Numerous satisfactory explanations from readers, of which the following two
pretty much cover it.
Reader A: “You’ve already seen the money pictures. The stuff that looks
like a really fast incoming high tide is it. Mega high waves are just
exceedingly rare and are typically the result of long run-out landslides.
Tsunamis from seismic activity typically look like a really fast high tide.
They are immensely destructive, but there’s not usually a ‘wall of water.’
It’s a myth. Out at sea, you don’t even notice the swell from a seismic
wave because it’s just a few feet high and the ocean’s so deep. The wave is
only noticeable at shore where the sea gets shallow. It’s more like a big
’slosh’ in a tub than a big wave.”
Reader B: “John — I was a little surprised too, and did a little
investigation and/or thinking. As far as the out-at-sea part goes, the
answer appears to be that in fact a 20-ft wave on shore isn’t all that big,
and the shock wave is propagating more or less as a compression wave, mostly
in the x-y direction. On the rest of it, well, I guess we’ve been misled by
the movies. Those great honkin’ walls of water we see from Hawaii and surf
movies are a function of a specific sort of subsurface topography. If you
don’t have that topography, you don’t get the big rolling breakers.”