Tsunami Warning - Part 1

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Miniscule chance of a bloody big wave

With headlines about the typhoon in Burma and earthquake in China still dominating world news, it is easy to lapse into the "It can't happen here" mindset.

"The weather sucks, but at least we don't have disasters like that," we tell ourselves, consolingly.

We are wrong to think that, because it already has happened here.

And the tsunami that came ashore in 1929, killing 27 people, could happen again. In fact, there are two potential triggers for such an incident one involving an earthquake and/or landslide near the Grand Banks, the other a volcanic eruption on the other side of the Atlantic.

The recent typhoon in Burma and its 3.5-metre storm surge reminded me of this story, which hasn't been discussed locally since after the devastating tsunami of 2004. Back then, Ted Blades of On the Go interviewed a scientist about it.

Most of us don't view earthquakes as a major threat around here, and, for the vast majority of time, they aren't. But have a look at this seismic map of Canada. Based on the earthquake of 1929, we rate one of the big red dots, indicating a fault zone capable of generating an earthquake of 6.5 or more on the Richter scale (the '29 quake was a powerful 7.2).

The red dot is located in the ocean, south of Newfoundland and east of Nova Scotia, on the edge of the Grand Banks.

First, some basic science. Tsunami are caused most often by underwater earthquakes, with the lifting of the earth resulting in a wave of equal height (that is, if one side of the fault rises by five metres, the water on the surface rises by five metres, creating a wave). Because there is an upper limit on an earthquake's magnitude, there is also a limit on how large the resulting tsunami can be.

But then there are suboceanic landslides, in which large sections of a continental shelf collapse and tumble into deeper water. Depending on its speed and shape, a slide of earth can knife through the water with very little effect on the water column or it can charge like a Mack truck, triggering a tsunami of a hundred metres and more.

The largest known continental slope failure happened thousands of years ago off the coast of Norway, when 3,000 cubic km of earth gave way, pushing residue up to 500 km offshore halfway to Greenland. It resulted in a tsunami at least 10 metres high. This is known as the Storegga slide.

There is only one recorded case of a suboceanic landslide in recent times. That happened in 1929 on the southern edge of the Grand Banks of Newfoundland. Scientists theorize that the resulting tidal wave that struck southern Newfoundland, killing 27 people, had a "landslide assist". (Waves from this event actually came ashore in Portugal.)

Here is how the incident is described in the scientific paper Suboceanic Landslides, by Steven N. Ward and Simon Day:

The first fully submarine landslide to be recognized dates to 1929 when between 300 and 700 cubic km of sediment slid off the top of the continental slope south of Newfoundland in a thin but broad flow that passed just west of the wreck of the Titanic. The mass of fluidized sediment plunged into the depths of the Atlantic at speeds near 80 km/h. During its course, the landslide mass turned into a giant flow of turbulent, sediment-laden water that successively broke several transatlantic telegraph cables connecting America and Europe. Interestingly, the timing of the cable breaks established the speed of the landslide. Little recognized for many years after the event was the landslide-generated tsunami that struck sparsely populated coasts of Newfoundland and Nova Scotia. Waves 10 m and more high killed nearly 30 people.

The claim of 300 to 700 cubic km of sediment is contradicted in this Natural Resources Canada web site, which is a great resource. It says that just 200 cubic km of earth moved.

The Storegga slide off Norway was studied intensively by scientists, to determine whether or not the remaining continental shelf was safe for offshore oil and gas installations. Scientists concluded that the slope had been destabilized by the tremendous weight of glaciers, before they retreated at the end of the last ice age, and thus there was no threat of further landslides until the next ice age.

As noted, we have a seismic fault on the Grand Banks capable of triggering significant earthquakes, so the possibility remains of another suboceanic landslide offshore which could trigger a major tsunami. This, however, is one of those lurking dangers that seldom gets discussed locally.

In tomorrow's post, I will talk about a volcano that some scientists claim could devastate the eastern coast of North and South America, under a tidal wave 25 metres high.

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