What’s Shaking on the Quiet Coast?
-a look into BC earthquakes
No one person will discover the solution to earthquakes. We all stand on other’s shoulders, yet sometimes it’s the most unorthodox that advance disciplines. Some of the crazy ones are the game changers. Dr. John Clague
Cascadia’s Fault, written by Jerry Thompson, and the subsequent video documentaries got everybody’s attention, around the previously relatively unknown subduction quake zone that lies off the west coast of Vancouver Island and goes all the way to Northern California. This fault ruptured in January of 1700, with an estimated 9 point mega-thrust earthquake and has been building pressure ever since. It will rupture again, the only question is when? But while the subduction quake has received a great deal of attention (for good reason) the southwest corner of BC and northern Washington is at risk from smaller, but still potentially damaging earthquakes called strike-slip and slab earthquakes. Although many people in Greater Vancouver vividly recall the 2001 Nisqualli earthquake in Olympia Washington, on the whole, there really aren’t as many as might be expected. It seems that the Georgia Strait, Puget Sound and the Lower Mainland in general are seriously fractured with faults. So why don’t we have as many quakes compared to many other places?
The Asian Journal put that question to Dr. John Clague who is a Professor of Earth Sciences at Simon Fraser University and an Emeritus Scientist of the Geological Survey of Canada.
John Clague: That’s a complicated question to answer. First you need to know that the Juan de Fuca plate, which is our oceanic plate, is moving northeast at about 45 mm per year, (almost 2 inches) beneath North America. Currently the North American and Juan de Fuca plates are locked against one another creating strain as one wants to slide below the other. In terms of the lesser quakes, we do get large earthquakes on some of those structures, but I think your question is why we don’t get more of them. There was a large one in 1946 near Courtney, and several down in the Puget Lowlands. Surprisingly there hasn’t been one in the Metro Vancouver area and we wonder about that. I should start by saying there are local quakes that are not subduction zone earthquakes, which occur on faults that extend up to the surface of the earth, although they don’t have to extend through he crust. They are called crustal faults and even if they come close to the surface they can slip and produce an earthquake, generally in the six to seven range, to seven and a half at the extreme.
Asian Journal: Clague then described how the Juan de Fuca plate is being torqued or twisted against the North America plate causing a distortion known as an arch.
John Clague: In our region we call it the Nisqualli Arch and it forms just south of the US border. It separates those two places where it’s being torqued and yet it wants to break apart. That’s why we get a lot of what we call slab earthquakes, which are deep earthquakes within the Juan de Fuca plate. The recent quake in Olympia Washington is one. We had a magnitude 5.5, which is getting up into the damaging range, off of Pender Island sometime in 1975 or 76. So these could occur beneath Vancouver or Victoria. The 1946 Vancouver Island earthquake was a fairly deep crustal earthquake on a crustal fault. There was also one in northern Washington near Lake Chalan in 1872, another crustal earthquake, magnitude about 6.5 to 7. So it’s not like they don’t occur. Some of the seismologists think those are the earthquakes of greatest concern because we’ve had more of those. Getting back to your original question, we haven’t had many earthquakes of these crustal faults and this is very strange because we should be getting more of those, and the question is; is this just the way it is or are we in a lull period when we’re just not getting very many earthquakes. So that’s a fundamental question and I don’t think seismologists have the answer to that.
Asian Journal: Tell me about the Georgia basin which underlies the lower mainland and can amplify earthquake energy to create bigger problems?
John Clague: We find these features all along the trough along British Columbia, Washington and down into Portland Oregon. It’s more than just a low-lying trough. It’s actually bounded by a series of faults that form a necklace of basins that are structural basins that’s been largely in-filled with other material. These are ancient and probably date back fifteen million years or more. The one here is defined by the southern Strait of Georgia, the Salish Sea. It’s a basin in a geologic sense filled with other types of sediment, some of which is fairly loose, in other words it’s rock. I don’t really understand the full details, but because that’s a closed depression, as seismic waves pass through it they tend to amplify because of the configuration of the basin and bounce back off the sides like waves in a boat basin. Above that at the surface you get higher levels of ground motion than you would outside of the basins. There are a bunch of those that extend down to Olympia. Some of those faults are active, but there’s no real evidence that the basin bounding faults in BC are active. But there is one in Seattle that is really active, called the Seattle Fault, that’s a basin-bounding fault and right through downtown Seattle. There’s really good geological evidence for young, big faults, the most recent about 900 years ago and was probably a magnitude 7 earthquake.
Asian Journal: Where are the most vulnerable areas in the northwest?
John Clague: I’m talking here about the local quakes so these would be the crustal earthquakes or in-slab earthquakes that are in the subducting quake zone. These actually produce the strongest ground motions, the ones that have the highest velocities in accelerations which are described in percentages of gravity to describe the strength of ground motions. If you have a 1g, ground motion, the ground is moving fast enough that you would be momentarily suspended in air. That’s catastrophic. They had 1g accelerations in Christchurch, New Zealand, and it was a miracle that only two buildings collapsed. So what kind of damage do these earthquakes do? You have the high ground accelerations, they put terrible stresses on buildings that aren’t properly engineered, and we particularly worry about older buildings such as you find in Gastown, New Westminster and Victoria, for example, many of which have been seismically refitted so they may be better able to take a quake, of course depending on how big the quake is going to be. But at least they are doing something. It’s very expensive to retrofit buildings, the BC government is spending $4 billion upgrading and replacing vulnerable schools. First, 50 years or more ago, we didn’t know very much about earthquakes and structures.