With regard to the formation of slope:http://earthsci.org/flood/j_flood04/masswa/masswa.html#SubmarineSlopeFailures
I have yet to see any terrain software create such erosive structures (Not even GeoControl) so i think you are out of luck. It would probably have to be painted by layers and blend modes to fake the features.
Faking would be possible in 3d software... But i am curious if it would be in a way easier with voxels from a programmer side (not that it would be easy to program)...hmm
Kadri
Are you trying to make a slope deform from an original position to the new slide debris field? It may be possible using the Animation version.
You would have to modify the erosion function of a heightfield terrain for each part and time scale of the slide. Objects can then be used on the surface to 'fall' down the slide using key framing. At least I think that would work. I don't know how to limit the area of the slide, unless the images were composited later.