There was a program a number of years ago on Nat Geo I believe that demonstrated how large chunks of vertical cliff breaking away and thus becoming high speed landslides can displace a huge amount of water.
They displace much more water than just the mass of the land itself, because of the speed at which the land falls into the water apparently also pulls air into the space where the water was very quickly.
They demonstrated this effect in a small scale tsunami tank in some university.
They then theorized that this explained how some fossils unique to the area of Hawaii were found at the top of some Australian mountains some 600ft up if I recall correctly.
But their general hypothesis was that high speed landslides are capable of creating massive waves that move at a high rate of speed and do not lose energy over very long distances.
A massive underwater uplift would be worse
You had me and then you lost me. An expanding wave on a surface must necessarily diminish in linear intensity as 1/d, since the wave has a certain amount of energy which is distributed over a perimeter of length 2pi d.
I saw a show some time ago depicting this type of wave, but confined to a channel, somewhere in Alaska. Of course, the narrative did not point this out explicitly, but it was quite clear that this was the case. They were just angling for hype. I took due notice!