Hi. "Tidal wave" suggests a wave associated with the ebbs and flows related to the relative positions of the earth, moon and sun. These rises and falls relative to mean sea level are periodic and easily predictable. But the enormous wave generated by an earthquake is probably even less easy to predict than an earthquake and of no regular (that is, periodic) recurrence that anyone knows of. But I think "tsunami" is not too bad a name for such earthquake-generated waves as tsunami does not suggest any strong link with the relative positions of the earth, moon and sun.
