A recent article I read somewhere said that if the predicted bit of
La Palma
falls off in one slab the resulting tsunami will lead to the
disappearance
of the Isles of Scilly (among other damage!)
Charlie Pridham writes

Lots of people have said that, but it seems unlikely. To create a tsunami
requires a high energy shock wave, a bit of land falling in would, however
large not be moving fast enough for the damage to be transmitted any
distance, although there would certainly be a large wave locally much as
when large icebergs break off.

I think you confuse speed with energy. If you drop a very very large
mass (say 5000 million tons) a few hundred feet (and I think in the case
of the canaries it drops a long way down to the ocean floor) then the
*energy* released is converted into a (relatively) smaller mass of water
travelling *very* fast. I don't know what the conversion factor is but
say 1% of the mass travelling at say 20 times the speed would still be
quite significant.
--
David