Well they will probably have a bomb "texture" / crater that is placed...? Lots of rts's do this (just speculation).
People are going to flame me, but the easiest way to do this is tesselation/dx11. Have a bomb create a crater with depth and size dependent on bomb weight. Then when you approach said crater it activates the tesselation and creates the geometry up close to look like a real crater. This would not require any terrain deformation programming, because it would just be a mesh that is placed and therefore is not dynamic (like it wouldnt be the same as in red faction, as soon as you move away it turns back into a texture so it doesnt technically change the geometry unless your ontop of it and no calculatios need to be done if you are dropping bombs and watching from a distance or are not directly looking at it).
Edit: with the specs released this is obviously not on the table
