Why some tsunamis are worse than others at distance

An article on tsunamis on the BGS web page attracted a comment on Facebook raising the question as to whether the main tsunami threat to Britian might be from a flank collapse in the Canary Islands, not mentioned in the posted article.

To explain why such an event would be unlikely to pose much of a problem to the British Isles, here is a thought experiment. Imagine you have a concrete slab 2m long, 1m wide and 30 cm thick. Using a sling and a hoist, you suspend it horizontally, 3m above the surface of a swimming pool. Then you drop it. The slab hits the water flat side on. You can imagine the sort of huge splash that will make.

Now repeat the experiment, except this time, you hold the slab vertically, so that when you drop it, the slab enters the water end on. That’s going to make rather a smaller splash.

Third experiment: this time, you have ground up the slab and you have an equivalent weight of gravel. Drop that into the pool. Again, a splash, but of quite a different character.

It’s not the weight of material that determines how effective the splash is, it is the geometry of the object as it enters the water (and also the speed). An earthquake tsunami is rather like the face-on slab – except the slab is hitting the water from below rather than from above. Any sort of landslide tends to be either like the end-on impact or the gravel sliding in. It may still make a splash, but the wave is much more localised and not so good at travelling long distances. To produce a really effective tsunami, a volcanic flank collapse would have to involve the side of the mountain tipping over so as to hit the water flat on with maximum force, and as a coherent block. And that is not generally how such events happen.

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