Based on geological evidence, Ian Dalziel believes what is now North America remained attached to the supercontinent Gondwanaland until the early Cambrian period, in contrast with current belief, which has the separation occurring earlier. …Interesting. There is another account, and it has mankind around when the continents separated.
And unto Eber were born two sons: the name of one was Peleg; for in his days was the earth divided; and his brother’s name was Joktan.The name Peleg comes from Hebrew palag, which means to split apart; the name itself means “earthquake”.
— Genesis 10:25
Ok now that solves the mystery. The tectonic plates under the ocean do cause earthquakes, which create tsunamis.