the units that are "placed" on the map (like you can see their dot on the minimap or you see the placement flash on the minimap) but don't show up usually don't have anim files for them
I got the cloud/snowstorm units in simply by making an anim file for them defining their graphic.
Other units though (like stone mines) simply don't exist at all, and have no texture/model and thats why they don't show up.
The earthquake cracks have textures (i've toyed with them before - don't know if they have a model. They should be set as particles .prt) so I think all they need is an anim.