Questions and Answers on Megathrust Earthquakes
What is a megathrust earthquake?
A megathrust earthquake is a very large earthquake that occurs in a subduction zone, a region where one of the earth's tectonic plates is thrust under another. The Cascadia subduction zone is located off the west coast of North America. From mid Vancouver Island to northern California the Juan de Fuca Plate is subducting beneath the North American Plate. The two plates are continually moving towards one another, yet become "stuck" where they are in contact. Eventually the build-up of strain exceeds the friction between the two plates and a huge megathrust earthquake occurs.
How often do megathrust earthquakes occur?
The recurrence time varies from subduction zone to subduction zone. In the Cascadia subduction zone 13 megathrust events have been identified in the last 6000 years, an average one every 500 to 600 years. However, they have not happened regularly. Some have been as close together as 200 years and some have been as far apart as 800 years. The last one was 300 years ago.
How big can they be?
Megathrust earthquake are the world's largest earthquakes. The last Cascadia earthquake is estimated at magnitude 9. A megathrust earthquake in Chile in 1960 was magnitude 9.5, and one in Alaska in 1964 was magnitude 9.2.
Where do megathrust earthquakes occur?
The Cascadia fault, on which megathrust earthquakes occur, is located mostly offshore, west of Vancouver Island, Washington, and Oregon, although it does extend some distance beneath the Olympic Peninsula of Washington State. The large distance between the Cascadia fault and the urban centres limits the level of shaking that the urban areas are exposed to.
How do we know that megathrust earthquakes have occurred?
The sudden submergence of the outer coast when a megathrust earthquake occurs kills vegetation which can be dated. Megathrust earthquakes also cause underwater landslides off the continental shelf into the deep ocean. The landslide deposits can be recognized in core samples taken from the ocean floor.
How do we know that we are going to have one in the future?
The deformation of the crust in a predictable pattern can be detected by very careful geodetic measurements using Global Positioning Satellites, precise levelling, micro-gravity measurements and changing distance measurements using laser technology.
If the shaking of a magnitude 7 is 10 times greater than a magnitude 6 and 100 times greater than a magnitude 5, is the shaking from a magnitude 9 100 times greater than a magnitude 7?
No. Earthquake shaking, in the frequencies that damage buildings, increases to a maximum between a magnitude 7 and 8 earthquake, then the shaking simply involves a bigger area. However, the duration of shaking for a megathrust earthquake is much longer. It can be several minutes. This long duration can result in damage to some types of buildings that might not be damaged at the same strength of shaking produced by a smaller earthquake.
If a magnitude 6.9 earthquake can devastate Kobe, Japan, what would a magnitude 9 megathrust earthquake do to Vancouver?
The Kobe earthquake was right beneath the city and the megathrust earthquake will be about 150 kilometres from Vancouver. The damage pattern would be very different. We can get a good example of the kinds of damage Vancouver can expect to experience if we look at what happened to Anchorage, Alaska, during the 1964 magnitude 9.2 megathrust earthquake. Anchorage is about the same distance from the Alaska subduction fault. Small buildings generally had little or no damage, unless they were affected by landsliding. Almost all the damage involved large buildings or large structures such as bridges.
Are megathrust earthquakes our biggest earthquake hazard?
No. Inland earthquakes, which are not as big but can be much closer to our urban areas and occur much more frequently, are our biggest earthquake hazard.
Why do megathrust earthquakes cause tsunamis?
The thrusting motion of megathrust earthquake causes large vertical movement on the sea floor and this displaces a large volume of water which travels away from the undersea motion as a tsunami.
Will Vancouver Island sink when a megathrust earthquake occurs?
No. Vancouver Island is part of the North American plate. The fact that there is water between Vancouver Island and the mainland is function of the current position of sea level. However, the west coast of Vancouver Island will drop as much as a metre or two when the next megathrust earthquake occurs.
Is all of coastal BC vulnerable to tsunamis from a megathrust earthquake?
No. Just the coast exposed to the open Pacific is vulnerable to damaging tsunamis waves. The areas vulnerable to tsunamis are indicated in the red-tabbed pages of the telephone books published for the coastal communities of British Columbia
If we have lots of little earthquakes will they relieve the stress building up for a megathrust earthquake?
No. It takes many, many small earthquakes to release the amount of energy equivalent to a large earthquake. The amount of energy released increases about 40 times every time there is an increase of one unit on the magnitude scale. Thus, if we consider a small earthquake at the felt level, about magnitude 2, there would have to be 40x40x40x40x40x40x40 of these earthquakes to release the amount of energy as one magnitude 9 event. That is about one million small earthquakes a day, every day, for 500 years. That level of earthquake activity is not observed.
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