I saw a documentory about this about 5 years ago, but I forget which island it was talking about.
With this particular island, they found that there were lots of vertical layers of porous and igneous rock and the water pressure building up in the porous rock could trigger a slide.
They were supposed to drill some holes into that part to relieve the pressure but I never heard anything else about that.
The island was volcanic, but the volcano itself it was supposed to be dormant.
The thing about stuff like this, is that nobody gives a crap until it's too late. The asian tsunami was a normal one, albeit caused by a massive earthquake.
Earthquake-generated tsunami's are relatively limited in their maximum size, but these so-called mega-tsunami' are a **** lot more scary - If you consider the damage and loss of life caused by this recent one, and then compare it to a worst-case 'mega-tsunami', well... it don't bear thinking about...