Charlie wrote:
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

This is contentious. Have you read
http://www.benfieldhrc.org/CentreNew...es/tsunami.htm or
http://www.bbc.co.uk/science/horizon...anscript.shtml

More recent suggestions that it would break up before falling, and
"only" cause a wave 2 to 3m high have been made:
http://news.bbc.co.uk/1/hi/sci/tech/3963563.stm . That page has links
to sites that support and oppose the original tale.
It boils down to "we can't know the future".