Thousands of years ago, two massive landslides broke free 60 metres beneath the surface of Douglas Channel on the British Columbia West Coast. Combined, the two slides moved 63 million cubic metres of earth.
Their impact was likely dramatic. A smaller undersea slide in 1975 produced waves more than eight metres high, damaging a First Nations village along waterways that are being considered for large tanker traffic, to carry away oil brought to nearby Kitimat, B.C., by the proposed Northern Gateway pipeline. Natural gas tankers, too, will frequent these waters if companies succeed in building a handful of gas export terminals in the area.
But the discovery of the two massive slides, documented by Natural Resources Canada in a report submitted for ongoing federal hearings into Gateway, has caused concern among nearby residents. They say it points to potentially dangerous seismic instability in the area, a belief buttressed by the NRCan conclusion that the slides’ “close proximity to an apparently active fault presents the possibility that they may have been triggered by ground motion or surface rupture of the fault during past earthquake events.”
Enbridge, however, disputes the very existence of such a fault.
“We find the evidence at present to be somewhat unconvincing,” said Drummond Cavers, a principal engineer at AMEC Environment and Infrastructure who serves as a consultant to Enbridge.
He said there is no clear indication that sediments have moved around the supposed fault. “There’s these old faults but there’s no evidence that they’re presently active,” he said, adding: “At this point, there’s nothing to indicate that we don’t have a safe pipeline route, and that there’s any elevated seismicity risk on the West Coast.”
The NRCAn document, however, suggests the area is seismically active, pointing to 11 small earthquakes within 20 kilometres of the slides in the last 25 years and, “in the absence of additional evidence, the fault must be considered a potential trigger for the submarine failure events.”
The document does not, however, detail the specific risk such slides might hold for ship traffic, saying detailed tsunami modelling is needed.