong it could be, citing the difficulty of extrapolating from a single event so far in the past.

“We don’t have a good idea what the hazard is right now,” lead author Nicolas Harrichhausen told the National Post. “It is certainly a risk we need to consider, but because we see only one earthquake in the at least last 12,000 years, calculating the hazard is difficult. We need more information, i.e., more studies and funding, to better determine its slip rate and how many earthquakes it has hosted in the past. Then we can use a hazard model to estimate the risk posed to the Victoria region.”

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He added: “As for now, we can only say it is active and it could possibly host an earthquake similar to the one we observed. There is no information that is telling us that an earthquake on the fault is imminent.”

The group’s find was made possible in part by the availability of new technology in the form of airborne lidar, Harrichhausen added.

“Lidar is basically a laser range finder from that is flown on a plane or drone, and it allows us to make very high resolution topographic models of the land surface,” he said. “Because the Pacific Northwest is heavily vegetated, and the faults in the upper crust are slow moving (not like the fast moving San Andreas), the signature they leave in the landscape is very subdued and hard to see, especially with the vegetation.”

The models allow scientists to see smaller features, revealing more faults. “B.C. just launched its OpenLidar portal, which provides a bunch of free lidar covering portions of B.C., which will allow for more of these discoveries to be made,” Harrichhausen said. But, he added, more funding for on-the-ground field work is still needed.

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