Location of debris from the 2011 tsunami. (NOAA)
By Evan
Perhaps nothing is more sobering to an aspiring earth scientist than the results of major disasters. I remember watching the footage of the 2004 Sumatra tsunami and thinking about the responsibility we have to the public to inform about the hazards associated with natural phenomena such as earthquakes and tsunamis. The last major megathrust earthquake that produced a large tsunami before the Sumatran event was the 1964 Alaska Earthquake, which caused few fatalities due to its remote location. People were unaware of the risks in Sumatra, partially because this style of earthquake does not happen frequently, and there had been few geological investigations to determine the risk there.
In Japan, there is a long recorded history of major tsunamis. However, the sheer size of the earthquake that struck there last year was unprecedented in the past 1000 years, and barriers that were designed to handle historically large tsunamis were insufficient. As a result, there was a massive loss of life, and large amounts of debris were swept to sea. The New York Times is reporting on a vessel carrying volunteers who are documenting the debris that was swept to sea. This project is of archaeological interest, and to determine what might be coming towards the North American coast. People on Haida Gwaii on the west coast of British Columbia are already describing the beaches there as “landfills”. As a goodwill gesture, the Japanese government is offering to help pay for the cleanup, which is probably going to be an immense endeavour.
Debris washed up on the shore of Haida Gwaii (CBC).
The debris field approaching the west coast of North America should be a reminder that they are not immune to massive tsunamis. Last month, Postmedia News reported on a recently published study giving the statistics on megathrust earthquakes off the North American coast. The last major earthquake on the Cascadia subduction zone happened on January 26, 1700, over 300 years ago. Dating the timing of large underwater landslides or turbidites for the past 10,000 years, the authors of the study determined the frequency of major earthquakes on the northern portion of the subduction zones is 500-530 years, and 240 years on the southern portion (the earthquake can be caused by a rupture in small parts of the subduction thrust, or the whole thing can go at once, like in the 1700 event). If you look look at the averages, it has been longer than 240 years since the last major earthquake on the Cascadia subduction zone. In the Postmedia article, Chris Goldfinger states that if there isn’t an earthquake in the next 50 years, the interval between major earthquakes will exceed 85% of events, and that the duration already exceeded the interval of 75% of events. This means that the chance of there being a major earthquake similar to the Sumatran and Japanese megathrust events is very high in the next 50 years. People on the west coast of North America need to be vigilant, because very soon it could be debris from their communities that is washing up on some distant shorelines.
Earth scientists may not be able to predict exactly when earthquakes will happen, but with a thorough geological investigation of past events, we can estimate when the risk is high. It is the responsibility of earth scientists to inform the public what those risks are, and the precautions that are needed to mitigate that risk.
