The majority of the population of the world have sent me a link to this cartoon about a superhero called Etymology Man. It is instructive in my uselessness, and I thoroughly recommend having a look.
However, it's not quite right. A tidal wave had always been a wave caused by the tide, until T.H. Huxley misused it in 1878. But there was a perfectly good English word for a huge rise in sea level caused by seismic activity: groundswell.
For some reason groundswell is now pretty much the prisoner of political journalists, as in a groundswell of support/disapproval/lust etc.
So, in cases of emergency you should shout Groundswell, and everybody on the beach with a copy of the OED in their swimming-trunks pocket will know to run for the hills. This is called Natural Selection.
This is not a groundswell