[EvaH (OP)]

Paul wants to know:

Would it be possible to devise some method of turning a tsunami upon itself so that it cancelled itself out, like noise cancelling headphones? One would need to split the wave in two, and a single wall going out into the sea would presumably be ineffective. If two walls were joined at an apex pointing out to sea that might have some effect, but if they are small walls the tsunami would just rise over the top. To negate a wave, would an opposing wave need to be 180 degrees out of phase? And then the question arises of how one delays one part of a tsunami and then turns it against the balance of the tsunami wave...

Another suggestion for small islands is to figure out a way for tsunami to bypass the island.

What do you think?

[evan_au]

The Japanese Tohoku earthquake that destroyed the Fukushima nuclear reactor had an energy around 2x10¹⁷ Joules, or around 50 million tons of TNT, or 50,000 Hiroshima bombs (but not all of that energy ended up in the Tsunami).

You can't cancel immense amounts energy unless you are willing to apply immense amounts of energy - and the doubled amount of energy will just produce more destruction elsewhere.

You may be able to divert energy without applying so much energy - and Fukushima reactor did have a seawall that reflected a fair amount of the incoming wave - but they had ignored geological evidence of huge (but infrequent) earthquakes, in favor of protecting areas that had smaller (but more frequent) earthquakes.

We often think of a tsunami as a smashing wave. When a force like a wave strikes a fixed obstacle (like a seawall), the wave rises up to double the previous height, before being reflected out to sea.

But there is a second component to a tsunami - a more gradual rise in sea level over the next 5-10 minutes, which will flow around any obstacle. So you can't just put a wall in the sea, as this second component will flow around it.

In the end, having relevant building codes to withstand the earthquake, training the population to know what to do (run to high ground), having signposted safety zones, and having an effective public warning system will be more effective than brute-force seawalls or a series of atomic bombs on the seafloor waiting for the next tsunami.

Let's face it, when it comes to tectonic forces, humans are lost in the noise.
See: https://en.wikipedia.org/wiki/2011_T%C5%8Dhoku_earthquake_and_tsunami