It depends where you are. I've been in a 6.2 and a 5.6 and the latter felt stronger.
I'm not denying that a 7.8 is not a big quake, I'm just saying it's still very different to a 9.1...
Central New Jersey sits atop clay and sand, was once the shoreline of an ancient ocean, and has various ancient fault lines running through. The 5.6 sounded like a truck ran into our school building. It cracked roads and gaslines. IIRC, clay and sand liquify in a quake.
If we are talking tidal waves then it is the type of quake that matters most. If the quake was an up/down quake surfs up, if not no Tsunami.