The history of megathrust earthquakes in British Columbia and how to prepare for the ‘Big One’
By Koy Tayler, Contributor
With motion pictures such as The Impossible, and the upcoming movie San Andreas, popular culture is bringing science into homes around the world. Through adaptations such as these, the public’s knowledge of large-scale earthquakes and subsequent megathrust events is expanding. Do situations such as these register in the minds British Columbians? Concerns regarding the “Big One” that could originate in close proximity to Vancouver Island have increased over generations. With such a disaster looming over residents’ heads, scientific research is laying out a clearer understanding of how the Pacific Northwest should react, potentially answering the question: will residents be able to withstand such a quake?
Such events like the megathrust earthquake of March 11, 2011 and the subsequent tsunami that wreaked havoc on the East coast of Japan have been making headlines around the world. A megathrust earthquake is no different from any other earthquake, only it’s larger and stronger—registering at an approximate magnitude of nine or higher on the Richter scale—and is situated along subduction zones as described on EarthquakesCanada.nrcan.gc.ca.
As seen on cbc.ca, legends from local First Nations in combination with scientific findings have concluded that on January 26, 1700 at about 9 p.m. a magnitude nine earthquake struck the Pacific coast. Observations of the area impacted by the events confirm the legends and indicate that there was a very powerful tsunami that followed the megathrust quake. The quake was so powerful that vibrations were felt as far east as Manitoba.
Susan Smythe, Geography Department Chair at Douglas College, explains that British Columbia’s coast is no stranger to large earthquakes. Scientists have found evidence to suggest that over approximately 10,000 years, 19 earthquakes of magnitude nine have been felt along the West coast. Smythe also explains that analysis of tsunami deposits in various places, along with the remnants of an old forest found on Vancouver Island, add to the conclusion that a tsunami created significant damage in the area. Unfortunately, data compiled regarding the magnitude nine earthquakes of past do not occur at even intervals, and therefore cannot be used to predict when the next one will hit the province.
However, the understanding of why British Columbia’s geographical terrain is so prone to earthquakes has improved tremendously. Along with the ability to compare similar areas such as the coast of Indonesia with that of British Columbia’s, geoscientists are better equipped to understand the effects of a megathrust earthquake. One of the most important observations about the Pacific coast is the presence of a subduction zone situated off the West coast of Vancouver Island. The subduction of the thinner but more dense oceanic Juan de Fuca Plate under the thicker but less dense continental North American Plate is not moving smoothly.
The world’s tectonic plates do not form a perfect puzzle, so often areas of subduction become locked together but continue to converge, creating a build-up of stress between the plates. Through advanced technology, experts observed that during Indonesia’s 2004 megathrust earthquake, one tectonic plate seemed to have moved up to 15 metres horizontally. Locally, the North American Plate is being pushed up due to its inability to slide over the Juan de Fuca Plate while both plates continue to converge. This area of subduction is called the Cascadia Subduction Zone, and geoscientists predict that the release of this energy—creating a kick-out—would produce an unzipping effect of the subduction zone. Smythe also adds that a kick-out could cause areas to drop vertically approximately 1-2 metres; a slight concern for areas such as Richmond which are already so close to sea level and are situated on a lot of river sediment.
Smythe also describes a relatively unknown but important result of a megathrust earthquake along the southwest portion of the province: local tsunamis. Vancouver Island forms a barrier separating mainland British Columbia and the Pacific Ocean. Unfortunately for the island residents, this landmass will bear the brunt of a tsunami, reducing concerns for those living in Metro Vancouver. Smythe explains that the Lower Mainland should focus their awareness towards the damage that aftershocks could create. It is predicted that large-scale earthquakes can produce aftershocks capable of reaching magnitude eight. If the epicentre of an aftershock occurs along the coast of the mainland, there is a potential for a seismic wave to produce a smaller, local tsunami. The local tsunami may have the capacity to reach the Gulf Islands in approximately five minutes and may reverse off those landforms and move towards the Lower Mainland. Although impacts would be small, it is an event to consider.
British Columbia enjoys the benefit of numerous rivers that converge into the Strait of Georgia but it is possible that they may negatively affect Metro Vancouver’s stability in the event of an earthquake. Although there is no past evidence, Smythe informs that the steep beds of unconsolidated sediment near the entrance of the Georgia Strait could potentially create an unstable foundation for the southwest coastline during a megathrust earthquake or large subsequent aftershocks. Another factor contributing to the worry about the megathrust earthquake set to hit southwest British Columbia is the shape of the Georgia Basin. Simply put, the shape of the basin would focus and amplify the seismic waves of an earthquake.
Where will you be in the event of an earthquake? Smythe explains that no matter where you are, having a family plan is extremely important as roads and phone lines could be unavailable. Families should create information cards about who to call or where to go if such an event were to occur, and to also have a discussion about having their homes stocked with a few days worth of food and water among other things. Smythe adds that many lives are not only lost from initial megathrust events but through situations where there could be falling debris or downed power lines. Although difficult, having the spatial awareness to recognize hazards after an event is key.
Building codes have greatly improved over the past few decades. Current standards are much more equipped to handle quakes but homeowners living in buildings constructed prior to the ‘60s and ‘70s should look into assessing their foundations as they may need some upgrading. Residents could also invest in things such as straps for the front of bookshelves to prevent heavy items from falling, as well as securely installing their water tanks into the walls of their homes. Little things such as bolting shelving units into studs or putting rubber mats under glassware to reduce movement, can make a difference in securing one’s safety.
Along with the city of Vancouver, which has implemented a five-year strategy to identify, prepare, and recover from a large-scale earthquake, Douglas College has also stepped up in its preparation. From training people in light urban rescue to looking into how plumbing would be affected by such an event, the college is contributing to the recovery efforts. If residents are not near the college in the event of an earthquake, Smythe suggests that individuals should stop and crouch down—preferably under a stable object while covering their neck and facing away from windows—for the duration of a quake. If outside, look for doorways to stand under, as these are extremely strong, and wait until shaking stops for approximately a minute. In case of a tsunami following an earthquake, people should make their way to high ground as soon as possible. Whether it is up the North Vancouver mountains or to the upper levels of a high-rise building, the higher you are, the better you are off.
Most on the Pacific coast have learned, in some capacity, how to prepare for and recover from an earthquake. Unfortunately, we can only anticipate so much for something that has not occurred in our lifetime, but taking the time to learn about how such an event will occur may benefit our preparedness and actions following it. In this instance, knowledge really is power. Despite the grim reality of a megathrust earthquake, it is a waiting game and generations may pass by before the Big One will take place.