Waves, plates, and earthquakes on B.C.'s West Coast
It’s a pretty horrifying scenario, right out of a summer disaster movie: a towering wave the size of a six-storey building would rise up out of the Strait of Georgia, roll west, and slam into the eastern coastlines of Galiano and Mayne islands, just a few kilometres away from Metro Vancouver.
Homes, cottages, boats, and docks would be destroyed. The lack of warning would mean there would be multiple human casualties, especially if it occurred at night, on a weekend, or in the summer.
No one had really thought about such a thing. After all, tsunamis didn’t, couldn’t occur in the strait. There were no active fault lines under the ocean there to cause earthquakes that could generate monster waves, after all. And the nightmarish consequences of the oft-mentioned “Big One”—the up to magnitude-nine-plus (Richter scale) quake predicted for this region that would bring shaking destruction and walls of seawater up to 30 metres high—would be deflected by the protective bulk of nearby Vancouver Island.
Residents of towns on the west coast of the Island, such as Bamfield, Ucluelet, and Tofino, were used to emergency planning and tsunami warnings. That was in anticipation of the megathrust quake that geologists have predicted has a 37-percent chance of occurring in the next 50 years along the 1,000-kilometre-long fault known as the Cascadia subduction zone just offshore. Tofitians knew they might have only 10 or 15 minutes to get to the school or community hall’s high ground—or the loftier Radar Hill, if they were feeling lucky.
But Gulf Islanders never gave it a second thought. That stuff came from the west, and they had their 460-kilometre-long breakwater over there to snuff such pip-squeak ripples.
No one ever thought about anything sneaking up on them from the other direction, out of the millpond that lay between them and B.C.’s Lower Mainland.
Until 2003, that is. That’s when lead author and research scientist Alexander Rabinovich published a study titled Numerical Modelling of Tsunamis Generated by Hypothetical Landslides in the Strait of Georgia, British Columbia. The paper looked at the possible outcomes of underwater sediment slippages in the central strait and off Roberts Bank on the foreslope of the Fraser River Delta, where that mighty watercourse deposits between 17 million and 20 million tonnes of sediment every year.
Under several scenarios, the study—sponsored by B.C. Hydro and the federal government’s Geological Survey of Canada (GSC)—came up with results that showed the theoretical slides displacing the strait’s water column sufficiently to result in potentially huge waves racing eastward.
Marine geologist Brian Bornhold, one of the paper’s four other authors, has been studying underwater landslides for most of his career, 25 years of it with the GSC, from which he is recently retired. “We took the scenario ‘What if this area failed on Roberts Bank? What would the wave look like?’ ” Bornhold told the Georgia by phone recently from Sidney on Vancouver Island. “When we did that, what we found was that you would get very, very big waves over on Galiano Island, on Mayne Island, of 15 to 18 metres.”
The study got lots of play in circles concerned with such things but little attention from the public, for some reason. Maybe it was because of the relatively small populations (about two thousand in off-peak months) of the islands concerned. Or maybe it was because of the paper’s rather prosaic title, or the fact that it contained the word hypothetical. Either way, other than a few newspaper articles, the report didn’t gain much popular traction.
In hindsight, this was probably a good thing—seeing as how that paper’s conclusions have since been refuted by other researchers and engineers doing geotechnical studies in the Roberts Bank area.
Bornhold, now a private consultant, isn’t apologetic, given how the study was theoretical in the first place.
“We basically said, ‘Okay, you’re telling us this could happen or this is where it could happen. Now we do the modelling. It turned out later, when much more geotechnical work was done—by other people, not us—they determined, in fact, that [slope] failure would not occur. We [just] did some hypothetical modelling, and this is where I want to allay any fears that may be out there: there is no basis for a deep-seated [sediment] failure being likely on the Fraser Delta.”
As with any good thriller, though, when one threat subsides, another one rises.
John Clague is a veteran geologist and a professor in SFU’s earth-sciences department. He wasn’t a big fan of the 2003 study. “I was very upset, actually, at that Rabinovich paper because it was unduly alarmist,” he told the by Skype from Taupo, New Zealand, where he was holidaying. “It was based on a modelling exercise without any discussion of the geological constraints on what’s possible.”
Clague said he not only thinks the sediment slope will not suddenly fail by itself, he is convinced that even a helping hand in the form of the expected devastating Cascadia subduction-zone earthquake would not push it over the edge. “We had one of those in AD 1700 and it, apparently, didn’t cause that kind of failure. We’ve had five [such massive earthquakes], or maybe six or seven…in the past 3,500 years or so, and we don’t see that, so I would say that it’s unlikely.”
He said the sandy delta regulates itself with constant small failures that “travel down canyons off the main channel of the delta slope”.
Clague is unwilling, though, to rule out the possibility of an earthquake-triggered underwater-landslide tsunami in the strait. “I don’t rule anything out,” he said, laughing. “We had a meteor, in fact, 65 million years ago that killed the dinosaurs. It would be foolish to say we can’t have this unprecedented possible event. But in terms of actual [emergency] planning, I’m not sure you would have to worry so much about that.”
Bornhold was also unwilling to use the word never with respect to a slide-caused wave in the strait. “We know that earthquakes can trigger underwater landslides, but this would likely not occur except if we had a truly giant earthquake, a nine-plus earthquake, around the Fraser Delta.”
Sidney-based seismologist John Cassidy is very familiar with the Georgia Basin area and is an expert in earthquake hazards and fault zones. He has worked for the Geological Survey of Canada for 20 years and is an adjunct professor at the University of Victoria.
Cassidy agrees with Clague that an offshore Big One would not likely trigger a Fraser Delta failure: “It doesn’t seem like the shaking level is high enough or strong enough to do that,” he said by phone, “because there is no evidence of this occurring, and we would expect to see evidence of several of these. They [the megathrust quakes] happen on average every 500 years.”
Clague is convinced, though, that despite their rarity, relatively weak, shallow local earthquakes should be more of a worry to residents of southern B.C. than the endlessly referenced Cascadia “Ring of Fire” megaquake that would probably take place dozens of kilometres below the ocean floor and about 75 kilometres off B.C.’s or Washington state’s coast.
“Probably more of a concern would be not so much the very large earthquakes,” Clague said, “[but] a more local earthquake on a fault closer to Vancouver that can produce much more intense ground shaking.…When I’m talking about local earthquakes, I’m talking about shallow earthquakes with centres closer to big cities.”
Cassidy, for his part, said that shallow quakes “in this area…[are] the rarest type of earthquake. The deep ones, 60 to 70 kilometres, those are the most common.”
The February 2011 earthquake in Christchurch, New Zealand, is a good example of a destructive yet fairly shallow temblor near a big city, Clague said. “I worry a little bit more about crustal earthquakes…that we can get Christchurch-type earthquakes. We can get a magnitude-six to -seven earthquake that can be quite catastrophic and wouldn’t necessarily trigger a tsunami. The Christchurch earthquake didn’t produce a tsunami and is unlikely to be preserved geologically, though you have this horrible earthquake that caused $20 billion damage and killed a couple hundred people.
“An earthquake like that in Christchurch, that close in, even that size—it was a magnitude-6.3 earthquake; it wasn’t a huge earthquake—and yet it caused so much liquefaction-related damage, and we could certainly expect a similar type of damage pattern in Richmond and parts of Delta.
“And you have to think of the infrastructure at risk there: there’s the airport; there’s the Delta-port; there’s the Tsawwassen ferry terminal; there’s electrical cables that deliver a significant chunk of Vancouver Island’s power—they cross the Fraser Delta and go across the Strait of Georgia—and there’s a proposal to expand the Deltaport. These are huge things, I think, in terms of economic infrastructure at risk.”
Although Cassidy noted the historical lack of shallow, crustal quakes in the region—“We haven’t seen any large earthquakes with a surface rupture; there are no known active faults beneath land in this area”—he did point out the relatively recent discovery of a fault running “right through downtown Seattle. That was discovered about 15 years ago, and that fault runs right up to the surface.”
He said the Lower Mainland could be hiding faults of the same type and we just haven’t seen them yet. “One of the ways of identifying faults is through earthquakes, and we’ve been recording earthquakes for a long time in British Columbia, for more than a hundred years. And the shallow earthquakes, there are very few of them.
“It’s difficult to find faults in this area, where we’ve had glaciation, a lot of [eroding] rainfall, and a lot of [covering] vegetation,” he said. Cassidy explained that urban construction projects sometimes find evidence of faults, and that a remote-sensing technology called LIDAR (light detection and ranging), often airplane-mounted, that analyzes backscattered laser light has been extensively and effectively used to analyze topographical features for the past two decades.
But absence of evidence, as they say, isn’t necessarily evidence of absence.
Clague pointed to the Seattle fault (“a known active fault” with “a history of large slips during postglacial time”) as an example to spur further research: “There’s no reason to think that these active faults end at the international boundary. We really don’t have the type of evidence that they have down in the Puget Lowland. We need additional tools, like LIDAR coverage of Metro Vancouver, to look for these active faults.”
But that takes federal money and a dedication to promoting science, a combination in seemingly short supply within the ruling Conservative party.
“I think we need to do more research of this sort, but it’s hard to find the resources to do it in the economic climate we live in,” SFU’s Clague explained. “It’s not a university-based initiative; we don’t have the resources in university to do that type of research. It has to be government-driven through the Geological Survey of Canada, and this government is not inclined to really support government science. Meaning the federal government.
“And I don’t mind being quoted on that, because I think this government is not… You know, it should be supporting its federal research capability better than it is.”
He doesn’t sound as if he’d be too upset if another meteor took care of some modern dinosaurs.